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Earth and Environmental Science
Preliminary course
Stage 6
Planet Earth and its environment
A five thousand million year journey
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Number: 43177
Title: Planet earth and its environment
This publication is copyright New South Wales Department of Education and Training (DET), however it may contain
material from other sources which is not owned by DET. We would like to acknowledge the following people and
organisations whose material has been used:
Photographs © R.A. Binns, CSIRO, taken from JAMSTEC submersible “Shinkai-6500”
Part 3 p 17
Photograph courtesy of Paul Brooks
Part 4 p 7
Photograph courtesy of Upgrade Business Systems Pty Ltd
Part 4 p 11
Photograph courtesy of NASA
Part 6 p 18
COMMONWEALTH OF AUSTRALIA
Copyright Regulations 1969
WARNING
This material has been reproduced and communicated to you on behalf of the
New South Wales Department of Education and Training
(Centre for Learning Innovation)
pursuant to Part VB of the Copyright Act 1968 (the Act).
The material in this communication may be subject to copyright under the Act.
Any further reproduction or communication of this material by you may be the
subject of copyright protection under the Act.
All reasonable efforts have been made to obtain copyright permissions. All claims will be settled in good faith.
Published by
Centre for Learning Innovation (CLI)
51 Wentworth Rd
Strathfield NSW 2135
_______________________________________________________________________________________________
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Copyright of this material is reserved to the Crown in the right of the State of New South Wales. Reproduction or
transmittal in whole, or in part, other than in accordance with provisions of the Copyright Act, is prohibited without the
written authority of the Centre for Learning Innovation (CLI).
© State of New South Wales, Department of Education and Training 2008.
Contents
Module overview ............................................................................................ ii
Indicative time....................................................................................... ii
Resources............................................................................................ iii
Icons .....................................................................................................v
Glossary............................................................................................... vi
Part 1: The evolution of the solar system ............................1–16
Part 2: The evolution of the Earth........................................1–26
Part 3: The evolution of life on Earth ...................................1–30
Part 4: The evolution of the atmosphere .............................1–38
Part 5: Carbon in the atmosphere and hydrosphere ...........1–22
Part 6: Climatic variation .....................................................1–36
Student evaluation
Introduction
i
Module overview
In this course you will be investigating the Earth and its processes.
There are four modules in the preliminary course. In this first module
you will explore the changes that have occurred throughout time–the
evolution of the universe, solar system and the Earth. In particular you
will focus on the evolution of the atmosphere and its relationship to life
on Earth.
Before you begin this module it is assumed that you can discuss the
currently accepted theory of the origin of the Universe. These ideas will
be briefly revised as work through the module.
The achievement of the outcomes, as listed below, will be assessed by:
•
you, when comparing your answers to those suggested
•
your teacher, using the exercises you are asked to complete and
return.
The best way to help your teacher help you is by:
•
attempting each of the questions and activities
•
returning your exercises regularly and on time.
Remember your teacher is there to help you!
Indicative time
There are four modules in the preliminary course in Earth and
Environmental Science. Each module should take you about 30 hours to
complete.
All modules in this course are composed of six parts. You need to spend
about five hours on each part though the learning in some parts might
take a little longer and in others could take a little less time.
ii
Planet Earth and its environment
Resources
Materials and equipment that you need to complete all activities in this
module are listed below. Remember that some of the activities are best
conducted at a practical session with your teacher. If you do not have
access to some of the materials do not become concerned. Your teacher
may suggest alternative activities or materials.
For Part 1 you will need access to:
No specialised equipment required.
For Part 2 you will need access to:
•
small baby food jar
•
oil (such as cooking oil)
•
water
•
iron sample eg. a bolt or nails (to simulate core material)
•
a rock sample preferably a non–porous rock such as granite (to
simulate crustal material)
•
graduated measuring flask or measuring cylinder, eg. baby bottle,
measuring cup
•
a set of scales, eg. kitchen cooking scales
For Part 3 you will need access to:
Introduction
•
a gas tight container (in which to carry out the fermentation) with a
tube passing to a smaller container
•
25 g of glucose C6H12O6 or sucrose (table sugar) C12H22O11
•
1 g of table salt
•
7 g dried yeast (found in the bread making or flour section of a
supermarket–usually in 7g, 12g or 28 g packets)
•
250 mL of water
•
a warm area to place the equipment (20–30°C is suitable)
•
limewater Ca(OH)2(aq) to fill the smaller container; made using the
following two items
•
lime (a chemical obtainable from a hardware store, bricklayer or
builder)
•
a sealable container eg. a glass jar with a lid
iii
For Part 4 you will need access to:
•
four glass jars (similar to vegemite or peanut butter jars)
•
steel wool
•
table salt
•
cooking oil
•
limewater prepared in Part 3
•
glass jar
•
straw
For Part 5 you will need access to:
No specialised equipment
For Part 6 you will need access to:
iv
•
ruler
•
pen
•
five metres of adding machine tape (or a roll of toilet paper or even a
roll of wide ribbon)
•
graph paper (supplied in the part)
Planet Earth and its environment
Icons
The following icons are used within this module. The meaning of each
icon is written beside it.
The hand icon means there is an activity for you to do.
It may be an experiment or you may make something.
You need to use a computer for this activity.
Discuss ideas with someone else. You could speak with
family or friends or anyone else who is available.
Perhaps you could telephone someone?
There is a safety issue that you need to consider.
There are suggested answers for the following questions
at the end of the part.
There is an exercise at the end of the part
for you to complete.
You need to go outside or away from your desk
for this activity.
Introduction
v
Glossary
The following words, listed here with their meanings, are found in the
learning material in this module. They appear bolded the first time they
occur in the learning material.
accrete
to build upon through the compilation and
compaction of material
accretion
product of accretion
aerobic
an environment that contains free oxygen
albedo
fraction of incident radiation scattered by a
surface
amino acids
are referred to as the basic building blocks of
life and link together by peptide bonds to
produce proteins
anaerobic
an environment that contains little to no free
oxygen
Archaea
the first cellular organisms on earth
archaeobacteria
very simple prokaryotic bacteria; are thought
to have been the very earliest forms of life on
Earth
asteroid
solid pieces of rock orbiting the Sun between
Mars and Jupiter
asthenosphere
is situated in the upper mantle and is thought
to contain between one and two percent
molten rock; the material that the lithosphere
moves over
astrophysicist
a scientist who studies objects within the
universe, in particular the formation and
destruction of stars
atmosphere
the gaseous environment surrounding the
surface of the Earth (or any other body in the
universe)
banded iron formation
particular formations whereby iron was
deposited in layers after having being
transported in the soluble (Fe2+) form
(BIF)
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bar
a unit used to measure pressure; one bar of
pressure is equal to 105 Newton of force per
square metre
black smokers
submarine volcanic vents that emit black
sulfides into the ocean
BP
before present
Planet Earth and its environment
Introduction
carbon
carbon is present in all living organisms.
Carbon can combine with itself and hydrogen
to form very long chains of hydrocarbons
which form our reserves of fossil fuels
carbon dioxide (CO2)
a gas that is emitted by volcanoes and is
expelled by living organisms through the
process of cellular respiration, and is used in
the process of photosynthesis
carbonate
carbonates are a group of minerals that
combine with the carbonate (CO3) radical
carbon flux
flow or movement of carbon in a carbon cycle
cellular respiration
the process whereby living cells break up
carbon molecules (glucose) to extract energy.
In this process carbon dioxide is also given
off
chemosynthesis
is the process where organic molecules are
manufactured from elements other than
oxygen in an oxygen free environment
chemosynthesise
undergo the process of chemosynthesis
chemosynthetic
organisms have the ability to chemosynthesise
and are classified as chemotrophs
chemotrophs
are organisms which can exist in the absence
of sunlight, and are capable of manufacturing
organic molecules from elements other than
oxygen in their environment
coccoliths
micro-fossil useful in the dating of rocks
combusts
a chemical reaction in which a substance
combines with oxygen producing heat, light
and flame
comets
small bodies of watery ice containing dust and
compounds of carbon, nitrogen, oxygen,
hydrogen
constellations
groups of stars with allocated names. Names
are often based on characters in Greek
mythological stories
condensation
the act of reducing a gas or vapour to liquid
form
cosmic particles
dust particles from space
Cretaceous
this period makes up the upper one third of
the Mesozoic era, beginning approximately
141 million years ago and extended through
until 65 million years ago
crust
the uppermost portion of the lithosphere; the
two types are continental and oceanic
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cyanobacteria
prokaryotic bacteria able to photosynthesise;
sometimes incorrectly labeled as blue-green
algae
daughter product
product of radioactive decay
degassing
the process of heating a solid body to release
molecules of gas into its atmosphere
diffraction
when a beam of radiation is split up into its
constituent rays by passing through such
objects as lens
electromagnetic
spectrum
the range of frequencies over which
electromagnetic radiation are displayed.
Included is the visible spectrum of light
electrons
negatively charged particles orbiting the
nuclei of atoms
energy levels
regions of energy around the nucleus of an
atom in which electrons orbit
equilibrium
a state of balance between opposing forces or
effects
evaporite
a sediment resulting from the evaporation of
saline water
evolved
to make changes to be able to adapt better to
the environment
flux
flow or movement
foraminifera
micro-fossil useful in the dating of rocks
fossils
an organic trace buried by natural processes
and subsequently preserved
fossil fuels
refer to hydrocarbons–coal, oil, oil shale and
natural gas
free (oxygen)
existing in the atmosphere as a gas molecule
(O2) and not combining with any other atom
or molecules
fumaroles
a type of volcanic vent that emits gases
containing mineralogy usually in the form of
sulfides
galaxy
a cluster of stars within the Universe; one of
the stars includes our Sun
geo-chemical analysis
the study of chemistry in rocks and fossils
glaciation
large ice sheet
glacier
ice masses that accumulate from compacted
snow on a seasonal basis; when the snow has
compacted sufficiently and it exerts enough
pressure, the body of ice will begin to
gradually move.
Planet Earth and its environment
Introduction
gravity
force of attraction between all masses; very
noticeable for small objects near extremely
large objects
halons
gases that contain either fluorine, chlorine, or
bromine
homogeneous
uniform mixture
hydrocarbons
organic compounds which contain which
contain only carbon and hydrogen, including
coal, oil, and natural gas
hydrosphere
includes all bodies of water on the planet,
oceans, rivers, streams, lakes, ground water
ice sheets
large deposits of accumulated ice
inert
not easily changed by chemical reaction
inner core
inner most layer of the Earth consisting of
solid iron
interglacial period
a period of time in history between ice ages
iron
is a metal which rarely occurs in its pure
form, due mainly to the fact it reacts readily
with oxygen to form oxides
isotopes
different types of atoms for the same element;
they only vary in the number of neutrons they
have in their nuclei.
life
for the purposes of this module refers to
cellular organisms whether they be single
celled or multi-celled; they may or may not
have a nucleus but do contain nucleic material
light year
the distance covered by light in one year.
Light travels at 300 000 km/s
lithosphere
the outermost rock layer of the Earth which
includes the uppermost mantle as well as the
continental and oceanic crust
LPTM
Late Palaeocene Thermal Maximum – high
temperature period about 55 million years
ago
Ma / mya
million years ago
mantle
layer of Earth between the outer core and the
lithosphere, containing about two-thirds of the
Earth’s mass; consists of iron and magnesium
bearing silicates.
marine sediments
sediments deposited in oceans
mass spectrometer
specialised equipment with the ability to
measure the mass of particular atoms
ix
x
meteorites
are solid fragments of asteroids
methane (CH4)
a natural gas; the simplest of the
hydrocarbons; one molecule contains one
carbon atom combined with four hydrogen
atoms
mineralogy
different types of minerals that go to make up
rocks, sediments, shells
molluscs
members of the phylum Mollusca, including
snails, pippies, squids and octopuses
nebulae
swirling gaseous cloud of cosmic dust that
begins to form stars
neutrons
atomic particles with no charge found in the
nuclei of atoms
oil
liquid hydrocarbon found in ‘oil traps’ in the
Earth’s crust
ostracod
micro-fossil useful in the dating of rocks.
Inhabited a fresh water environment
outgassing
see degassing
outer core
surrounds the inner core and is thought to be
composed of liquid iron and nickel based
minerals
oxide
a compound produced by another substance
combining and reacting with oxygen
oxidised
to combine and produce a compound with
oxygen
oxygen
the most abundant element present in the
Earth’s crust. Reacts readily to produce
oxides and makes up approximately 20% of
the Earth’s atmosphere
parent material
the original unstable radioactive material
before it decays into its more stable daughter
product
partially molten
state in between liquid and solid
peptide bonds
these bonds link amino acids together to form
proteins
periodic table
table of elements showing atomic structure
photodissociation
the decomposition of water vapour into
hydrogen and oxygen gas by the ultraviolet
radiation from the Sun
photolysis
see photodissociation
Planet Earth and its environment
Introduction
photosynthesis
the process whereby sunlight combines with
carbon dioxide to produce glucose (energy
rich) molecules and oxygen gas
photosynthetic
organisms that are capable of
photosynthesising
planetesimals
planets in their infant stage of development
undergoing accretion of material
precipitation
means to ‘fall out’ or ‘drop out’; may refer to
rain, hail, sleet, or snow; in chemical terms it
refers to solid being formed after a chemical
reaction which will then sink if in a solution
prokaryotic
cell with no nucleus
proteins
are large molecules formed by the linking of
amino acids. All proteins contain carbon,
hydrogen, oxygen and nitrogen; used for the
growth and repair of cells
protons
atomic particle with a positive charge
occupying the nuclei of atoms
pyruvate
an important three carbon molecule formed
from the division of glucose
Quaternary
most recent Period of time, extending from
two million years ago to the present day
radioactive decay
the breakdown of elements into other
elements by the emission of charged particles
from the nuclei (radiation)
reactant
chemical that reacts
reservoir
see sinks
respiration
energy releasing process in cells
sedimentary
to be deposited as a sediment. the sediment
will have been eroded and transported from
another source
shells
common name given to energy level
sinks
storage of a particular substance, in particular
carbon
Solar System
system of nine planets circling the Sun
including the Earth
soluble
a particle (solute) that is dissolved in a solvent
stromatolites
layered structures formed by the action of
bacteria and the entrapment of fine mud.
Alternating layers of calcium salts and mud
make up the stromatolite structure
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subduction zone
a convergent lithospheric plate boundary
where one plate, due to its greater density,
will be subducted beneath an opposing plate;
the subducting plate is remelted into the upper
mantle
sulfides
are compounds of sulfur that contain sulfur
atoms or sulfide ions
the Dreaming
Aboriginal belief system involving the telling
of mythological stories which are often linked
to the land; the Dreaming has spiritual
significance
Universe
contains the total amount of matter, energy
and space
uraninite
uranium dioxide (UO2) - a mineral
visible spectrum
a colourful band of waves emitted from the
Sun; wavelengths range from 0.4-0.7mm
volcanic vent
openings from volcanoes through which,
gases, dust and igneous material is ejected;
can be above or below sea level
volcanism
includes any emissions onto the Earth’s
surface (including under the oceans) or into
the atmosphere of gas, or igneous material
from a vent or fissure in the Earth’s crust
weathered
to break down either physically or chemically
to produce a type of sediment
Planet Earth and its environment
Gill Sans Bold
Earth and Environmental Science
Preliminary Course
Stage 6
Planet Earth and its environment
Part 1: The evolution of the Solar System
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Eon
Era
Period
Quaternary
Epoch
Pleistocene
Pliocene
Holocene
(last 10 000
years)
Change
of scale
10
Miocene
20
Cenozoic
30
Tertiary
Oligocene
40
Millions of years before present (Ma BP)
Eocene
50
60
Palaeocene
Phanerozoic
mass
extinction
70
Change
of scale
Cretaceous
Mesozoic
100
Jurassic
200
Triassic
Permian
Carboniferous
300
Palaeozoic
400
500
mass
extinction
Devonian
Silurian
Ordovician
Cambrian
Ediacaran
600
Change
of scale
Precambrian
1000
Proterozoic
age of BIFs
2000
3000
4000
Archaean
Hadean
oldest stromatolites
oldest evidence indicating life
Gill Sans Bold
Contents
Introduction ............................................................................... 3
Placing the Earth in Perspective within the Universe ................ 4
Origin of the Universe ..........................................................................4
Galaxies................................................................................................6
Solar System ........................................................................................7
Suggested answers................................................................. 13
Exercises–Part 1 ..................................................................... 15
Part 1: The evolution of the Solar System
1
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Planet Earth and its environment
Gill Sans Bold
Introduction
What are the features of the Universe? How did it form? These are the
questions you will be seeking answers to in this part.
In this Part you will be given opportunities to:
•
explain the existence of matter in the universe using current
scientific ideas, and describe the process of accretion of such matter
to form stars and planets
•
identify a sequence of events described by scientists to outline the
formation of the Solar System
At the end of Part 1, you should:
•
gather, process and present information that outlines a sequence of
events that led to the formation of the Solar System.
Extract from Earth and environmental science Stage 6 Syllabus © Board of
Studies NSW, amended November 2002.
The original and most up–to–date version of this document can be found on the
Board’s website at:
http://www.boardofstudies.nsw.edu.au/syllabus_hsc/syllabus2000_liste.html#e .
Part 1: The evolution of the Solar System
3
Placing the Earth in perspective
The Universe includes all of space. It is composed of all known matter
and energy, including billions of galaxies each of which contain up to
100 000 billion stars. To put this into perspective the Sun is considered
to be a smaller than average star.
Have you ever wondered where all this matter came from? How did all
these stars, planets, moons, and other objects such as meteors and comets
come to exist? These questions lead to other questions such as, how
significant is our own planet? And, could life exist on other planets?
Questions such as these have been teasing and challenging humans since
our beginnings. Stonehenge, for example was built approximately
5000 years ago in Wiltshire England to trace the paths of the Sun and
the Moon.
The study of astronomy took a great leap forward during the age of
learning in the Renaissance. Galileo Galilei (1564–1642) invented the
refracting telescope which greatly enhanced the study of stars and other
objects in the night sky.
Origin of the Universe
What are your thoughts on the origin of the Universe? How do you think
it came to be?
The current scientific thinking is that the Universe is thought to be
expanding away from a central group of galaxies which began about
11 to 15 billion years ago. One theory suggests that this occurred after a
huge explosion. This theory is known as the Big bang theory.
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Big bang theory
The observations of modern day astronomers suggest that the Universe
including space and matter is still expanding from an infinitely dense and
energetic state that existed some 11 to 15 billion years ago.
The implication of the Big bang theory is that before the Big bang there
was no matter. All matter was created during the Big bang. Initially the
heat energy of that concentration of matter was high. As time went on
space continued to expand and the matter created in the Big bang became
more dispersed, temperatures decreased and eventually condensation
resulted in galaxies being formed. These galaxies included stars (such as
our own Sun) which formed from smaller condensations and were
sometimes accompanied by planets.
As the Universe continues to expand it drags the galaxies with it.
Therefore galaxies do not maintain a fixed position within the Universe.
This theory presents a ‘closed model’, in that the same matter that is
currently expanding will eventually collapse in on itself under its own
gravity. The result would be another big collision and explosion, similar
to the initial Big bang, and the cycle will begin once again.
1
2
3
4
5
The Big bang theory seeks to explain the formation of the Universe. An
explosion releases all energy and matter which condenses to form galaxies.
The process goes through a cycle of steps from 1 to 4 before beginning again.
Flat Universe theory
A new slant on the Big bang theory idea is an idea that the Universe may
continue to expand forever and may not undergo gravitational collapse.
Under this model the fate of the Universe is a black cold place with
no illumination.
This theory also states that the Universe began initially with a large
explosion. However, it assumes that the material is less dense, and as
such the material will not collapse in on itself, as in the model above.
Instead, the material will keep on moving away from the point of
explosion. This will effectively mean that the Universe will continue to
expand and is therefore considered an ‘open model’.
Part 1: The evolution of the Solar System
5
1
2
3
4
5
The flat Universe theory states that the Universe began with a huge explosion
and the materials released will continue to expand.
Comparing the theories
1
Which of the above theories seems to you to give the best explanation
for the origin of the Universe? Has this information changed your
original ideas? Why? Why not?
______________________________________________________
______________________________________________________
______________________________________________________
2
Describe your preferred theory to another person. Tell them how this
theory differs from one of the other theories outlined above.
3
In your opinion, what part of your preferred theory needs more
explanation? Where do you think you would go to try and find more
information about this theory?
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
Galaxies
Galaxies are the largest structures within the Universe, and are classified
according to shape. Galaxies are made up from stars, gas and dust.
Our galaxy is known as the Milky Way. A galaxy is a concentration of
matter in the emptiness of space.
When you look up into the sky at night, all the stars that you see are not
revolving around the Sun. Our Sun is only one of 100 billion stars that
orbit about a common centre of gravity. This collection of stars make up
our Milky Way galaxy. The Milky Way is one of an estimated 125
billion galaxies in the Universe.
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How long do you think it would take to travel from one end of our galaxy
to the other?
If you were in a special space ship that could travel at the speed of light
(300 000 kilometres per second), it would take you 100 000 years to
cross our galaxy. Keep in mind that our galaxy is only one of many
billions of galaxies in the Universe!
The Solar System
Our Solar System comprising our star, the Sun, and its planets lies within
the Milky Way galaxy and is composed of a number of objects.
Before moving on, try and see how much you already know about the Solar
System. Answer the following questions as best you can.
1
Apart from planets, what other objects are included in the Solar System?
_____________________________________________________
_____________________________________________________
2
How many planets make up our Solar System?
_____________________________________________________
3
What are the names of the planets in our Solar System? List them in
order from the Sun to the outermost planet.
_____________________________________________________
_____________________________________________________
_____________________________________________________
Check your answers.
Satellites
In your answer to question one above, you should have mentioned
satellites. What do think of when someone mentions the term satellite?
You probably picture a metal device with reflectors orbiting the Earth
after having been launched from the back of the space shuttle. This is
just one type of satellite, classified as an artificial satellite.
The first artificial satellite was built and launched from Russia in 1957.
It communicated information about the upper atmosphere back to Earth.
Satellites refer to bodies that rotate in orbits around other bodies of
greater mass, and are held in a gravitational field. Natural satellites
Part 1: The evolution of the Solar System
7
include moons, such as our own, rotating around planets. Our Solar
System contains many of these natural satellites.
The origin of our Solar System
The Solar System has fascinated humankind for centuries. Prior to the
invention of the telescope Mercury, Venus, Mars, Jupiter and Saturn,
together with the Sun and Moon, could be studied at length with the eye
due to their brightness.
Aristarchus of Samos (280–264 BC) stated that all the planets rotated
around the Sun and that the Earth was one of the planets. Unfortunately,
this idea went ignored and it took almost 2000 years before it was
considered to be correct. During this 2000 year period the most accepted
theory was that the Earth held the central position of the Universe. It was
not until Nicholas Copernicus in 1543, published his book stating that the
Sun was the centre of the Universe, did modern astronomy take off.
It was only during the last two hundred years that the development of the
telescope enabled the discovery of Uranus, Neptune, and Pluto.
Sun
The Solar System is made up of the Sun, nine planets, satellites, minor planets
and comets.
The Solar System also contains asteroids. These are relatively small
compared to say our Moon, and are made up from solid fragments of
matter that orbit the Sun largely between Mars and Jupiter.
Meteorites, which are solid fragments of asteroids, and comets, made up
of ice and dust, are also included in the contents of the Solar System.
There are many theories proposed for the formation of the Solar System.
Some of these are outlined below.
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Nebular hypothesis
In 1755 the German philosopher Immanuel Kant proposed a model
whereby rotating clouds of gas (nebula) condense into individual
globular bodies which, as they do so, would increase their
rotational speed.
This phenomenon obeys the Law of conservation of angular momentum,
and can be illustrated by an ice skater spinning on his or her own axis.
If an ice skater begins spinning with their arms outstretched and then
they bring their arms in close to their body, they will increase their rate
of spin.
Therefore, the centrifugal force that results from these globular bodies
condensing, flings gases off into a succession of rings orbiting the Sun.
These rings are then thought to eventually condense to form individual
planets.
The speed of an ice skater spinning changes with the position of the arms.
Approximately 100 years later physicists James Maxwell and Sir James
Jeans from Britain disagreed with this theory stating that there was not
enough mass in the gaseous rings to produce a gravitational field strong
enough to condense the gases into planets.
More recent ideas
The Chemical condensation sequence model really builds upon the ideas
of the nebular model outlined above. This model goes some way in
explaining and predicting the different compositions and densities of
materials in the different planets.
Part 1: The evolution of the Solar System
9
Chemical condensation sequence model endeavours to explain and predict the
different compositions and densities of different planets. It builds on the nebular
hypothesis. A spinning cloud of gas and dust surrounds a developing sun.
Planets formed as this mass cooled.
In this model a spinning hot disc of gas and dust surrounds the
developing star. As this cooled, small particles begin to come together
forming larger particles and eventually planetesimals (planets in their
infant stage).
Depending on how close these planetesimals are to the star, different
materials will not be able to condense because of the heat, and will be
blown away as a gas by solar radiation. Therefore, those closest to the
Sun will condense materials such as metals and rocks which have high
boiling points.
This would explain Mercury’s high iron content, and the lighter rock
forming minerals such as magnesium, silicon and oxygen, accumulating
on planets which are at a greater distance from the Sun. Planets which
were being formed on the outer edge of the Solar System furthest away
from the Sun, would accumulate materials such as water, methane and
ammonia as ice.
Jupiter and Saturn are said to be large enough to have a gravitational
attraction which could hold on to all of the materials present in their
original nebula, including hydrogen and helium.
The planetesimals, due to their gravitational attraction, accumulated
material through a process known as accretion. As more and more
material bombarded these planetesimals they grew in size, and eventually
formed into planets.
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The accretionary process.
This accretionary process is thought to have been a major factor in
generating heat within the Earth’s interior. Heat is also generated
through the decay of radioactive elements held within the Earth’s rocks.
To see a link site that contains a set of archived stories about planetesimals
and evidence for the formation of solar systems about stars viewed with
space telescopes such as the Hubble space telescope see a site on the EES
links page at http://www.lmpc.edu.au/science.
The latest generation of space telescopes has enabled scientists to test
theories of solar system formation through searching for examples of
dust discs surrounding young stars. The success in finding the evidence
to support what was once only theory has been nothing short of
spectacular.
Create a series of labelled diagrams summarising the sequence of events
outlined above under the heading of More recent ideas. You should create
at least four diagrams to do this activity correctly. The use of colour and a
key is often helpful in diagrams such as these. You should also include a
small description under each of the diagrams to help explain each diagram.
Turn to the exercises at the end of this part and complete Exercise 1.1 and
1.2.
What’s next?
In this part you learned about how scientists now believe the Solar
System evolved.
In the next part you will focus on events closer to home: the evolution of
the early Earth and its changing composition.
Part 1: The evolution of the Solar System
11
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Suggested answers
Solar System
1
The Solar System consists of: the Sun; nine planets; many satellites
(bodies of rock revolving around planets); and a large number of
minor planets, meteoroids and comets.
2
There are nine planets.
3
The name and order of the planets from the Sun are Mercury, Venus,
Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto.
Part 1: The evolution of the Solar System
13
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Exercises – Part 1
Exercise 1.1 and 1.2
Name: _________________________________
Exercise 1.1: How the Earth came to be
In your own words, and with the aid of diagrams, explain how you
understand the Solar System to have formed.
In your explanation you will need to include the following terms:
nebular, satellites, meteorites, asteroids, comets, telescope, accretion,
planetesimals, star, Uranus, Neptune, Pluto, gravity, ice, radioactive.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Part 1: The evolution of the Solar System
15
Exercise 1.2: Evidence for how the Solar System
formed
Using the Internet or very recent publications gather then present some of
the evidence discovered by space telescopes describing the formation of
planets and solar systems. If you do not have access to the Internet or a
library of recent astronomy publications your teacher may supply you
with some reference materials . Make sure you reference any
information you present. Use of the following key words in any Internet
search will get you started:
nebular, satellites, space telescope, accretion, planetesimals, star, solar
system formation.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
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Planet Earth and its environment
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Earth and Environmental Science
Preliminary Course
Stage 6
Planet Earth and its environment
Part 2: The evolution of the Earth
0
20
I
er
b
to T S
c
O EN
g
in D M
t
a
r EN
o
p
or AM
c
n
2
0
1
2
3
4
5
Eon
Era
Period
Quaternary
Epoch
Pleistocene
Pliocene
Holocene
(last 10 000
years)
Change
of scale
10
Miocene
20
Cenozoic
30
Tertiary
Oligocene
40
Millions of years before present (Ma BP)
Eocene
50
60
Palaeocene
Phanerozoic
mass
extinction
70
Change
of scale
Cretaceous
Mesozoic
100
Jurassic
200
Triassic
Permian
Carboniferous
300
Palaeozoic
400
500
mass
extinction
Devonian
Silurian
Ordovician
Cambrian
Ediacaran
600
Change
of scale
Precambrian
1000
Proterozoic
age of BIFs
2000
3000
4000
Archaean
Hadean
oldest stromatolites
oldest evidence indicating life
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Contents
Introduction ............................................................................... 3
Evolution of the Earth ................................................................ 4
The earliest stage.................................................................................4
Fpormation of a layered Earth................................................... 7
The role of gravity.................................................................................9
Theory to explain a layered Earth .....................................................10
The Earth’s layers ..............................................................................12
A traditional Aboriginal perspective ......................................... 18
The evolution of the atmosphere............................................. 21
The composition of the atmosphere ..................................................21
The early atmosphere ........................................................................21
Suggested answers................................................................. 23
Exercises–Part 2 ..................................................................... 25
Part 2: The evolution of the Earth
1
2
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Introduction
At the end of Part 2, you will have been given opportunities to learn to:
•
compare cultural beliefs with the views of astronomers and other
scientists that may arise in discussion of the origins of the Earth
•
explain the role of gravity in the formation of the Earth
•
describe the relationship between the density of Earth materials and
the layered structure of the Earth
•
describe the composition of the early (pre–oxygen) atmosphere and
compare it with the composition of the present atmosphere.
At the end of Part 2, you will have been given opportunities to:
•
gather and process information that compares a cultural explanation
with an astronomical or scientific model of the origin of the Earth
•
perform a first–hand investigation to measure the density of a
selection of earth materials representative of core, mantle and crust
•
identify data sources, process and present information from
secondary sources to compare Earth’s earliest atmosphere with the
present atmosphere.
Extract from Earth and environmental science Stage 6 Syllabus © Board of
Studies NSW, amended November 2002.
The original and most up–to–date version of this document can be found on the
Board’s website at:
http://www.boardofstudies.nsw.edu.au/syllabus_hsc/syllabus2000_liste.html#e .
Part 2: The evolution of the Earth
3
Evolution of the Earth
How old do you think the Earth is and what would you use to measure
its age?
If you take the age of the oldest known sedimentary rocks on the planet
as being the age of the Earth, then the Earth would be approximately
3 800 million (3.8 billion) years old. However, if you accept the position
of many of the world’s leading astrophysicists, that the Solar system,
including the Earth and meteorites all formed at the same time, then the
age of the Earth could then be estimated at approximately 4 700 million
(4.7 billion) years old.
The age of the Earth has been derived from the dating of known
meteorites that have landed on the Earth.
The earliest stage
For the first billion years of its life the Earth existed as a relatively cool
and homogenous mass of silicon compounds; and iron and magnesium
oxides. These materials would have been distributed relatively evenly
throughout the Earth’s interior at this very early stage.
Temperature and pressure
As materials began to react with each other producing new material,
planetesimals (small planets), began to accrete (gather and build upon
each other) and the temperature began to increase. This increase in
temperature is due to the energy being carried and released upon the
impact of accreting material, and also due to the increase in pressure as a
result of the accretion itself.
This event occurred approximately 4 700 million (4.7 billion) years ago.
Refer to the diagram on the next page.
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The accretionary model. Materials react to form new products which accrete
(build on each other). This results in an increase in temperature.
It has been estimated that the average temperature reached as high as
1000°C as a result of accretion and compression of material during this
early stage of the Earth’s development.
(An increase in pressure causes an increase in temperature. Have you
ever felt how hot your push bike pump is after you have pumped up one
of your tyres on a pushbike?)
Temperature and radioactive decay
Radioactive elements such as uranium, thorium and potassium, only
make up a small percentage of the Earth’s elements however, these
play an important role as they contributed to the heating of the Earth’s
interior. How do you think radioactive elements are able to
generate heat?
When radioactive elements are first formed they are said to be ‘unstable’.
This is because as soon as they are formed they begin to change into a
different element, which is more ‘stable’. Part of this transformation of
an unstable radioactive element into a stable non–radioactive element
involves the production and emission of radiation and heat.
This process of change is called radioactive decay. The unstable
radioactive element is also called the parent material, and the stable
non–radioactive element produced from the decay of the parent material
is known as the daughter product.
Part 2: The evolution of the Earth
5
Parent material
Daughter product
uranium
lead
potassium
argon
thorium
lead
rubidium
strontium
Radioactive elements decay to form daughter products.
Parent material and daughter products
The table above shows some examples of radioactive elements and their
daughter products. Remember, when each of the above parent materials
decay to their stable daughter products they give off radiation and heat.
It is this heat which contributed to the Earth’s initial heat source and is
still contributing to heat the Earth’s core today.
The heat generated through accretion and the heat generated through
decay of radioactive elements was being produced at a faster rate than it
could escape. Because rocks are poor conductors of heat, the Earth’s
interior began to increase in temperature.
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Formation of a layered Earth
Did you know that the Earth is made up of a series of layers? Try this
self–correct exercise to see what you already know. Draw and label a
diagram showing a cross–section of the Earth.
Check your answer.
Did the Earth always have this layered structure? If it did not, then how
did it come to develop this structure?
What is density?
The ideas behind how the Earth formed in layers relies heavily on the
concept of density. It is therefore important to understand thoroughly
what density is. The density of a substance is the amount of matter that
substance contains in a given volume.
1 tonne feathers
1 tonne steel
One tonne of feathers has the same mass as one tonne of steel.
Part 2: The evolution of the Earth
7
Every one knows that one tonne of feathers weighs the same as one tonne
of steel. However, the steel would take up considerably less volume than
the feathers would. This is because the atoms that go to make up the
matter in steel are packed much closer together than the atoms that go
to make up the feathers. Therefore, steel is said to be more dense
than feathers.
The diagram on the next page shows a range of substances. It compares
the density of these different materials. Each of the boxes in the diagram
has the same volume–one cubic metre. The mass of the matter in each of
the boxes is measured in kilograms and is shown on the top of each box.
750 kg
feathers
wood
re
1000 kg
a s in g d e ns
i ty
inc
1.5 kg
45 kg
air
water
11 300 kg
1600 kg
lead
7800 kg
2700 kg
steel
aluminium
coal
Different materials have different densities.
1
8
List the materials shown in the diagram in order from the least dense
through to the most dense.
a) ____________________
b)________________________
c) ____________________
d)________________________
e) ____________________
f) ________________________
g) ____________________
h)________________________
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2
The average density of the Earth is 5500 kg per cubic metre.
Which of the materials in your list above would best represent the
average Earth material?
_____________________________________________________
_____________________________________________________
Check your answers at the back of this part.
The role of gravity
What is gravity? Is gravity the same everywhere?
Gravity is the force that one object applies on another object and is
determined by the amount of mass or matter in that object.
Isaac Newton proposed the Law of Gravitation, that states ‘every particle
in the Universe attracts every other particle with a force that is directly
proportional to the combined masses of the particles and inversely
proportional to the distances between the particles’. In other words, the
more massive the particles the greater is the gravitational force those
particles can supply.
For example, the Earth has more matter, and therefore has a greater mass
than the Moon. As a result the Earth has a stronger gravitational force.
In fact the Earth’s gravitational force is six times stronger than that of
the Moon.
This force attracts or ‘pulls’ objects towards the centre of the mass that
created it. The Earth, therefore, attracts objects towards its centre.
If the Earth’s gravitational attraction is stronger than the Moon’s, why
doesn’t the Moon come crashing into the Earth?
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Check your answer.
We are also attracted towards the centre of the Earth but are prevented
from accelerating towards it by a solid and rigid continental crust.
However if we were to attempt to stand on something less rigid such as
the ocean we would be drawn closer to the Earth’s centre.
Part 2: The evolution of the Earth
9
Implications for a developing Earth
One implication is that gravity formed the driving mechanism for the
accretionary model of the Earth. Material is attracted and accreted from
out of space due to the Earth’s gravitational force. As the Earth gained in
mass so did the amount of gravitational attraction, which in turn led to
more material being attracted and accreted.
Another implication is that once the Earth heated up as a result of the
impact and pressure from this accretionary process, (and from the decay
of radioactive elements) material became partially molten. This allowed
the denser materials to be drawn down more easily to the centre of the
Earth by the Earth’s gravitational force. This attracting force allowed the
very first layering of Earth materials.
What possible conclusions can you make about the composition of the
Earth’s interior, given that the average density of all the Earth’s material is
5.5 grams per cubic centimetre, and the average density for continental
crustal material is approximately 2.5 grams per cubic centimetre?
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Check your answers.
Theory to explain a layered Earth
As you know, heat was generated from the impact of material accreting
as well as from radioactive elements decaying down to their daughter
products. The temperature eventually rose enough for iron to melt.
The melted iron then began to accumulate and, due to its greater density,
began to sink towards the centre of the Earth forcing other material not as
dense to be positioned above it. This movement of iron in itself would
generate heat due to the friction created during movement.
Approximately one–third of the Earth’s material sank towards the centre
of the Earth. Due to this material being heated it was not in a completely
solid state. However, it was also not in a completely liquid state either,
as we would perhaps imagine. This material is said to be in a partially
molten state, which is somewhere in between liquid and solid.
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The lighter molten material rose to the surface and cooled in the process,
forming an early type of crust. Between the iron core and this early crust
remained a mantle. During this time gases were most probably released
from the Earth’s interior which, in turn led to the formation of the
atmosphere and oceans. The result is a layered planet.
The diagram below illustrates the basic sequence of events leading to the
Earth’s layered structure.
Label the layers of the last diagram by referring to the previous information.
As well, you may need to refer to the following information The Earth’s
layers. Please note this diagram of the Earth’s layers does not include the
atmosphere, and that the crust is included in the lithosphere layer.
The early Earth – a homogenous mixture.
Iron sank to the centre.
The result is a zoned planet.
Check your answers at the back of this part.
Part 2: The evolution of the Earth
11
The Earth’s layers
The inner core is thought to be solid and consisting almost entirely of
iron. The outer core is thought to be liquid and is mostly composed of
iron and nickel based minerals.
The mantle is thought to be composed mainly from the oxides of silica
and magnesium as well as some oxides of iron and aluminium.
The mantle is separated into upper and lower regions based on the
density differences between minerals.
Since the lower mantle is deeper, minerals produced in this region have
done so under greater pressure than the minerals produced in the upper
mantle. As a result their crystal structure will change but not necessarily
their chemical composition, producing a more dense structure. Carbon is
a well–known example of a material changing its structure according to
differing pressure at the time of its formation. If the pressure is high
enough the carbon will take the form of diamond, if the pressure is lower
it will take the form of graphite.
The boundary between the upper and lower mantle is mainly based upon
the different structures and density of material produced rather than
differences in chemical composition.
Asthenosphere
The asthenosphere is situated in the upper mantle and is thought to
contain between one and two percent molten rock. This small percentage
of molten material reduces the friction between rocks and allows rocks
above it (the lithosphere) to move over the top. (See the diagram below)
The asthenosphere has been likened to conveyor belt–creating a mobile
zone for material to move above it.
oceanic crust
0
Depth (km)
50
70
oceanic crust
continental
crust
lithosphere
(rigid – includes crust)
100
150
asthenosphere (partially molten)
200
250
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Lithosphere
The lithosphere is composed of the uppermost part of the mantle and the
overlying crust, and comprises of relatively cool and rigid rock situated
above the asthenosphere. The lithosphere is a mobile layer and can move
across the top of the partially molten asthenosphere.
The crust, which forms the upper most part of the lithosphere, is either
continental or oceanic.
The continental crust is less dense than the asthenosphere and therefore
cannot be puled down into the lower mantle. Continental crustal
material generally ranges from 20 to 70 kilometres in thickness. In most
cases, however, the continental crust is approximately 35 km in
thickness. This crustal material is largely composed of oxides of silica,
aluminium, iron, magnesium, potassium and calcium. (See the diagram
on previous page.) The minerals that make up the continental crust do
not undergo transformation to new minerals that increase in density as
they are placed under greater pressure.
Oceanic crust does not vary in composition as much as continental crust.
Oceanic crust is denser than continental crust and is composed of basaltic
rock that is rich in iron, magnesium and silica. The minerals that make
up the oceanic crust are converted to other more dense minerals when
placed under great pressure. This increased density aids oceanic crust to
be subducted or sink into the lithosphere. Oceanic crust is relatively thin
ranging from approximately 6 to 10 km in most places. Its thickness is
much more uniform than that of the continental crust. Lying on the
oceanic crust are the oceans and seas that make up the majority of the
hydrosphere. (The remainder of the hydrosphere is made up from
rivers, creeks and all other bodies of water on the Earth’s crust.)
The outermost layer of the Earth is known as the atmosphere, and is
composed largely from nitrogen (78%) and oxygen (21%).
The atmosphere is both the outermost and the least dense layer of
the Earth.
Write a statement linking the different density of Earth’s materials with their
location, as described in the layered model of the Earth above.
_________________________________________________________
_________________________________________________________
_________________________________________________________
Check your answer at the back of this part.
Complete Exercise 2.1 at the end of this section.
Part 2: The evolution of the Earth
13
Density of liquids and layering
Aim:
To create a model that represents the layering of the Earth based on density
differences between different materials.
Materials:
•
small baby food jar
•
oil (such as cooking oil)
•
water
Procedure:
1
Fill approximately one third of the jar with oil.
2
Fill another third of the jar with water.
3
Allow the jar to sit until all the fluids in the jar have settled.
Results:
1
Make a labelled sketch of the jar immediately after you have poured
in the water and before the liquids have settled.
2
Make a labelled sketch after the liquids have settled.
Sketch before settling
14
Sketch after settling
Planet Earth and its environment
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Conclusion:
How can these results be used to model the formation of the Earth’s
interior?
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Density of Earth materials
Aim:
The purpose of this activity is to find material which would simulate the
composition of the Earth’s crust and mantle, and then perform an
investigation to compare their densities.
Background information:
The density of a material is determined by the amount of material in a
given volume. Density is often given in:
•
kilograms per cubic metre (kg/m3)
•
grams per cubic centimetre (g/cm3 or g/cc).
Therefore, the density of an object can be calculated by dividing the mass
of an object by its volume.
density (D) = mass (m) ∏ volume (V)
It is also important to note that, when dealing with volume there is a
direct relationship between cubic centimetres (cm3) and millilitres (mL).
1 cm3 = 1 mL
1000 cm3 = 1 L
The mass of an object can be found by using a mass balance or a set of
scales marked in kilograms or grams.
The volume of a small object can be found by lowering the object into a
container of water with measuring graduations on its side.
Remember the narrower the container (with finer graduations on its side)
the more accurate the reading will be. That is why a measuring cylinder
is normally used in laboratories. When the object is fully immersed, the
rise in water level gives the volume of the solid.
Part 2: The evolution of the Earth
15
Method:
1
You need to collect the following:
•
iron sample eg. a bolt or nails (to simulate core material)
•
a rock sample preferably a non–porous rock such as granite
(to simulate crustal material)
•
graduated measuring flask or measuring cylinder eg. baby
bottle, measuring cup
•
a set of scales eg. kitchen cooking scales.
Equipment required to determine the density of a bolt.
2
With the above information and the equipment you have collected,
write out a series of steps which will enable you to calculate the
density of your simulated core material sample, and your simulated
crustal material sample. Use diagrams where appropriate.
Place these steps under the heading of Procedure.
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
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Results
Record your findings under the heading of Results. Remember to use the
correct units for density.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Conclusion
Under the heading of Conclusion, make a statement about the density of
each of your samples linking them with the formation of the Earth.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Part 2: The evolution of the Earth
17
A traditional Aboriginal perspective
The Aboriginal belief system is known as the Dreaming. In the
Dreaming, meaning is handed down from generation to generation by the
telling of mythological stories, which are often linked to the land.
Their religion is the land and therefore has spiritual significance.
The following stages give a brief outline of the formation of the Earth as
described in one version of the Dreaming.
18
1
At the beginning the enormous flat mass of Earth was surrounded by
evil and murky water and everything was enveloped in darkness.
Sand and soil covered the surface and gigantic rocks were hidden
beneath the surface. These rocks were supported by many huge tree
trunks. Without this support the Earth would have collapsed in on
itself and be lost forever.
2
Suddenly the Sun was born and he forced his way up through the
land mass and still water. This was thought to be the first man and
his face was a blaze of fire which shed light on the very flat and
dusty plains. The day ended when the Sun became weary and
gradually sank underground leaving everything in darkness.
3
After resting the Sun returned to the east through an underground
passage which he had made. The Sun felt he must once again see
this strange place. The Sun’s habits have been regular since this first
day.
4
The Sun did not know that mysterious things were hidden
underground and only needed light to make them flourish. The soil
awakened and every day was different. Springs began to trickle out
of the dirt and these springs eventually formed creeks, lagoons and
lakes. Grasses, plants and trees rose from the soil covered with fruit,
berries and flowers. The Sun saw more and different plants every
day, while the trees grew in height.
5
One day he saw an unbelievable change when the Earth beneath him
appeared to erupt. Level ground was pushed toward him as hills and
mountains appeared. Large trees and rocks rose from the soil and
flowing water was spread in all directions. The Sun saw that living
creatures were emerging from underground. They had been asleep,
but now they had light, water and plant growth, and it was time to
waken.
Planet Earth and its environment
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In the boxes below make diagrammatic representations of how you visualise
the five stages described above.
1.
2.
3.
4.
Part 2: The evolution of the Earth
19
5.
Discuss with members of your community, family or class why it is
important to include a cultural perspective when considering the origin of
the Earth.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Complete Exercise 2.2 at the end of this section.
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The evolution of the atmosphere
The composition of the atmosphere
As stated earlier, the Earth’s current atmosphere is largely composed of
two gases:
•
nitrogen (78%)
•
oxygen (21%).
The remaining 1% is mostly made up of argon (0.93%), carbon dioxide
(0.03%) and trace elements of other gases.
This composition is the result of a continual evolutionary process.
The early atmosphere
Our planet was unable to keep an atmosphere of gases such as hydrogen,
as the radiation given off by the Sun would have driven these very light
gases away and they would have escaped into space.
Our early atmosphere is thought to have developed from within the
Earth’s interior in a process known as outgassing or degassing.
During this process gases are released from the Earth’s interior due to
internal heat gained from the process of accretion and the decay of
radioactive elements.
The earliest evidence we have for the composition of Earth’s primitive
atmosphere originates from rock samples. The oldest rocks on the planet
are approximately 3800 million years old and they provide with some
clues about the atmosphere at the time of their formation. This leaves a
question mark over what our very first atmosphere was like, 800 to 900
million years before these rocks were formed. We have no
direct evidence.
The current scientific view is that the early atmosphere probably
consisted of methane (CH4), water vapour (H2O), nitrogen (N2), carbon
dioxide (CO2), ammonia (NH3), and hydrogen sulfide (H2S). Methane is
Part 2: The evolution of the Earth
21
thought to have been the dominant gas at this early stage. The role of
methane as a greenhouse gas in the early atmosphere was probably
critical to the development of the planet. In the early days of the planet
the Sun was much less luminous than it is today so the transfer of energy
to Earth’s surface as solar radiation would have been much lower.
Without the greenhouse effect of the methane atmosphere the presence of
liquid water essential to early life and sedimentary processes would not
have been possible on Earth’s surface.
The 3800 million year old rocks referred to above were found in
Greenland and were originally deposited as sediments in water. It can be
inferred that:
•
the Earth had at least cooled to a surface temperature below 100°C
by this stage and that the greenhouse effect keeping the surface
temperature in the range for liquid water was operating
•
these sediments must have been derived from the erosion of even
older rocks.
Comparison of atmospheric composition
Compare the composition of the early atmosphere to the gases present in our
current atmosphere. Describe the similarities and differences between the
early atmosphere and the current one.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Check your answers.
Ideas and evidence for the composition of the Earth’s earliest atmosphere is
an area of active research by scientists. Check out some of the links to
studies of this fascinating area on the EES website links page at:
http://www.lmpc.edu.au/science.
Next…
So far you have learned about the origin of the Universe and the Earth.
In the next part you will find out how scientists believe living cells
originated.
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Suggested answers
Formation of a layered Earth
lower mantle
inner core
outer core
lithosphere
asthenosphere
upper mantle
What is density?
1
a) air
b) feathers
c) wood
d) water
e) coal
f)
aluminium
g) steel
h) lead
2
A material more dense that aluminium and less dense that steel
would represent the density of the average Earth material.
The role of gravity
Even though the Earth has a stronger gravitational attraction than the
Moon, the Moon will not be drawn into a collision with the Earth.
This is because it also wants to be hurled outwards away from the Earth
due to its orbit around the Earth (centrifugal force). These two forces,
gravity and the centrifugal force cancel each other out resulting in the
Moon remaining in its present orbit.
Part 2: The evolution of the Earth
23
Implications for a developing Earth
The fact that the Earth’s average density is virtually double that of the
rocks that go to make up continental crust, means that the Earth’s interior
must be made up of material that is far greater in density than the
material in the Earth’s crust. This therefore supports the layered model
of the Earth.
Theory to explain a layered Earth
solid iron core
(4980–6370 km)
liquid outer core
(2900–4980 km)
lithosphere
(0–70 km)
asthenosphere
(70–250 km)
lower mantle
(700–2900 km)
upper mantle
(250–700 km)
The result is a zoned planet.
Lithosphere
The very dense elements such as iron, nickel, cobalt and the radioactive
elements are concentrated mostly in the inner and outer core.
Material gradually becomes less dense in the mantle until the least dense
material is concentrated in the Earth’s crust which ‘floats’ on the more
dense upper mantle. The atmosphere could even be considered the
Earth’s outermost layer and is by far the least dense layer.
Comparison of atmospheric composition
It is difficult to say for certain what the early atmosphere was like,
however, it did contain methane, nitrogen, carbon dioxide and water
vapour like our current atmosphere. The present atmosphere contains
much less methane, ammonia and hydrogen sulfide.
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Exercises – Part 2
Exercises 2.1 to 2.2
Name: _________________________________
Exercise 2.1: Formation of the Earth
Read carefully the previous sections on The earliest stage, The formation
of a layered planet and Theory to explain a layered Earth. You may
even wish to do some of your own research for this exercise.
Create a flow diagram summarising the events that led to the formation
of the Earth. These diagrams should show a logical sequence from the
Earth’s earliest beginnings through to its present day structure.
Ensure these diagrams are well labelled and contain a summarising
caption beneath each diagram.
Your flow diagram should contain at least six individual diagrams and
should not be too small (approximately 2 to 3 diagrams per page.)
Part 2: The evolution of the Earth
25
Exercise 2.2: Research assignment
Research, and in no more than two A4 pages, report on the beliefs of a
particular cultural group on the Earth’s origin.
You may wish to take another story from the Dreaming, or you might
like to take a particular religious viewpoint or perhaps even draw from
another culture altogether.
In your two page report, include such things as relevant diagrams and
quotes.
Your research may include: accessing the internet, visiting your local
library or even conducting an interview with an appropriate elder.
Below are some questions you may use to help guide your research.
26
•
Where did the culture or religion you have chosen originate from?
•
How is this perspective of the origin of the Earth communicated to
others? (eg. story, song, dance, symbols, pictures, rituals, writing)
•
Has this means of communication changed over time?
•
At what age and to whom is this perspective communicated?
•
Does it have religious significance?
•
Does it have any significance for the way people live from day to
day?
•
Is there any similarity with any other culture (or religion)?
•
Are there any other messages being communicated, apart from an
explanation for the origin of the Earth?
Planet Earth and its environment
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Earth and Environmental Science
Preliminary Course
Stage 6
Planet Earth and its environment
Part 3: The evolution of life on Earth
2
0
0
In
r2
e
b S
o
t
c NT
O
ng DM E
i
t
ra E N
o
rp A M
o
c
0
1
2
3
4
5
Eon
Era
Period
Quaternary
Epoch
Pleistocene
Pliocene
Holocene
(last 10 000
years)
Change
of scale
10
Miocene
20
Cenozoic
30
Tertiary
Oligocene
40
Millions of years before present (Ma BP)
Eocene
50
60
Palaeocene
Phanerozoic
mass
extinction
70
Change
of scale
Cretaceous
Mesozoic
100
Jurassic
200
Triassic
Permian
Carboniferous
300
Palaeozoic
400
500
mass
extinction
Devonian
Silurian
Ordovician
Cambrian
Ediacaran
600
Change
of scale
Precambrian
1000
Proterozoic
age of BIFs
2000
3000
4000
Archaean
Hadean
oldest stromatolites
oldest evidence indicating life
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Contents
Introduction ............................................................................... 3
Origin of the atmosphere and life .............................................. 5
The early atmosphere ..........................................................................5
Studying the Earth’s atmosphere ........................................................7
The origin of life....................................................................... 10
The basic building blocks of life.........................................................11
The Urey/Miller experiment................................................................12
Extraterrestrial material......................................................................14
The earliest forms of life .......................................................... 15
Archaea ..............................................................................................15
Obtaining energy ..................................................................... 18
Respiration..........................................................................................18
Chemosynthesis.................................................................................19
Fermentation ......................................................................................21
Appendix ................................................................................. 25
Suggested answers................................................................. 27
Exercises–Part 3 ..................................................................... 29
Part 3: The origin of life on Earth
1
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Introduction
In the last part, you examined theories and ideas about the formation of
the Earth. Now, in this part, you will be studying how the Earth has
evolved and produced an atmosphere that could ultimately support life.
How do you define ‘life’?
Life has not always existed as we know it today. In the previous section
of work you learnt that the Earth’s earliest atmosphere consisted of gases
other than oxygen. Therefore the question becomes obvious. How did
oxygen come to occupy 21% of our present day atmosphere? From this
stems the next obvious question. How did life originate and eventually
evolve into dominantly oxygen dependent organisms?
This part will help shed some light on these questions and hopefully give
you some stimulus for your own thoughts and ideas.
At the end of Part 3, you will have been given opportunities to learn to:
•
summarise the experiments of Urey and Miller and consider the
importance of their findings to developing an understanding of how
amino acids may have originated on Earth
•
outline the evidence that indicates how the first cellular organisms
(Archaea) may have developed and describe their mode of
respiration (anaerobic fermentation)
•
outline the role of chemosynthesis in providing a suitable energy
source for early organisms
•
explain how the existence of Archaea near fumeroles and submarine
vents can be used to support ideas on early development of life.
Part 3: The origin of life on Earth
3
At the end of Part 3, you will have been given opportunities to:
•
gather and process information from secondary sources about the
synthesis of amino acids caused by discharging an electric spark in
mixtures of methane, ammonia, hydrogen and water
•
perform a first–hand investigation to demonstrate fermentation of
sugar by yeast and use an appropriate chemical test to identify the
produced gas as carbon dioxide
•
gather and process second–hand information about both ancient
Archaea and present day Archaea that live near fumeroles and
submarine vents known as black smokers
Extract from Earth and environmental science Stage 6 Syllabus © Board of
Studies NSW, amended November 2002.
The original and most up–to–date version of this document can be found on the
Board’s website at:
http://www.boardofstudies.nsw.edu.au/syllabus_hsc/syllabus2000_liste.html#e .
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Origin of the atmosphere and life
Try and recall information from your last part to answer the following
questions.
1
What gas currently makes up 78% of the Earth’s atmosphere?
_____________________________________________________
2
What gas currently makes up 21% of the Earth’s atmosphere?
_____________________________________________________
3
What currently makes up the remaining 1% of the Earth’s
atmosphere?
_____________________________________________________
_____________________________________________________
Check your answers.
Our atmosphere has not always been made up of these gases.
It has evolved into its present form.
The early atmosphere
As stated in the previous section, the earliest evidence for our atmosphere
comes from our oldest rocks, those being deposited some 3800 million
years ago in a watery environment.
1
What do you think is significant about the fact that the Earth had a
watery environment so long ago?
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
Part 3: The origin of life on Earth
5
The current scientific view, is that that the early atmosphere
consisted of methane (CH4), water vapour (H2O), nitrogen (N2),
carbon dioxide (CO2) ammonia (NH3), and hydrogen sulfide (H2S).
(Refer to the section of work Evolution of the Earth’s atmosphere in
Part 2.)
These gases are thought to have existed before the evidence shown
in the 3800 million year old Greenland rocks.
2
Where do you think scientists obtained their evidence to support
such a scientific view?
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
Check your answers.
As mentioned above, sediments that were deposited in a watery
environment formed the Earth’s oldest rocks from Greenland.
Where do you think this water originally came from?
Initially water did not exist! But, the elements which make up the
compound, water did. These elements, as you know, are oxygen and
hydrogen. However, the atoms of these elements were locked up in
minerals such as potassium–aluminium mica–KAl3 Si3O10(OH)2.
As the Earth heated up, a small proportion of the rocks in the
asthenosphere become partially molten. (See section Theory to explain a
layered Earth in the previous part.)
Water was released as steam, together with lava from volcanoes.
The steam quickly formed into clouds and eventually cooled enough to
rain down on the Earth’s surface creating streams, rivers and oceans.
The following activity should help you to develop an understanding of
the current atmosphere of the Earth by comparing it with those from
some of our neighbours.
You may need to refer to the periodic table (in the Appendix) to help you
locate and identify the particular elements mentioned.
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Studying the Earth’s atmosphere
Use the information in the following table to answer the questions that follow.
Mars
Earth
Venus
210K
288K
737K
Distance from the Sun
227 940 000 km
149 600 000 km
108 200 000 km
Atmospheric pressure
0.007 bars
1.013 bars
92 bars
Surface temperature
Composition of
atmosphere
•
CO2: 95.3%
•
N2: 78%
•
CO2: 96.5%
•
N2: 2.7%
•
O2:21%
•
N2: 3.5%
•
Ar: 1.6%
•
•
•
trace amounts
of: O2, CO, H2O,
NO, Ne, Kr, Xe
water vapour:
1%
•
trace amounts
of: Ar, CO2, Ne,
He, CH4, Kr, H2
trace amounts
of: SOs, Ar, H2O,
CO, He, Ne
Total atmosphere
compared to Earth
0.0070
1.0
41.0
% of greenhouse
gases
95.0%
1.5%
96.0%
Greenhouse
effectiveness
0.267
1.0
0.071
Greenhouse strength
0.077
0.65
121.0
•
The unit used for temperature is the Kelvin (K). Kelvin equals ∞C – 273.15.
•
One of the units for measuring pressure is the bar. One bar of pressure is
equal to 105 Newton of force per square metre.
•
‘Total amount of atmosphere’ is a measure of how much gas is present in
the atmosphere compared to the Earth’s atmosphere.
•
‘% greenhouse gases’ represent the percentage of the total amount of gas in
the atmosphere that is made up of greenhouse gases.
•
‘Greenhouse effectiveness’ refers to how effective the gas is in trapping
heat. (Water vapour is very good at trapping heat.)
•
‘Greenhouse strength’ refers to the total amount of heat that a planet traps
as a relative amount.
Part 3: The origin of life on Earth
7
1
Relate the ‘distance from the Sun’ and ‘surface temperature’ with the
orbits of each of the planets.
_____________________________________________________
_____________________________________________________
2
Give an explanation as to why Earth would have a low amount of
CO2 in its atmosphere, compared to both Venus and Mars.
Think about the main difference between Earth and the other
two planets.
______________________________________________________
______________________________________________________
______________________________________________________
3
Which planet has the greatest amount of atmosphere?
______________________________________________________
4
Which planet has the greatest proportion of greenhouse gases?
______________________________________________________
5
Which planet has the highest greenhouse effectiveness?
______________________________________________________
6
Which planet has the greatest overall greenhouse strength?
______________________________________________________
7
Why is Earth’s greenhouse strength smaller than that of Venus, even
though Earth has a larger greenhouse effectiveness?
______________________________________________________
______________________________________________________
______________________________________________________
8
Why is the greenhouse strength of Mars smaller than that of Earth,
even though Mars has a greater percentage of greenhouse gases?
______________________________________________________
______________________________________________________
______________________________________________________
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9
Why does Earth have a greater greenhouse effectiveness than Mars
or Venus, even though it has a smaller percentage of greenhouse
gases?
_____________________________________________________
_____________________________________________________
10. Why is Venus too hot and Mars too cold for generally available
liquid water, where as Earth’s conditions are perfect?
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
Check your answers.
Part 3: The origin of life on Earth
9
The origin of life
Scientific investigation into the origin of life has come a long way when
you consider that in the early 1800s it was widely believed that old socks
and rotting meat would create lice and maggots from no previous
organisms !
Charles Darwin (1809–1882) was a naturalist and is considered the
founder of the modern day theory of evolution. His findings are still part
of the basis of modern day research into evolution and the inheritance of
characteristics from one generation to another.
Darwin speculated that life may well have been generated from within a
warm, phosphate–rich pond. It was not until approximately 100 years
later that similar ideas were being proposed and given serious
consideration.
JBS Haldane, a Scottish biochemist working with AP Oparin from the
former Soviet Union, proposed that the basic chemicals for life could be
made from an atmosphere of ammonia, methane and water, if electricity
was used to initiate chemical reactions. The electricity could be used to
simulate lightning in early storms.
In 1953, approximately twenty five years after Haldane and Oparin
proposed their hypothetical model for the origin of life, a student at the
University of Chicago named Stanley Miller, under the direction of
chemist Harold Urey, began to experiment in earnest with Haldane’s and
Oparin’s model. The details of this experiment are outlined in the
following section.
It is important to remember that when considering the origin of life, a
scientific point of view is only one perspective. Other perspectives may
include a religious point of view, or the idea that life has always existed
in one form or another.
What are your own thoughts on the origin of life? You may also wish to
consider exactly what your definition of a living thing is!
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The basic building blocks of life
Amino acids are the basic building blocks of life. So what are amino
acids?
Amino acids are made up from a basic structure containing nitrogen,
carbon, oxygen and hydrogen. They are commonly referred to as the
basic building blocks of life. This is because they combine together in
different arrangements to form different types of proteins. Proteins in
turn are responsible for the formation, repair and function of all cells in
living organisms.
These amino acids are linked by peptide bonds to form many different
types of proteins.
peptide bond
amino acids
An amino acid chain. There are four amino acids shown in the key. These are
linked by peptide bonds. These chains of amino acids form proteins.
There are only 20 different amino acids used in the assembly of
thousands of different proteins. The human body, for example contains
about 150 000 different proteins.
To help illustrate this, think about our 26–letter alphabet. How many
different words can you produce from these 26 letters? The answer of
course is millions.
Answer the following questions and then check your answers at the end of
this part.
In the analogy of the alphabet given above:
1
What do the 26 letters represent?
_____________________________________________________
2
What do the words represent?
_____________________________________________________
Check your answers.
Part 3: The origin of life on Earth
11
The Urey/Miller experiment
As stated above, theories on the composition of the Earth’s primitive
atmosphere led scientists to believe that it was largely made up from
methane (CH4), water vapour (H2O), ammonia (NH3),
and hydrogen (H2).
Based on this information, Stanley Miller and Harold Urey in 1953,
set up equipment to try and emulate this ancient atmosphere, and then
performed a series of experiments to try and replicate the environment of
the time. The experimental set up is shown in the diagram below.
water vapour
sparks
H2O
water vapour
CH4
H2
simulated reducing
primitive atmosphere
NH3
positive
electrode
negative
electrode
condenser
boiling water with
CH4, NH3 and H2
cold water
sample for
chemical analysis
Urey/Miller experiment.
They passed electricity through a mixture of the gases thought to exist in
the early atmosphere to simulate lightning from ancient storms.
The result produced a collection of organic compounds including amino
acids, which have the potential when combined to form proteins.
(See The basic building blocks of life above.)
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These results of this experiment were obtained in an atmosphere without
free oxygen (oxygen existing as a gas and not bonded to any other
element). Oxygen has the effect of breaking down these newly formed
chemicals. This experiment energised further research into the area of
possible primitive atmospheres and how amino acids could be produced
in such environments.
At present the relevance of this experiment is under investigation, since it
is debatable whether the atmospheric conditions used in this experiment
actually reflect the composition of the atmosphere of the early Earth.
To answer the following questions, please refer to the diagram opposite,
then check your answers at the end of this part.
1
In the experiment outlined above, what part of the experimental set up
represented:
a) the ancient oceans
_________________________________________________
b) lightning
_________________________________________________
c) the ancient atmosphere?
_________________________________________________
2
What is the role of the ‘condenser’ in the experiment and what is it
supposed to simulate in the real world?
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
3
Why are the results of the Urey/Miller experiment so important?
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
Check your answer.
Part 3: The origin of life on Earth
13
Summarise Urey and Miller’s experiment by drawing a diagram showing a
scenario of how amino acids may have been first produced on Earth.
Refer to the conditions that Urey and Miller were trying simulate, and
ensure your diagram is labelled carefully.
Extraterrestrial material
Asteroids are very small planets not exceeding 500 km in diameter that
orbit the Sun between Mars and Jupiter. These asteroids, together with
comets and meteorites (bodies of solid rock) and even traces of cosmic
dust are now considered as an alternative mechanism for the formation of
the earliest atmosphere.
This idea gained momentum when in 1997 studies of the comet
Hale–Bopp revealed to scientists that its contents contained: water,
ammonia, formaldehyde and hydrogen cyanide which can react to form
amino acids, the building blocks for proteins necessary for life.
(See notes above.)
It is known that very early in the Earth’s history the Earth was
bombarded heavily with asteroids approximately between 4500 and 3800
million years ago. During this time, life developed and once the asteroid
bombardment eased, life accelerated at a great rate. Meteorites and
comets also deposit their mineralogy as they either land on the Earth or
pass through our primitive atmosphere.
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The earliest forms of life
What do you think was the very first life form on Earth?
Archaea
The earliest cellular organisms are thought to have been very simple
organisms belonging to the Archaea. The term archeo means beginning,
so these bacteria are thought to be the very first bacteria. They are
classified as prokaryotic, pro meaning before and karyo meaning
nucleus. The cells of these bacteria therefore had no nucleus. These first
prokaryotic cells are thought to have existed in the absence of oxygen.
The evidence of the existence of living things or life can be found in the
Earth’s oldest sedimentary rocks, dating back approximately 3800
million years. However, it is important to realise that this evidence has
been obtained from stable carbon isotopes (different versions of the
carbon atom) from ancient carbonaceous films in a metamorphic rock
called a schist and not nicely preserved fossils. Therefore, scientists
assuming these remains are truly organic will never really know what
these early organisms looked like.
The earliest cellular fossil
The earliest cellular fossils now known, are the filamentous microfossils
from the Apex Chert (chert is a silica rich sedimentary rock) near Marble
Bar in northwestern Australia. These fossils were discovered by the
palaeobiologist J W Schopf in 1992, and have been dated at
approximately 3465 million years.
These fossils have been described as being ‘cyanobacterium–like’,
implying then that they may have been able to photosynthesise. If these
organisms were a type of photosynthetic bacteria, then this would be the
earliest evidence of oxygen being produced by organic means.
Part 3: The origin of life on Earth
15
Present day Archaea
Is there life existing on Earth today that does not need oxygen to survive?
It might surprise you to hear that oxygen is in fact poisonous to a number
of life–forms on Earth today. There are a number of areas where very
simple microscopic bacteria flourish in the absence of oxygen.
What do you think of when someone says to you, ‘you need to do more
aerobic exercise’? You should be thinking in terms of exercise that
involves a high heart rate and lots of breathing such as running, boxercise
and swimming. The word aerobic means in the presence of free oxygen.
We are therefore considered aerobic organisms, since we need oxygen to
extract the energy necessary for living.
What therefore, do you think the term anaerobic organisms means?
An environment, which has very little to no oxygen, is said to be
anaerobic. There are a number of anaerobic environments such as
anaerobic marshes, sewage treatment plants, and in the digestive tract of
animals including ourselves. In each of these environments Archaea
happily exists. These bacteria are known as anaerobic organisms.
Modern day Archaea have been found living in high–temperature, deep
marine environments such as those associated with submarine (beneath
the water) volcanic vents known as fumaroles. These present day
Archaea, due to their very simple structure and their ability to live in very
harsh, hot and oxygen depleted environments are thought to be related to
the very first life forms on Earth because it is hypothesised by scientist
that early Earth was like that.
The best way of understanding these early life forms is to study present
day archaeobacteria. The volcanic vents on the deep ocean floor,
produce structures known as black smokers. The name black smoker is
derived from the way dissolved sulfides are emitted into the ocean when
hot lava from the submarine volcanic vents, come into contact with the
surrounding sea–water and precipitate out from the liquid as a black fine
sediment. The result is a black cloud of sulfides and sulfates, which
become deposited around the vent and eventually build up to form a
submarine ‘chimney stack’. All this volcanic activity produces water
temperatures in excess of 300∞C.
If you can obtain access to the video The Living Machine, from the Planet
Earth series, it has some vary rare footage of these black smokers in action.
This is an excellent film.
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Archaea can be found underwater around volcanic vents known as ‘black
smokers’. No light penetrates to the water depths at which these black
smokers are found so photosynthesis cannot form the base of the food chain in
these environments. The Archaea use a process called chemosynthesis to
obtain their energy. The Archaea are using the energy contained in the
chemical bonds in the sulfides and sulfates to form the base of the food chain of
this strange undersea community. You can see the presence of organisms such
as snails, tubeworms and clams on the photograph on the left. The photograph
on the right shows the sulfides being emitted as ‘smoke’.
Photo courtesy of Dr Ray Binns, CSIRO.
On the EES website page there is an interview with a CSIRO scientist,
Dr Joanna Par, who does much of her work on modern day black smokers
on the ocean floor. The interview is a Real© audio interview and can be
found at: http://www.lmpc.edu.au/science on the EES page.
Part 3: The origin of life on Earth
17
Obtaining energy
Almost 4000 million years ago, the Earth is thought to have been a very
different planet to the Earth we know today. The Sun was shining with
only 70% of its present strength and each day lasted less than 18 hours
due to the more rapid rate of rotation of the Earth on its axis. There was
no free oxygen in the atmosphere and no continents as such, only
volcanic island chains, similar to the Tongan islands, Indonesian islands
and Hawaii.
Today most cells have evolved in an oxygen rich atmosphere, to enable them
to ‘burn’ sugars to obtain energy. This process is known as respiration.
Respiration
What do you think of when you hear the term respiration? Do you think
about lungs and the inhaling/exhaling of air? If you do you are really
thinking about the process of breathing and not cellular respiration.
Breathing involves oxygen being absorbed into the blood stream from the
lungs and the carbon dioxide waste (CO2) being extracted out of the
blood stream and being exhaled from the lungs.
However, cellular respiration is far more complex than just breathing.
Cellular respiration is the process whereby cells use oxygen in order to
extract energy and create carbon dioxide as a waste product. This can
occur in animal and plant cells.
The equation below is a summary of the many chemical reactions that
occur in the respiration process within a cell:
glucose + oxygen Æ carbon dioxide + water + energy
C6H12O6 + 6O2 Æ 6CO2 + 6H2O + energy.
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As you can see from the equation above, one of the ‘ingredients’ or
reactants needed for respiration is oxygen. In an environment depleted
of oxygen organisms need to use other means to obtain their energy.
A number of theories have been put forward to explain how early
bacteria could have survived in such an anaerobic environment.
These theories are based on processes that are occurring today.
Chemosynthesis
Chemotrophs are organisms which can exist in the absence of sunlight,
and are capable of manufacturing organic molecules using energy
obtained by mediating chemical reactions using certain chemicals (other
than oxygen) from elements in their environment. This is known as
chemosynthesis. Chemosynthesis is the process whereby carbohydrates
are manufactured from carbon dioxide and water using chemical bond
energy rather than sunlight as used in photosynthesis.
Organisms from the Archaea which chemosynthesise, are found in
locations associated with black smokers where the water is super heated
to temperatures exceeding 300°C. These chemosynthetic Archaea form
the basis of entire ecosystems on the sea floor. At these temperatures the
sulfates, which previously existed in a stable condition under normal
temperatures, are converted into hydrogen sulfide.
Chemosynthetic bacteria use the hydrogen sulfide as an energy source
instead of obtaining energy from sunlight as plants do in photosynthesis.
Water remains as a liquid at these temperatures (around 300°C) due to
the great pressure associated with depths of approximately 2500 m or
more.
Outline the difference between the terms chemosynthesis, chemosynthesise,
and chemosynthetic.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Check your answer.
Part 3: The origin of life on Earth
19
Anaerobic fermentation
The early Earth’s surface was probably a very inhospitable place.
Chemosynthetic bacteria at depths or in the ancient ocean or living in a
very unusual environment such as a hot spring environment may well
have been the first life forms on the planet. Bacteria living in anaerobic
environments can generate their own energy through anaerobic
fermentation. Initially, this involves the decomposition of a glucose
sugar molecule into two pyruvate molecules (C3H4O3) in the cytoplasm
of the cell.
To see links to sites that describe these deep sea communities that have
chemosynthetic Archaea as the basis of their food chain see sites on the EES
links page at http://www.lmpc.edu.au/science.
Glucose is a six carbon molecule and pyruvate is a three carbon molecule
This process of breaking the glucose into two smaller molecules releases
two units of energy. This also occurs in aerobic respiration. Refer to the
diagram below.
glucose (C6H12O6)
2 units
of energy
water
pyruvate (C3H4O3)
Aerobic
with oxygen
36 units
of energy
Anaerobic
without oxygen
carbon
dioxide
water
carbon
dioxide
2 units
of energy
carbon
dioxide
carbon
dioxide
ethanol
(CH3CH2OH)
Different pathways for the production of energy in cells.
20
Planet Earth and its Environment
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Anaerobic fermentation involves further decomposition of these pyruvate
molecules which go on to form ethanol. The following equation
summarises this process.
C6H12O6 Æ 2 CH3CH2OH + 2 CO2 + 168 kJ of energy
glucose Æ ethanol + carbon dioxide + energy.
Now let’s see if you can put some of this theory into practice by
conducting an experiment that produces ethanol, through anaerobic
fermentation.
Anaerobic fermentation of sugar by yeast
Yeast is a familiar and economically important fungus. Yeast cells will
survive in oxygen depleted environments by carrying out anaerobic
fermentation. The waste products from this fermentation are carbon
dioxide and ethyl alcohol (ethanol).
For the next activity and for an activity in Part 4, you will need some
limewater. Prepare the limewater at least one day before you do the
experiment.
Fermentation
What you will need:
•
a gas tight container (in which to carry out the fermentation) with a tube
passing to a smaller container
•
25 g of glucose C6H12O6 or sucrose (table sugar) C12H22O11
•
1 g of table salt
•
7 g dried yeast (found in the bread making or flour section of a
supermarket–usually in 7g, 12g or 28 g packets)
•
250 mL of water
•
a warm area to place the equipment (20–30°C is suitable)
•
limewater Ca(OH)2(aq) to half fill the smaller container; made using the
following two items
•
lime (a chemical obtainable from hardware store, bricklayer or builder)
•
a sealable container eg. a glass jar with lid
Part 3: The origin of life on Earth
21
Keep lime and limewater away from eyes and skin. Lime or limewater in
eyes must be immediately washed away with lots of water.
Limewater is made by adding a small amount of lime (a chemical
obtainable from a hardware store, bricklayer or builder) to a
sealable container half filled with water. (Do not use lime from a
nursery or agricultural supplier—this is usually powdered
limestone, calcium carbonate CaCO3).
Shake the lime with the water then add more water until the
container is almost full. Some of the solid lime, Ca(OH)2(s), will
dissolve to form a water solution, Ca(OH)2(aq). Prepare the
limewater at least one day before you start the activity because
Ca(OH)2(s) is not very soluble in water and takes a while to form
Ca(OH)2(aq).
Seal the container and always keep it topped up with water to
minimise contact with air. When the limewater is needed it is
decanted as a colourless, transparent solution and the container
resealed.
Limewater turns cloudy when exposed to CO2 because
Ca(OH)2(aq) + CO2(g) Æ CaCO3(s) + H2O(l).
The small particles of calcium carbonate (CaCO3) formed reflect
light like clouds do.
Limewater turns cloudy when exposed to CO2. This a product of the
fermentation reaction along with the ethanol alcohol.
When you have prepared your limewater go on with the activity on the
next page. You may have to wait a day or so to make sure your
limewater is prepared. Keep your remaining limewater well sealed in an
air tight container for an activity in Part 4 of this module.
22
Planet Earth and its Environment
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bend straw to fit
in small plastic
container
seal straw
push straw
through the lid
ensure straw is
immersed in water
cut hole for
straw in
plastic bottle
stir in a
circular
motion
fill 1§3 full with water
dissolve sugar
place powdered yeast into sugar solution
Laboratory equipment
Home equipment
Method
1
Place the fermentation mixture of sugar, water, salt and yeast into
the larger container and swirl to mix.
2
Half fill the smaller container with limewater.
3
Put the two containers in a warm area and leave for a day or so until
obvious bubbling is occurring in the larger container.
4
Place the end of the gas transferring equipment so that any gas
produced in the large container enters the limewater about 1 cm
below the limewater surface.
5
Record the appearance of the fermentation mixture and limewater in
the results section below.
Results
_________________________________________________________
_________________________________________________________
Part 3: The origin of life on Earth
23
Conclusion:
1
What colour did the limewater change to over a day or so of
bubbling happening?
______________________________________________________
2
What type of gas does this change in the limewater indicate is being
produced during the fermentation?
______________________________________________________
Now turn to the exercise at the back of this part. Complete the research
according to the suggested guidelines then send your report to your teacher.
24
Planet Earth and its Environment
Part 3: The origin of life on Earth
21
Ra
Radium
Fr
Francium
24
25
Uranium
Protactinium
Thorium
U
Actinium
92
91
Pa
90
Th
89
Ac
Neptunium
Np
93
Promethium
Neodymium
Praseodymium
Cerium
61
Plutonium
Pu
94
Samarium
Sm
62
Hassium
Bohrium
Pm
Hs
Iridium
Osmium
5
6
65
96
Americium
Curium
Cm
95
7
8
9
Californium
Berkelium
Einsteinium
99
Fermium
Fm
100
Erbium
Holmium
Es
68
69
70
Mendelevium
Nobelium
No
102
101
Md
Ytterbium
Yb
Thulium
Tm
Ununhexium
Ununquadium
Er
Uuh
116
Polonium
Bismuth
115
84
Po
54
Lawrencium
Lr
103
Lutetium
Lu
71
118
Ununoctium
Uuo
Radon
117
Rn
86
Xenon
Xe
Astatine
At
85
Iodine
Tellurium
Bi
I
Krypton
Bromine
Te
36
Kr
Argon
Ar
18
Neon
35
53
83
Antimony
10
Ne
Br
Chlorine
52
Selenium
Uuq
114
Lead
Pb
82
Tin
Sb
51
Sn
Arsenic
50
67
98
Dysprosium
34
Se
33
As
Sulfur
Cl
17
S
Fluorine
16
F
Oxygen
O
Phosphorus
P
15
Nitrogen
N
Germanium
Ge
Ho
Cf
97
66
Dy
113
Thallium
Bk
Terbium
Tb
Ununbium
Uub
112
Tl
81
In
49
Gallium
Ga
80
Hg
Mercury
64
Gd
Silicon
Aluminium
Indium
Gold
Uuu
Si
Al
32
14
31
Carbon
13
C
Boron
B
Cadmium
Cd
111
Unununium
Gadolinium
Am
79
Au
Ununnilium
Uun
110
Platinum
Pt
Europium
Eu
63
Meitnerium
Mt
109
Ir
78
Silver
Palladium
Rhodium
Ag
Pd
48
Zinc
47
Copper
Rh
Nickel
30
Zn
29
Cu
46
77
Bh
28
Ni
45
Cobalt
76
108
60
27
Co
Os
Ruthenium
Ru
107
Rhenium
Nd
Lanthanum
59
Pr
58
Ce
Seaborgium
57
Dubnium
Sg
106
Db
Tungsten
105
Rutherfordium
Rf
75
Re
74
Tc
Iron
44
43
Technetium
W
26
Fe
Name
Symbol
Hydrogen
Atomic number
1
H
Manganese
Mn
Molybdenum
Mo
Tantalum
Ta
73
Niobium
Nb
42
Chromium
Cr
La
ACTINIDES
89-103
104
Hafnium
88
87
LANTHANIDES
Barium
Caesium
72
57-71
Hf
Zirconium
Yttrium
56
Strontium
Rubidium
Zr
Y
Ba
Sr
Rb
41
Vanadium
Titanium
40
V
Ti
39
Scandium
23
22
55
38
37
Sc
Cs
Calcium
Potassium
Magnesium
Sodium
20
Mg
Na
Ca
12
11
K
Beryllium
Lithium
19
4
Be
3
Helium
Hydrogen
Li
2
He
1
H
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Appendix
The periodic table.
25
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Planet Earth and its Environment
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Suggested answers
Origin of the atmosphere and life
1
Nitrogen
2
Oxygen
3
The remaining 1% is mostly made up of argon (0.93%), carbon
dioxide (0.03%) and trace elements of other gases.
The early atmosphere
1
The Earth would need to have cooled down sufficiently for water
vapour to condense into water and flow on the Earth’s surface. Also
water is an absolute necessity for life, therefore having water at this
early stage does not rule out the possibility that life existed.
2
Some of the evidence would have been gained from the study of
meteorites that are older than our oldest rocks, and some evidence is
also gained from the study of the contents of comets. Also studying
the gases being emitted from present day volcanoes can help
scientists make assumptions about the early atmosphere.
Studying the Earth’s atmosphere
1
The greater the distance the orbit is away from the Sun, the lower is
the surface temperature for the inner rocky planets.
2
Because plants take in carbon dioxide and produce oxygen in the
process of photosynthesis. Carbon dioxide is also dissolved in our
oceans, reducing the amount available in the atmosphere.
3
Venus
4
Venus
5
Earth
6
Venus
7
Venus has a far larger amount of greenhouse gas in its atmosphere
than does Earth, enabling greater amounts of heat to be trapped
underneath it.
Part 3: The origin of life on Earth
27
8
Because Mars does not have as much atmosphere as Earth. Earth
also has a large amount of water vapour to trap in the heat.
9
Because it has a large amount of water.
10 Venus is closer to the Sun, so any water would have evaporated off
and out of the atmosphere.
Mars is too cold, and any water would be in the form of ice since
Mars is further away from the Sun
Earth is just the right amount of distance from the Sun so the
temperature enables the water to exist as a liquid and is not
evaporated off or frozen as ice.
Basic building blocks of life
1
The 26 letters represent amino acids.
2
The words represent the types of proteins produced.
Urey/Miller
1
a) The ancient oceans are represented by the flask with water in it.
b) Lightning is represented by the electrical spark at the electrodes.
c) The ancient atmosphere is represented by the gases present at
the time the electrical spark is generated.
2
The condenser cools the gases down and converts the ‘atmosphere’
into liquid. This simulates clouds forming and rain in the real world.
3
The results from this experiment are important because, even though
it has become controversial in recent times, it gives scientists a
starting point to hypothesise about how life came to be and then to
begin experiments.
Chemosynthesis
Chemosynthesis is the process where organisms produce organic
material through the generation of energy derived from chemical
reactions without the use of sunlight.
Chemosynthesise is to have undergone the process of chemosynthesis
described above.
Chemosynthetic organisms have the ability to chemosynthesise, they are
chemotrophs.
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Exercise – Part 3
Name: _________________________________
Exercise: The generation of first life
From the information given in this part of work, and any other research
you may wish to conduct on this matter, write a two–page response to the
following statement:
Fumeroles associated with black smokers are responsible for generating
the Earth’s first ecosystem based on chemosynthesis via Archaea.
Resources
Apart from these notes you may wish to search:
•
the Internet
•
National Geographic magazines– past issues, in particular October
1999, and January 2000
•
your local library
•
your school library, including the audio–visual section.
Points to include
Your response should include the following information.
•
Definitions of fumeroles, black smokers and ecosystem.
•
Comments about aerobic and anaerobic environments.
•
Discussion on different ways organisms can generate
energy–respiration, anaerobic fermentation and chemosynthesis.
•
Alternative ideas for the formation of first life
Part 3: The origin of life on Earth
29
More stimulus
The following points are designed to help you stimulate thoughts on the
above quote. You may wish to comment on these points directly or just
use them to generate your own thoughts.
30
•
Fumaroles versus the Sun as the origin of food chains.
•
Other organisms such as: giant clams (half a metre long); colonies of
tube worms (up to one and a half metres in length); and long necked
barnacles, co–exist with bacteria around black smokers.
•
Making assumptions about the past, based on the present.
•
The mid–ocean ridge where black smokers and hot springs are
common today are over 40 000 km long in today’s ocean and may
well have been even longer in the earliest Earth environment.
Planet Earth and its Environment
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Earth and Environmental Science
Preliminary Course
Stage 6
Planet Earth and its environment
Part 4: The evolution of the atmosphere
2
0
0
I
2
r
be S
o
t
c NT
O
ng DM E
i
t
ra E N
o
rp A M
o
nc
0
1
2
3
4
5
Eon
Era
Period
Quaternary
Epoch
Pleistocene
Pliocene
Holocene
(last 10 000
years)
Change
of scale
10
Miocene
20
Cenozoic
30
Tertiary
Oligocene
40
Millions of years before present (Ma BP)
Eocene
50
60
Palaeocene
Phanerozoic
mass
extinction
70
Change
of scale
Cretaceous
Mesozoic
100
Jurassic
200
Triassic
Permian
Carboniferous
300
Palaeozoic
400
500
mass
extinction
Devonian
Silurian
Ordovician
Cambrian
Ediacaran
600
Change
of scale
Precambrian
1000
Proterozoic
age of BIFs
2000
3000
4000
Archaean
Hadean
oldest stromatolites
oldest evidence indicating life
Gill Sans Bold
Contents
Introduction ............................................................................... 3
Early life forms .......................................................................... 5
What are stromatolites? .......................................................................6
The early atmosphere ............................................................. 12
Evidence of an anaerobic environment.............................................12
What is a BIF? ....................................................................................13
Changing atmospheric composition ........................................ 18
How was oxygen first produced? ......................................................18
The removal of methane ....................................................................20
The removal of carbon dioxide ..........................................................21
The production of carbon dioxide ......................................................24
Carbon cycles.....................................................................................26
Carbon reservoirs...............................................................................27
Suggested answers................................................................. 31
Exercise–Part 4 ....................................................................... 35
Part 4: The evolution of the atmosphere
1
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Planet Earth and its environment
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Introduction
In the last part you looked at the evolution of the earliest atmosphere
from anaerobic conditions to aerobic. You were also introduced to the
earliest cellular fossils and what is considered to be the earliest
representation of life–the Archaea.
In this part you will be dealing with anaerobic and aerobic atmospheres
in greater detail. You will be looking at the processes responsible and
evidence for the evolution of the current atmosphere. In addition you
will examine the impact that living organisms and photosynthesis in
particular have had on the evolution of the atmosphere.
By the end of Part 4, you will have been given opportunities to learn to:
•
discuss the impact of photolysis on the composition of the early
(pre–oxygen) atmosphere
•
identify photosynthetic bacteria as the first organisms to release
oxygen into the environment
•
discuss the roles of precipitation and photosynthesis in the removal
of carbon dioxide from the early atmosphere
•
predict and explain the differences in composition of the oceans
before and after the evolution of photosynthesis
•
explain that reactions between oxygen and other elements would
readily occur producing oxide minerals and thus moderate the
release of oxygen into the oceans and atmosphere
•
describe the forms in which carbon is now locked up in the
lithosphere and biosphere.
By the end of Part 4, you will have had opportunities to:
•
perform a first–hand investigation to gather information about the
conditions under which iron reacts with oxygen to form iron oxides
•
perform a first–hand investigation to demonstrate the precipitation of
carbonate minerals in solution by bubbling carbon dioxide through
limewater
Part 4: The evolution of the atmosphere
3
•
gather and process information from secondary sources and use
available evidence to analyse differences in the composition of the
oceans before and after the evolution of photosynthesis
•
process and present information from secondary sources to list and
describe the forms in which carbon is now ‘locked up’ in the
lithosphere and biosphere.
Extract from Earth and environmental science Stage 6 Syllabus © Board of
Studies NSW, amended November 2002.
The original and most up–to–date version of this document can be found on the
Board’s website at:
http://www.boardofstudies.nsw.edu.au/syllabus_hsc/syllabus2000_liste.html#e .
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Early life forms
The following questions will help you to recall some background material
on this topic from your previous learning.
1
What compounds are considered the basic building blocks of life?
_____________________________________________________
2
What particular group of molecules do these ‘building blocks of life’
join together to form?
_____________________________________________________
3
What group of organisms are considered to be the earliest forms of
cellular life here on Earth?
_____________________________________________________
4
What is the approximate age of the oldest sedimentary rocks on
Earth? Did these rocks contain fossils?
_____________________________________________________
_____________________________________________________
5
How old are the oldest cellular fossils? Describe these fossils
_____________________________________________________
_____________________________________________________
_____________________________________________________
Check your answers.
The sometimes ancient fossils known as stromatolites are similar to
structures currently forming in Shark Bay, Western Australia.
These structures are also stromatolites, and are known as ‘living fossils’.
Part 4: The evolution of the atmosphere
5
What are stromatolites?
Ancient stromatolites provide evidence for early life. These ancient
structures have been found in the Pilbara region in Western Australia and
have been dated at 3450 million years old. Stromatolites dominated the
fossil record between 2000 and 1000 million years ago. They are about
the only fossil known.
Stromatolites are layered structures, thought to have resulted from the
activities of bacteria, which produce lime (calcium rich salts) and trapped
silt (very fine mud). Stromatolites display a variety of shapes from
mushroom shapes to egg–carton shapes, which have all been constructed
from these alternating layers of lime and silt.
A cross section of a fossilised stromatolite. This sample is on display at
Macquarie University. (Photo: Tim Reid)
The photograph above is a cross section of a fossil stromatolite.
The stromatolite in this photograph has been placed on its side.
Note the fine layering in what appears to be concentric rings. In fact,
these layers were produced in a dome shape.
Modern day stromatolites have been found forming in Hamelin Pool,
Shark Bay in Western Australia. The photograph below shows these
stromatolites in Shark Bay and the environment in which they are forming.
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Stromatolites forming in Hamelin Pool, Shark Bay in Western Australia.
(Photo courtesy of Paul Brooks)
Write a brief description of the environment that these structures are forming
in, shown in the above photograph.
_________________________________________________________
Check your answers.
Part 4: The evolution of the atmosphere
7
The formation of present day stromatolites
Present day stromatolite structures are formed by the action of
cyanobacteria. These cyanobacteria are also prokaryotic in structure.
Remember the term prokaryotic means ‘before nucleus’. Refer to your
work in the previous part under the heading of Archaea. Present day
stromatolites are also photosynthetic, which means they liberate oxygen
as a by–product. Because of their photosynthetic nature, these simple
bacteria are sometimes incorrectly referred to as blue–green algae.
Below is a summary of the chemical equation for photosynthesis.
light
CO2 + H2O Æ C6H1206 + O2 + H2O
light
carbon dioxide + water Æ glucose + oxygen + water
Look at the photosynthesis equation shown above. Now, refer to your notes
from the previous part on respiration. Look carefully at the equation for
respiration and answer the following questions.
1
In what way are the two equations similar?
______________________________________________________
______________________________________________________
2
How do the two reactions compliment each other? (In other words,
how does one reaction affect the other?)
______________________________________________________
______________________________________________________
______________________________________________________
3
In what way do the two reactions differ?
______________________________________________________
______________________________________________________
Check your answers.
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Formation of ancient stromatolites
Debate about the origin of ancient stromatolites is usually based on
comparisons with the living stromatolites such as the ones found in Shark
Bay. If the ancient stromatolites were formed in an identical fashion to
the modern examples, then this would have major implications regarding
the production of free oxygen in the early atmosphere since ancient
stromatolites have been around since almost 3.5 billion years ago.
What do you think these major implications would be? To help you answer
this question go back and have a look at the proposed composition of the
earliest atmosphere. Now have a look at the formula for photosynthesis
above. You should be looking at the gases being used in photosynthesis and
those being emitted, and then comparing the early atmosphere with our
present day atmosphere. (Remember CO2 is produced by volcanoes).
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Check your answers.
There is one difficulty in drawing a comparison between fossilised
stromatolites and the stromatolites currently forming today.
That problem is a lack of sufficient, direct evidence.
Although the two structures appear similar in structure, there is a distinct
lack of fossilised organisms that are said to have formed the ancient
stromatolites. There is evidence that ancient stromatolites are formed by
filamentous organisms but exactly what the nature of these organisms
was is a good question. In other words, the structures are prevalent, but
traces of the organisms which are said to have formed them are scarce.
If we continue the assumption that the stromatolites forming today
represent the method of formation for ancient stromatolites, then the
following sequence holds true for the formation of all stromatolites.
Part 4: The evolution of the atmosphere
9
sunlight
cynobacterial filaments
bacteria
Step 1. Initially cyanobacteria accumulate and grow during the day, using the
Sun’s light for photosynthesis.
night
silt deposited
and trapped
within filaments
Step 2. During the night, photosynthesis ceases and fine silt accumulates on
the cyanobacterial mats.
sunlight
bacterial
growth
Step 3. New bacteria will begin to grow during the following day on top of the
thin silt layer, once again photosynthesising.
10
Planet Earth and its environment
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night
decaying
cynobacteria
CaCO3 precipitation
Step 4. As the bacteria in step 1 die and decay, it encourages the precipitation
of calcium carbonate.
Present day stromatolites are found in environments in which other
organisms find it very difficult to survive. These environments include
saline lakes and near hot springs.
A modern stromatolite from the saline Lake Thetis in Western Australia.
Photo: Upgrade Business Systems Pty Ltd. This stromatolite is probably on the
order of 1200 years old and although not as famous a location as the Shark Bay
area, indicates the variety of locations in which stromatolites exist today.
Part 4: The evolution of the atmosphere
11
The early atmosphere
From your work in the previous part can you remember the term used to
describe an oxygen poor environment?
Evidence of an anaerobic atmosphere
The term you should have recalled is ‘anaerobic’. The early atmosphere
is described as anaerobic and therefore had very little to no free oxygen.
The oxygen that did exist was ‘locked up’ with other elements as
molecules. An example of this are the oxides, such as iron (II) oxide
(FeO2) and aluminium oxide (Al2O3). Another example is water (H2O).
Any free oxygen that was produced was immediately consumed and
formed into oxides.
Uraninite
One of the main pieces of evidence for this early anaerobic atmosphere
comes from the study of uraninite (uranium dioxide–UO2).
This mineral no longer accumulates as a free particle in our present
atmosphere to be washed down streams and deposited in sediments.
Why? Because it changes its structure when exposed to oxygen forming
uranium oxide (U3O8).
The following equation shows the basic chemical reaction that occurs
with uranite and oxygen. The oxygen on the left–hand side of the
equation would need exist as ‘free’ oxygen in the atmosphere for this
reaction to proceed.
3UO2 + O2 Æ U3O8
uraninite + oxygen Æ uranium oxide
Massive deposits of uraninite which were deposited as sediments in
rivers and streams as long as 2300 million years ago, have been found in
12
Planet Earth and its environment
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South Africa and Canada. The fact that they could be deposited as
sediments suggests that they did not have an opportunity to react with
oxygen. This therefore, is good evidence of an oxygen deficient
atmosphere.
Sulfides
The presence of sedimentary rocks rich in sulfide minerals prior to about
2400 to 2100 million years ago when the change to less sulfide minerals
are found in deposits suggests that the oceans and atmosphere would
have been low in oxygen. Sulfides react with oxygen to form sulfates
that are mostly soluble. Sulfates are also the basis of the chemical energy
chain for sulfate reducing bacteria, a primitive type of Archaea.
Low sulfate concentrations in the oceans probably means that in the early
oceans these bacteria did not thrive. Their absence would have left to
door open for other bacteria such as the methane producing anaerobic
Archaea to thrive in the world oceans.
What is a BIF?
BIF stands for banded iron formation. The occurrence of these
formations only from about 2400 million years ago are further evidence
for an oxygen deficient environment in the time before the age of BIFs
which extended from about 2400 million to 1800 million years ago.
During that interval of time extensive world wide deposits of iron in BIFs
occurred. Deposits such as those found in the Hamersley Ranges in
Western Australia (which make up the world’s largest iron ore deposit)
were deposited in ancient watery environments more than 1800 million
years ago.
BIFs such as these can be found in rocks of many ages. They’re found in
the oldest rocks on Earth, though they are not usually found in
formations younger than 1700 million old. The interval 2400 to 1800
million years ago probably indicates a time when oxygen was increasing
in the atmosphere due to the photosynthetic process of prokaryotic
bacteria form the Archaea.
Formation of iron oxides
BIFs were formed when soluble iron (II) ions (Fe2+) were dissolved,
transported and deposited in shallow oceanic waters without reacting
with oxygen (becoming oxidised). Iron (II) ions (Fe2+) form a green
solution.
Part 4: The evolution of the atmosphere
13
BIFs could not be deposited in the oceans of today as the oxygen, which
is now dissolved in our oceans, would quickly react with the iron to
oxidise it into the iron (III) (Fe3+) state. The iron would then be
transformed into orange–red iron oxide.
In the age of BIFs sediments of alternatively higher and lower
concentrations of iron oxide were precipitated out on the ocean floors.
They later metamorphosed into massive banded–iron formations showing
the characteristic banding of BIFs. The banding was possibly caused by
fluctuations in the iron oxide deposits as a result of a combination of
seasonal upwelling of iron–rich waters from the depths of the ocean
encouraging photosynthetic blue–green algal blooms (Archaea) and
seasonal variations in the algal photosynthetic activity.
It may be that the layers in the BIFs may represent a sort of seasonal non
living growth ring, indicating increases in oxygen production by the
Archaea in longer daylight seasons and lower oxygen production in
shorter daylight seasons. Nobody knows for sure and the formation of
BIFs continues to be a controversial and well researched scientific area.
A sample of a polished section of a BIF showing the well developed banding
typical of such rocks. This specimen shows considerable folding of the bands.
Photo: Ric Morante.
An everyday example of iron oxide is rust.
1
a) List three objects that rust.
_________________________________________________
b) What is common to these three objects? Why do they rust?
_________________________________________________
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Planet Earth and its environment
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2
a) List three objects which do not rust.
__________________________________________________
b) What is common to these three objects? Why don’t they rust?
__________________________________________________
__________________________________________________
Check your answers.
Reactions with iron and oxygen
In the following activity you will be investigating the conditions under
which iron reacts with oxygen. You may find it helpful to refer to
experimental procedure in the resource book before attempting the
following activity.
If you wish, you may want to use the following as an example and come
up with your own activity, or modify the activity below to investigate
conditions under which iron reacts with oxygen to form iron oxides.
With your activity, ensure that you choose appropriate materials and
write up your activity in accordance with the outline of experimental
procedure in the Resource book.
Aim:
To observe and analyse the conditions in which oxygen reacts with iron to
form oxides.
Materials:
•
4 glass jars (similar to vegemite or peanut butter jars)
•
Steel wool
•
Table salt
•
Cooking oil
Procedure:
1
Place a ball of steel wool, about the size of a table tennis ball, into
each of the four glass jars.
2
Boil some water in a pot on the stove for 5 to 10 minutes. Let this
water settle and then fill the first glass jar with the warm to hot
water.
Part 4: The evolution of the atmosphere
15
3
Pour a small amount of cooking oil on top of the water to seal it
from the air.
4
Fill the next glass jar with tap water.
5
Fill the third glass jar with tap water and stir in a tablespoon of salt.
6
Leave the steel wool by itself in the fourth and final jar.
7
Leave these jars for four days.
8.
Record your observations in the following table.
oil
boiled
water
tap
water
salt
water
1
2
3
4
Results:
Day
Jar 1 (boiled water)
Jar 2 (tap water)
Jar 3 (salt water)
Jar 4 (no water)
1
2
3
4
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Conclusion:
1
When iron is oxidised into the iron (III) (Fe3+) state it turns an
orange–red colour. If the iron is in the iron (II) (Fe2+) state it should
have a blue–green colour. In which of these states would iron exist
if it were to be in an anaerobic environment? _________________
2
In which of the glass jars did the steel wool oxidise or react the
quickest? Jar _____
3
In the glass jar you were referring to in question two, was the
environment aerobic or anaerobic? What evidence did you base
your answer on?
_____________________________________________________
_____________________________________________________
_____________________________________________________
4
In which of the glass jars did the steel wool oxidise or react the
slowest?___________________
5
What effect does salt have on the oxidation of iron in steel wool?
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
6
Why was steel wool placed in a jar by itself? In scientific terms,
what is this known as?
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
Discuss your results with others. You may choose your friends, family,
teacher or a fellow student. In your discussion think of how reactive oxygen
is and how many molecules actually contain oxygen.
Part 4: The evolution of the atmosphere
17
Changing atmospheric composition
In your previous discussion at the end of your experiment on the
oxidation of steel wool, you should have discussed the reactivity of
oxygen. Now consider the amount of oxygen that needs to be produced
to enable oxygen to exist ‘freely’ in the atmosphere as it does today.
How was oxygen first produced?
How has all this oxygen been produced, considering our Earth began
with an anaerobic environment?
What processes are responsible for the production of oxygen?
The production of oxygen is thought to have initially occurred by the
decomposition of water vapour into hydrogen and oxygen gas, by the
ultraviolet radiation from the Sun. This process is known as
photo–dissociation or photolysis.
2H2O Æ H2 + O2
water Æ hydrogen + oxygen
The hydrogen, due to its extremely low density, was not be held by the
Earth’s gravity and was lost in space. The oxygen, on the other hand,
due to its high reactivity would have quickly combined with other
elements to form oxides.
True free oxygen probably did not occur in the atmosphere until the
advent of abundantphotosynthetic organisms. In photosynthesis, oxygen
is released after carbon dioxide has combined with water using the
energy from sunlight.
light
CO2 + H2O Æ C6H1206 + O2 + H2O
light
carbon dioxide + water Æ glucose + oxygen + water
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What do you think happened to the carbon dioxide (CO2) and oxygen (O2)
levels in the atmosphere as increased numbers of organisms carried out
photosynthesis?
_________________________________________________________
_________________________________________________________
_________________________________________________________
Check your answers.
Because carbon dioxide is taken up and used in the process of
photosynthesis the concentration of carbon dioxide in the atmosphere
decreased.
Think about the photosynthetic organisms of today. What organisms do
you think were photosynthetic thousands of millions of years ago?
The earliest known photosynthetic organisms are the cyanobacteria.
These very primitive, single celled organisms which have been found in
rocks dating back to about 3465 million years ago.
The release of oxygen into the atmosphere increased dramatically with
the advent of the highly photosynthetic algae, that first appeared
approximately 1500 million years ago. This date approximately
coincides with the end of the banded iron formations. It was from this
point on that the atmosphere became abundant in free oxygen.
Oxygen’s effect on the ocean
Oxygen gas is an extremely reactive element. As soon as oxygen was
produced in the ancient oceans it would have reacted immediately with
hydrogen, carbon, and iron to form oxides. These would have included
water, calcium carbonate which is deposited as limestone, and iron ores
such as hematite and magnetite that are insoluble as oxides in water and
hence would have precipitated out of solution to form BIFs or other large
iron ore deposits. Without the presence of oxygen the ocean would have
been richer in dissolved iron and calcium. Its fundamental composition
would have differed. Once photosynthesis evolved the chemical
structure of the oceans would have changed forever. Most soluble forms
of iron were removed rapidly. This is perhaps reflected in the absence of
iron ore deposits such as BIFs after about 1500 million years ago. It
could be interpreted that once all the free iron dissolved in the ocean was
precipitated that oxygen from photosynthesis in the oceans was able to
begin building up in the atmosphere in earnest.
Part 4: The evolution of the atmosphere
19
To see links that outline some of the theories about the evolution of an
oxygen rich atmosphere see the EES links page at:
http://www.lmpc.edu.au/science.
The role of ozone
As the amount of oxygen (O2) increased in the atmosphere so too did the
production of ozone (O3) in the upper atmosphere. Ozone is a poisonous
gas formed by the action of ultraviolet radiation on oxygen molecules.
This ozone layer helps prevent intensive ultraviolet radiation from
penetrating into the lower atmosphere and onto the surface of the Earth.
This protective ozone layer enabled life to evolve into the shallower
waters and eventually on to land. The ozone shield probably first
appeared at about the same time as sedimentary rocks began to be
deposited containing a lot of oxidised iron in them indicating significant
oxygen in the atmosphere. These rocks are known as redbeds and were
first deposited from about 2000 million years ago. This was about 400
million years after the initiation of the BIF depositional time at 2400
million years ago.
The removal of methane
The early atmosphere was thought to contain large amounts of methane
(CH4). As a result, the sky would have appeared reddish in colour due to
the scattering of light. The large amount of nitrogen in our present
atmosphere is responsible for our blue sky.
Oxygen which had been produced by the photo–dissociation (splitting) of
water molecules by UV radiation from the Sun and through early
photosynthetic Archaea would have assisted in the removal of that
methane. An interesting note here is that the higher the oxygen level in
the atmosphere the higher the ozone content and ozone is a gas that
filters UV from reaching the surface. Low oxygen in the atmosphere
means a low ozone level which would therefore mean an accelerated
photolytic breakdown of water into hydrogen and oxygen. Methane is a
reactive gas and combusts (burns) in the presence of oxygen.
The following equation shows how oxygen and methane react together to
produce carbon dioxide and water vapour.
CH4 + O2 Æ CO2 + H2O
methane + oxygen Æ carbon dioxide + water
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Planet Earth and its environment
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1
What effect do you think this reaction would have on the composition of
the atmosphere over long periods of time?
_____________________________________________________
_____________________________________________________
2
As photosynthesising organisms evolved and increased in number,
so too did the rate of photosynthesis. What effect do you think this
increase in photosynthesis would have had on the amount of
methane in the atmosphere?
_____________________________________________________
_____________________________________________________
Check your answers.
The removal of carbon dioxide
Carbon exists in many different forms. In Part 5 you will be looking at
the carbon cycle in detail. However, in this section you will be dealing
with carbon in the form of carbon dioxide.
The role of photosynthesis
Carbon dioxide was initially produced from volcanism (in emissions
from volcanoes and associated vents) and by reactions with oxygen and
methane. The carbon dioxide produced as a result would have been a
large component of the atmosphere but it was gradually removed from
the atmosphere as more organisms carried out photosynthesis to produce
nutrients and energy.
Carbon dioxide was hence removed from the atmosphere through the
burial of carbon from the remains of the organisms in sediments.
Photosynthesis involves carbon dioxide reacting with water to form
organic compounds and oxygen.
Look carefully at the diagram on the following page.
1
Where would you expect photosynthesis to occur? Include the equation
for photosynthesis.
_____________________________________________________
_____________________________________________________
Part 4: The evolution of the atmosphere
21
2
As non photosynthesising organisms developed, cellular respiration
producing net amounts of CO2 became more common. Refer to your
previous work on respiration and its corresponding equation. What
would be the effect on the carbon dioxide cycle once the occurrence
of this type of organism increased? Explain your answer.
______________________________________________________
______________________________________________________
______________________________________________________
Check your answers.
The role of weathering
Carbon dioxide was removed from the atmosphere, and continues to be
removed , through the erosion of calcium and magnesium silicate rocks
uplifted as mountains.
The breakdown of these silicate rocks by the weathering action of
rainwater is a very efficient process. Carbon dioxide dissolves in the
rainwater to produce a weak acidic solution. This then reacts with the
rock. The rock is weathered chemically. During periods when the
amount of fresh calcium and magnesium silicate rocks available to react
at the surface of the Earth such as during mountain building episodes, the
level of CO2 removed from the atmosphere would be dramatically
accelerated.
The reaction is shown below using the erosion of the rock, calcium
silicate as an example. Note that CO2 is consumed in this reaction.
CaSiO3 + CO2 + H2O Æ Ca2+ + 2HCO3– + SiO2
You can see that calcium silicate reacts with carbon dioxide and water to
produce calcium ions, bicarbonate ions and silicon dioxide (silica).
Refer to the diagram of the carbon dioxide cycle below.
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CO2
CO2
weathering of silicate rocks draws
CO2 from the atmosphere
CaSiO3 + CO2 +H2O
Ca2+ + 2HCO3– + SiO2
ions carried by
rivers to oceans
HCO3–
2+
Organisms use ions to
build calcium carbonate
shells (CaCO3)
Ca
Ca2+ + 2HCO3–
CaCO3 + CO2 + H2O
CO2
subduction
CaCO3 + SiO2
CaSiO3 + CO2
increased pressure
and temperature
Carbon cycle note that it has input and output paths that over the longer term
smooth out fluctuations in the level of atmospheric carbon.
It is important to realise that although the figure above looks like a
system in equilibrium (what is produced is equal to what is being used),
this would definitely not be the case with the earliest atmosphere for
CO2. The active volcanism and fresh Ca and Mg silicate rock surfaces
not protected by a coat of soil in the earliest Earth would have been much
greater than today. The result would have been a removal rate of CO2 in
excess to its replacement Carbon dioxide would also have been removed
from the atmosphere at a far greater rate than it would have been
replaced due to the relatively small amount of animal life in the oceans
compared to today. This would, in effect, give reduced amounts of
carbon dioxide being released into the atmosphere from the volcanoes
above the subduction zones.
Estimates have been made as to the amount of time it would take one
atom of carbon to complete the cycle, from atmosphere to ocean and
back into the atmosphere, taking into account the current rate of tectonic
plate movement. The estimate is about 60 million years although this
estimate could be two orders of magnitude too short and the real time for
the completion of such a cycle could be on the order of billions of years.
Part 4: The evolution of the atmosphere
23
The production of carbon dioxide
Although carbon dioxide is being extracted from the atmosphere by two
processes (weathering and photosynthesis) carbon dioxide is also being
replaced through the emissions of gas from volcanoes and the
precipitation of carbonates.
The precipitation of carbonates
The calcium ions (Ca2+) and bicarbonate ions (HCO3–) produced in the
weathering of rocks are quite reactive. The ions are used by
shell–forming organisms eg molluscs to make shells that are mainly
composed of calcium carbonate (CaCO3).
Ca2+ + 2HCO3– Æ CaCO3 + CO2 + H2O
These shells, along with those made of silica, are deposited as thick
layers of sediment on the floors of lakes, seas and oceans.
It is important to realise that once these products of erosion eventually
get washed into the oceans, organisms then use these ions to build
carbonate shells, which are eventually deposited on the ocean floor.
This process then releases carbon dioxide back into the atmosphere.
In the following activity you will observe the precipitation of calcium
carbonate.
Aim:
To model the precipitation of a carbonate in solution.
Materials:
•
Limewater prepared in Part 3
•
Glass jar
•
Straw
Procedure:
24
1
Put some limewater into the glass jar
2
Using the straw, blow into the limewater. Do this for only a minute or
so.
Planet Earth and its environment
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3
Record your observation as a diagram of the glass jar, Before and
After.
Results:
Before
After
Conclusion:
What conclusion, or comments can be made about this experiment and
the deposition of carbonate in oceans?
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Emission of gases
Periods of non–volcanic mountain building (fold mountains) are directly
responsible for the uptake of massive amounts of carbon dioxide (CO2)
from the atmosphere. Mountains encourage weathering processes, and in
doing so consume carbon dioxide from the atmosphere. This process is
still occurring today in places such as the Himalayas.
The ocean floor is continually being recycled back into the mantle at
subduction zones. (Refer to the diagram of the carbon dioxide cycle).
At these zones the oceanic plate, due to its high density, is subducted
underneath an opposing plate and the carbonate sediments which were
Part 4: The evolution of the atmosphere
25
deposited on top are carried down with it. Much of this subducted
material is melted back into the mantle at temperatures well in excess of
1000K.
As a result of this remelting, calcium silicates (CaSiO3) and carbon
dioxide are produced. The carbon dioxide is emitted through the
volcanoes above these subduction zones and released back into the
atmosphere once again.
Carbon cycles
nis
bustio
m
sto r a ge
carbonate v o
rock
Carbon c o
compounds
for example
fossil fuels
to
sp
iratio
n
s y nt he
sis CO2(g)
lca
et
long
term cycle
CO2(g)
2(g)
n
medium
term cycle
m
C2H12O6 r e
ph
o
short
term cycle
te s CO
m
a l s ili c a
There are really three cycles involving carbon.
Years
10 2
10 3
decade
century
millenium
10 4
10 5
10 6
10 7
10 8
10 9
10 10
age of Earth
average human lifetime
month
day
billion years
10 1
million years
1
year
10 –2 10 –1
You are probably most familiar with the relatively short term carbon
cycle involving photosynthesis and respiration that operates on a years or
days timeframe.
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Planet Earth and its environment
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In other words, a carbon atom involved in photosynthesis and respiration
takes years or days to travel once around the cycle.
There is another medium term carbon cycle that involves carbon storage
and combustion operating on timeframes from 105 to 108 years.
These cycles are dwarfed in terms of the quantities of carbon involved
when the long term carbon cycle involving carbonate rock storage, shale
deposition and volcanism and metamorphism is considered. The long
term carbon cycle operates on timeframes from 107 to 109 years.
Carbon reservoirs
Carbon reservoirs are broad ways or locations where carbon is stored
(usually as a compound combined with other elements).
The actual amount of carbon in the Earth is not clearly known.
The mantle for example, has huge quantities of carbon but exactly how
much is uncertain. Because of extremely limited contact with the mantle
this source of carbon probably can be ignored over meaningful
timeframes.
The table below describes the main reservoirs of carbon that are
accessible over the time frames of the long term carbon cycle or shorter
intervals. The amounts of carbon in some of the reservoirs are quite
imprecisely known because estimating the size of the carbon reservoir is
extremely difficult. Reservoirs such as the atmosphere are currently
growing whereas other reservoirs such as the marine biomass are
relatively constant.
A quick look at the table might surprise you. For example, the level of
land based biomass being so much greater than that of marine based
biomass is not generally known. This is of course largely because of the
presence of trees and extensive forests. The mass of people in terms of a
carbon storage is really a very insignificant amount compared to other
carbon reservoirs. All the people in the world for example have a carbon
mass of about only 0.036 ¥1012 kg.
Part 4: The evolution of the atmosphere
27
Look at the table below that describes global carbon reservoirs then answer
the questions below.
12
Reservoir
Amount of carbon ( ¥10 kg)
Land biomass
About 700
Atmosphere
About 600
Marine biomass
3
Surface ocean
900–1000
Methane as clathrate
10 000
Deep ocean
38 000
Soil
1500 –1600
Sediments including shales, coal and
limestones
77 000 000
1
How much carbon is contained in living things?
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
2
How does the amount of carbon in living things compare to that in
non–living things?
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
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Planet Earth and its environment
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The information contained below has been compiled from information
you have covered in previous parts.
Read carefully through the following and fill in the blank spaces by
selecting the appropriate word from the list below:carbonate, oxygen,
carbon dioxide, photosynthesis, free, oceans, iron, evolved, increases,
volcanic, weathered, carbon, life, oil, dead, warm, condensation, oxides.
3 The early oceans probably originated from the ………………… of
water vapour (produced from the degassing of the Earth’s interior),
and would have been relatively ………… compared to the oceans of
today. The oceans became saline as rocks were ………………… to
sediment and deposited in oceans from rivers and streams. Hot
springs, associated with submarine ……………… activity, are
another major source of salt within the oceans.
As life ……………… and …………………… took hold, oxygen
became more abundant within the oceans and was in the first
instance ‘locked up’ as ……………. with many elements and
minerals within the seas. This resulted in banded …………
formations being deposited in the oceans. This production of
……………, eventually led to the existence of ‘free’ oxygen within
the atmosphere.
Another implication of increased photosynthesis within the oceans,
is the subsequent reduction of …………… …………… dissolved in
the water. Carbon dioxide is used as a reactant in the photosynthetic
equation, and so as photosynthesis, and therefore oxygen,
…………… dissolved carbon dioxide concentration decreases.
As life continued to evolve and increase in quantity and diversity,
……………… layers began to be deposited on ocean floors from
accumulated shell and bone material, originating from …………
organisms. These carbonates eventually formed limestone and
dolomite layers. This increase in carbonate within the ……………
was, and still is important, as it provides a mechanism which enables
the carbon to be ‘locked up’. (Coal, ………, oil shales and natural
gas also serve the same purpose in continental crustal material).
If the …………… was not removed in this manner, then it would
combine with the oxygen, reducing the amount of ………… oxygen
in the oceans and atmosphere, which is a necessary requirement for
the process of most………….
4 Recall the equation for photosynthesis and predict how
photosynthetic organisms may possibly change the composition of
the oceans.
_____________________________________________________
_____________________________________________________
_____________________________________________________
Check your answers.
Part 4: The evolution of the atmosphere
29
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Planet Earth and its environment
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Suggested answers
Early life forms
1
Amino acids are the basic building blocks of life.
2
Amino acids join to form proteins.
3
Archaeobacteria are considered to be the earliest form of cellular life
on Earth.
4
The oldest sedimentary rocks are estimated to be 3800 million years
old.
No, these rocks do not contain fossils. But, they contain carbon
deposits from living organisms that were dated using carbon
isotopes.
5
The oldest cellular fossils are estimated at 3465 million years old.
These fossils have been described as ‘cyanobacterium–like’
implying then that they may have been able to photosynthesise.
What are stromatolites?
The stromatolites are forming in a shallow marine environment.
The waters as a result would probably be relatively warm.
1
The same types of molecules appear in the equations for both
processes.
2
Respiration requires oxygen which is produced by photosynthesis.
Photosynthesis requires carbon dioxide which is produced in
respiration. But, these processes are not the same, in reverse.
3
Light energy is required for photosynthesis to proceed, respiration
releases energy.
Formation of ancient stromatolites
Carbon dioxide was abundant in the early atmosphere. If these early
stromatolites were photosynthetic, then oxygen would have been
produced and begin accumulating in the atmosphere.
Part 4: The evolution of the atmosphere
31
Formation of iron oxides
1
a) There are many metal objects that can rust eg. the bodywork of
cars, unpainted metal hand rails, old metal gates, nails, fish
hooks, bicycle wheel rims and spokes, sheet metal roofing.
b) Rusted objects are all bare metal (made of iron) and exposed to
the atmosphere. Don’t forget the atmosphere is approximately
20% oxygen and also contains water vapour.
2
a) Objects that do not rust include: plastic containers; painted or
galvanised metal surfaces; wooden furniture; objects that are
made out of metals other than iron eg. copper, gold, silver and
aluminium; metal objects in space.
b) Objects that do not rust either do not contain iron or are
protected from the atmosphere which is rich in oxygen. Iron is
protected from rusting by painting or galvanising.
How was oxygen first produced?
The amount of carbon dioxide would decrease and the amount of oxygen
would increase as a result of this process.
Removal of methane
1
The amount of methane would reduce and the amount of carbon
dioxide would increase.
2
The amount of methane would reduce even further since
photosynthesis produces oxygen. This oxygen would react with
methane reducing methane levels in the atmosphere.
Removal of CO2 from the atmosphere
1
Photosynthesis would occur in photosynthetic organisms on the land
and in the oceans.
light
CO2 + H2O Æ C6H1206 + O2 + H2O
light
carbon dioxide + water Æ glucose + oxygen + water
2
C6H12O6 + O2 Æ CO2 + H20
glucose + oxygen Æ carbon dioxide + water
As a result of increased respiration the amount of carbon dioxide in the
atmosphere would increase. However, this should be offset to some
degree by an increase in photosynthetic organisms, which would take in
the carbon dioxide as a reactant.
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Carbon reservoirs
1
12
About 703 ¥10 kg.
2
It is very tiny. The non living carbon reservoirs are much larger.
They therefore have the capacity to affect carbon cycles
dramatically.
3
The early oceans probably originated from the condensation of
water vapour (produced from the degassing of the Earth’s interior),
and would have been relatively warm compared to the oceans of
today. The oceans became saline as rocks were weathered to
sediment and deposited in oceans from rivers and streams. Hot
springs, associated with submarine volcanic activity, are another
major source of salt within the oceans.
As life evolved and photosynthesis took hold, oxygen became more
abundant within the oceans and was in the first instance ‘locked up’
as oxides with many elements and minerals within the seas. This
resulted in banded iron formations being deposited in the oceans.
This production of oxygen, eventually led to the existence of ‘free’
oxygen within the atmosphere.
Another implication of increased photosynthesis within the oceans,
is the subsequent reduction of carbon dioxide dissolved in the
water. Carbon dioxide is used as a reactant in the photosynthetic
equation, and so as photosynthesis, and therefore oxygen, increases
dissolved carbon dioxide concentration decreases.
As life continued to evolve and increase in quantity and diversity,
carbonate layers began to be deposited on ocean floors from
accumulated shell and bone material, originating from dead
organisms. These carbonates eventually formed limestone and
dolomite layers. This increase in carbonate within the oceans was,
and still is important, as it provides a mechanism which enables the
carbon to be ‘locked up’. (Coal, oil, oil shales and natural gas also
serve the same purpose in continental crustal material).
If the carbon was not removed in this manner, then it would
combine with the oxygen, reducing the amount of free oxygen in the
oceans and atmosphere, which is a necessary requirement for the
process of most life.
4
CO2 + H2O Æ C6H12O6 + O2+ H20
carbon dioxide + water Æ glucose + oxygen + water
Marine algae and other photosynthetic marine organisms would
reduce the concentration of carbon dioxide and increase the amount
of oxygen in the oceans.
Part 4: The evolution of the atmosphere
33
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Planet Earth and its environment
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Exercise – Part 4
Name: _________________________________
Exercise 4.1 Changes in the Earth’s atmosphere
1
Describe what you understand the word ‘anaerobic’ to mean.
_____________________________________________________
2
Explain, in detail, how oxygen came to be so abundant on Earth.
In your answer you should make reference to particular time periods
and processes involved.
_____________________________________________________
_____________________________________________________
_____________________________________________________
3
Identify and outline some of the earliest indicators of oxygen having
being produced. You must state why the indicators you have chosen
could be used to show the existence of oxygen, and give their
estimated age of indicating oxygen.
_____________________________________________________
_____________________________________________________
_____________________________________________________
4
Describe ozone and explain why it was important to the evolution of
life.
_____________________________________________________
_____________________________________________________
5
Where is the ozone layer?
_____________________________________________________
_____________________________________________________
Part 4: The evolution of the atmosphere
35
Exercise 4.2 Changes in the composition of oceans
In this exercise you will investigate the composition of oceans before and
after the evolution of photosynthesis.
After reading the notes above and going over your notes in this Part,
complete the following table in point form. You may even wish to do
some of your own research.
Resources
Some useful texts you may have access to include:
•
Frank Press, F and Siever, R. Earth. Freeman.
•
Emiliani, C. Planet Earth. Cambridge University Press.
•
Clark, IF and Cook, BJ. Perspectives of the Earth. Australian
Academy of Science.
•
Environmental science. Australian Academy of Science.
You may have access to the Internet. If you do you may find the sites listed
on the Earth and Environmental Science pages of the following site helpful.
http://www.lmpc.edu.au/science
Ensure that you also carry out your own searches whether it is on the
Internet or your local or school library. You may find the following key
words useful when conducting your searches:
•
evolution of the Earth
•
evolution of the Earth’s atmosphere
•
hydrosphere
•
photosynthesis
•
evolution of life
Procedure
Place your response in the table outlined below. On the first page record
your findings about the composition of the oceans before the advent of
photosynthesis in the table. In the left column place, in point form, notes
about the composition of the ocean. And in the right column list the
evidence or what led you to write that point. You should also list the
source of your information in the right column.
On the second page answer the questions about the composition of
oceans.
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Composition of oceans before
evolution of photosynthesis
Part 4: The evolution of the atmosphere
Supporting evidence
37
Analysis of research
1
Was oxygen being produced from the oceans before the advent of
photosynthesis? (Refer to your notes regarding photolysis of water.)
______________________________________________________
______________________________________________________
2
Comment on the interrelationship between the atmosphere and the
ocean.
______________________________________________________
______________________________________________________
3
What gases were more abundant in the ocean before the evolution of
photosynthesis?
______________________________________________________
______________________________________________________
4
What gases became more abundant in the ocean after the evolution
of photosynthesis?
______________________________________________________
______________________________________________________
5
What evidence do scientists use to indicate the presence of oxygen in
ancient oceans?
______________________________________________________
______________________________________________________
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Earth and Environmental Science
Preliminary Course
Stage 6
Planet Earth and its environment
Part 5: Carbon in the atmosphere and hydrosphere
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Eon
Era
Period
Quaternary
Epoch
Pleistocene
Pliocene
Holocene
(last 10 000
years)
Change
of scale
10
Miocene
20
Cenozoic
30
Tertiary
Oligocene
40
Millions of years before present (Ma BP)
Eocene
50
60
Palaeocene
Phanerozoic
mass
extinction
70
Change
of scale
Cretaceous
Mesozoic
100
Jurassic
200
Triassic
Permian
Carboniferous
300
Palaeozoic
400
500
mass
extinction
Devonian
Silurian
Ordovician
Cambrian
Ediacaran
600
Change
of scale
Precambrian
1000
Proterozoic
age of BIFs
2000
3000
4000
Archaean
Hadean
oldest stromatolites
oldest evidence indicating life
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Contents
Introduction................................................................................ 3
Carbon occurs in many forms.................................................... 4
The release of carbon dioxide .............................................................4
The carbon cycle ..................................................................................5
The greenhouse effect .............................................................. 9
Greenhouse gases .............................................................................11
Global temperature variations............................................................12
Predictions for the future....................................................................13
Suggested answers................................................................. 15
Appendix ................................................................................. 17
Exercises–Part 5 ..................................................................... 19
Part 5: Carbon in the atmosphere and hydrosphere
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Planet Earth and its environment
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Introduction
Previously you followed the development of the atmosphere from
anaerobic through to aerobic, and considered the effects of the evolution
of life on the development of the atmosphere.
In this part you will consider the relationship between CO2 and the
temperature of the surface of the Earth. You will also have a more
detailed look at how carbon exists on Earth and the role carbon cycling
plays in relation to variations in the Earth’s surface temperature.
At the end of Part 5, you will have been given opportunities to learn to:
•
outline the links between the concentration of atmospheric carbon
dioxide and average global temperature over geological time.
At the end of part 5 you will have been given opportunities to:
•
identify data, process and analyse information from secondary
sources and use available evidence to assess claims of a relationship
between changing carbon dioxide concentrations and changes in
average global temperatures.
Extract from Earth and environmental science Stage 6 Syllabus © Board of
Studies NSW, amended November 2002.
The original and most up–to–date version of this document can be found on the
Board’s website at:
http://www.boardofstudies.nsw.edu.au/syllabus_hsc/syllabus2000_liste.html#e .
Part 5: Carbon in the atmosphere and hydrosphere
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Carbon occurs in many forms
Carbon presents itself in different compounds on the Earth, and is
‘locked up’ within different molecules. Much discussion until now has
centred on carbon in the form of carbon dioxide. However, the carbon
that makes up carbon dioxide has originated from elsewhere and is part
of a cyclic process. Carbon can take the form of:
–
inorganic (not from living material) material in rocks and minerals
–
organic material from living or once living organisms
–
gases such as carbon dioxide (CO2), carbon monoxide (CO), and
methane (CH4).
These stores of carbon are referred to as carbon sinks and reservoirs.
The release of carbon dioxide
Trees and fossil fuels such as coal, oil and natural gas, are important
carbon sinks. Industrial activities such as automobiles, burning of
forests, home heating, and the generation of electricity, have seen these
fossil fuels burn and release their storage of carbon into the atmosphere
as carbon dioxide.
The oxygen in the atmosphere has allowed human kind to burn these
fossil fuels to obtain energy.
As stated above carbon is released from the burning of fossil fuels.
This carbon then combines with oxygen during combustion to release
carbon dioxide into the atmosphere.
Write a word equation showing this combustion reaction. Write an equation
using chemical formulas as well.
_________________________________________________________
_________________________________________________________
Check your answer.
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Planet Earth and its environment
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The burning of fossil fuels has been particularly prevalent since the
nineteenth century and is claimed by many to have had a major impact on
climatic change. This period of time is known as the industrial
revolution. Some of the carbon dioxide produced is dissolved in the
oceans but much of it ends up as atmospheric carbon dioxide.
Once released, carbon dioxide produced from industrialised activities
remains in the oceans and atmosphere until it is used in biological
processes or weathering and deposited as sediment on the ocean floor.
This is discussed in more detail below.
Carbon can be easily converted to carbon dioxide gas through the
combustion of fossil fuels, however it takes a much greater time for this
carbon then to be converted back into fossilised fuel.
Read over the last paragraph once again. Comment on the effect of carbon
being easily converted from fossil fuels to carbon dioxide and the time taken
for fossil fuels to be produced.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Check your answer.
Estimates are have shown that by the year 2500, atmospheric carbon
dioxide levels could be as high as 1000 to 2000 parts per million,
compared with 350 parts per million of today. Now read over your
response to the last self–correct exercise.
This increase in carbon dioxide, should it occur, will affect global
climates and is discussed in detail below.
The carbon cycle
The different stores of carbon are connected through the carbon cycle.
Carbon in one form and over time can be removed, transported and
converted into other forms. The carbon cycle on the next page illustrates
the interrelationship between these different stores of carbon.
Part 5: Carbon in the atmosphere and hydrosphere
5
atmospheric CO2
fossil fuel combustion
photosynthetic
assimilation
decomposition
respiration
land plants
coal and oil
dead organic
matter
ocean plants
ocean
dead organic matter
sediments
sedimentation
The carbon cycle.
Carbon exists in carbonates (CO32–) deposited on the bottom of oceans,
lakes and rivers, by being bonded to ions such as calcium (Ca2+), sodium
(Na+) and magnesium (Mg2+). Limestone, composed largely from
calcium carbonate (CaCO3), is deposited on ocean floors from shells of
marine organisms such as plankton. Coral, calcareous algae and shellfish
accumulate carbonate in shallow water and may form reef structures.
Coal and natural gas (methane) are hydrocarbons (compounds of
carbon and hydrogen), and are formed when dead organisms (mainly
plants) accumulated in a swamp–type environment. Oil is formed mainly
in a marine environment, and formed by decay of decay of animal and
plant material.
Photosynthesis incorporates carbon into organic molecules by taking
carbon dioxide in from the atmosphere. Carbon in this form is passed
along the food chain by other organisms that eat these plants.
Once these organisms die the carbon is released into the soil as organic
compounds and as carbon dioxide into the atmosphere.
The cells of most living organisms use oxygen in respiration to release
energy stored in organic molecules and in so doing release carbon
dioxide into the atmosphere. Generally, plants take in more carbon
dioxide for use in photosynthesis than they lose in respiration.
This is used in growth and development of the plants.
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The diagram shown above is one example of a diagrammatic representation
of the carbon cycle.
1
Draw your own version of the carbon cycle showing the flux or
transfer of carbon from one carbon sink to another using the
information given in the table below. You may decide to use
drawings or you may decide to use symbols on your carbon cycle.
Use arrows to show the transfer of carbon from one form into
another. Be sure to label your diagram to show the amount of
carbon transferred in tonnes.
Movement of carbon
Amount of carbon transferred
(billions of tonnes)
Atmosphere to ocean surface
100
Atmosphere to land plants (photosynthesis)
700
Vaporisation—ocean to atmosphere
97
Respiration–land plants to atmosphere
10
Use of fossil fuels—industry to atmosphere
5
Decaying land plants to soil
25
Decaying organic matter–soil to atmosphere
700
Respiration–ocean to ocean plants
40
Decaying ocean plants to organic sediment
40
Part 5: Carbon in the atmosphere and hydrosphere
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2
The table below shows some of the main stores of carbon known as
sinks and the amount of carbon stored in each of these sinks.
Carbon stores (sinks)
atmosphere
oceans
sedimentary rocks
coal and oil
land plants and animals
ocean plants
dead organic matter on ocean floor
Carbon present (billions of tonnes)
700
38 000
77 000 000
10 000
700
3
3 000
Identify which of these sinks you would anticipate were increasing at
the present time and which sinks are decreasing in size.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
3
What effect do you think the industrial revolution, in the late
eighteenth century, had on the carbon cycle in terms of increasing
CO2 levels in the atmosphere? You may wish to comment on:
combustion; the advent of the motor car; logging of forests; the
mining of coal, oil and natural gas; or any other activity you feel is
relevant.
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
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The greenhouse effect
Without the greenhouse effect the average surface temperature of the
Earth would be –20°C.
There is approximately 700 billion tonnes of carbon in the atmosphere.
This amount is not fixed and has varied through the history of the Earth.
At present it is on the increase due to transport, industrial emissions and
deforestation.
In what form does carbon mainly exist in the atmosphere?
_________________________________________________________
Check your answer.
What effect does all this carbon dioxide have on the atmosphere?
It enhances the greenhouse effect.
The diagram below shows how the greenhouse effect contributes to an
increase temperature.
infrared waves
trapped by
atmosphere
infrared waves
trapped by glass
visible
light
waves
greenhouse
atmosphere
The greenhouse effect. Visible wavelengths penetrate to the surface and are
absorbed and re–radiated as infrared radiation. That infrared radiation is
trapped by the atmosphere and re–radiated back to the surface of the Earth so
increasing the temperatures at the surface. A similar thing happens in
greenhouses for plants hence the name.
Part 5: Carbon in the atmosphere and hydrosphere
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A greenhouse is a glass building designed to protect and cultivate plants
in cool climates. Sunlight passes through the glass so that these plants
can photosynthesise and grow. However, the glass also traps heat in the
form of re–radiated infrared radiation and the temperatures increase in
the greenhouse as a result. This increase in temperature is a result of this
‘greenhouse’ effect.
Another example where this greenhouse effect operates is in the heating
of a motor cars’ interior when it is left parked in the sun. Sunlight passes
through the glass areas of the car because the glass does not impede the
visible wavelengths (light). The light energy is partly absorbed by the
seats, dashboard and other parts of the cars’ interior and re–emitted, not
as visible light energy, but as infrared energy. This energy does not
completely pass back through the glass but is partly absorbed by it and is
partly reflected back into the interior of the car, heating the interior.
Why should you place the windscreen sun protector on the outside of a car’s
windscreen rather than on the inside?
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Check your answer.
In the example of the car above, the windscreen of the car represents the
atmosphere, and the car’s interior represents the Earth.
This greenhouse effect has been used to help describe the increase in
global temperatures over the past two hundred years.
10
•
Visible rays from the Sun can readily pass through the Earth’s
atmosphere to the surface.
•
At the surface, the radiation is reflected back as invisible infrared
radiation.
•
Most of the infrared radiation is reflected back into space. However,
some of this radiation is absorbed by clouds (water vapour), carbon
dioxide and other greenhouse gases and is therefore prevented from
escaping into space.
•
The net effect is that the Earth increases its overall average
temperature as the heat is radiated back from the atmosphere towards
the Earth’s surface. This is similar to the greenhouse in that the heat
is radiated back from the glass heating the interior of the greenhouse.
Planet Earth and its environment
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Greenhouse gases
Apart from naturally occurring carbon dioxide and water vapour in the
atmosphere, other gases also contribute to the trapping of infrared
radiation, resulting in an increase in surface temperatures.
In the agricultural industry nitrogen based fertilisers are used extensively.
The nitrogen reacts with oxygen to produce nitrous oxide, which is then
released into the atmosphere. This nitrous oxide absorbs infrared
radiation in the same way as carbon dioxide.
Similarly, halons (gases that contain either fluorine, chlorine, or
bromine) and CFCs (chlorofluorocarbons) have the same heat trapping
properties. These gases were once widely used as solvents, in aerosols,
fire extinguishers and refrigeration units.
Other gases that contribute to the greenhouse effect are methane
(produced in marshes, rice paddies and cattle) and carbon dioxide from
combustion in industry and transport.
ing
om
Inc
Most infrared radiation goes back
into space
adi
ar r
sol
n
atio
Some heat radiation
escapes but greenhouse
gases trap the rest
methane
s
ca
rb
lo
on
id
ox
us
tro
ni
aerosols
fire extinguishers
refrigeration
fertiliser
Activities that
release gases
and contribute
to greenouse
effect
di
ox
i
cattle
paddy fields
e
ha
de
the re
phe
tropos
ns
CFC
coal
oil
petrol
the Earth
The greenhouse effect is enhanced by increased levels of greenhouse gases
such as carbon dioxide, halogens, CFCs, methane and nitrogen oxides.
Part 5: Carbon in the atmosphere and hydrosphere
11
Since the acceleration of the industrial revolution at the end of the
nineteenth century the concentration of carbon dioxide in the atmosphere
has increased considerably. The carbon dioxide is released when fossil
fuels such as coal, oil and natural gas are burned (combusted) to provide
energy for industry.
Although some of this carbon dioxide is absorbed into the oceans, a large
percentage remains in the atmosphere where it acts to absorb and
re–radiate the infrared radiation back towards the Earth’s surface.
Concern has arisen as to the increased greenhouse effect and the rise in
the Earth’s average temperature that appears to be happening now.
Global temperature variations
From your work above you now realise the level of carbon dioxide
concentrations in the atmosphere contribute to the overall impact of the
greenhouse effect.
Correlation between CO2 and temperature
An analysis of past carbon dioxide concentrations and temperature
fluctuations has shown a correlation. Although this does not necessarily
prove that temperature increases or decreases are the result of
fluctuations in carbon dioxide, it does, however, raise some important
points for further investigation.
There is strong evidence to link higher atmospheric carbon dioxide
fluctuations with global temperature fluctuations. However, the
interrelationship between the Earth’s atmosphere, oceans and land is so
complex that it is difficult to say that one factor was the sole cause.
Complexities that must be considered include:
•
the degree to which oceans dissolve carbon dioxide
•
the degree to which carbon dioxide is distributed throughout the
oceans by currents
•
the effects of different clouds eg. some clouds cool the Earth whilst
other types of clouds warm the Earth.
Even given these complex issues, it is generally accepted that an increase
in carbon dioxide will increase average global temperatures due to an
increase in the greenhouse effect.
Do the return exercise now.
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Comparisons with our neighbours
One argument which supports the idea that increased carbon dioxide
levels are related to increased temperatures on Earth are the observations
made about Earth’s neighbour Venus.
Venus receives less light energy at its surface than Earth does due to its
large volume of reflective clouds. However its surface temperature is
much hotter that that of Earth’s.
It is also known that the atmosphere of Venus consists of 300 000 times
more carbon dioxide (CO2) than Earth’s. This may indicate that the
greenhouse effect is operating to a much greater degree on Venus and is
heating the planet up.
Predictions for the future
Climate modelling by scientists indicate that a doubling of the amount of
carbon dioxide (CO2) will lead to a average global surface temperature
increase in the range of 1 to 5.5∞C due largely to the enhanced
greenhouse effect. It is also estimated by some scientist based on
projection modelling with computers that over the next century carbon
dioxide (CO2) levels should double those of the nineteenth century levels.
Other aspects apart from climate need to be considered when looking at
increased levels of atmospheric carbon dioxide and their effect on the
planet. One area in particular is the effect on photosynthetic organisms
in both aquatic (water) and terrestrial (land) environments. This has
widespread implications on food chains and crop production.
Experiments on plants with increased concentrations of atmospheric
carbon dioxide have shown both positive and negative effects, depending
on the species of the plant, and the amount the carbon dioxide was
increased. This area requires further research.
Discuss the implications of increased amounts of carbon dioxide for our
future. Where should scientific funding be allocated? Should we be
learning to live with increasing amounts of atmospheric carbon dioxide or
should we be learning how to avoid increasing carbon dioxide output?
You may decide to discuss this with your family, friends or teacher, or you
may even decide to locate a relevant scientist in this field and find out what
approach they are taking to studying the enhanced greenhouse effect.
Part 5: Carbon in the atmosphere and hydrosphere
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Planet Earth and its environment
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Suggested answers
Carbon occurs in many different forms
carbon + oxygen Æ carbon dioxide.
C + O2 Æ CO2
Our society relies heavily on fossil fuels for energy. Fossil fuels are
popular because they are relatively easy to extract and we obtain large
amount sof energy when they are burned. However, combine this rapid
rate of fossil fuel consumption with the millions of years it takes for these
fossil fuels to form we can conclude will deplete the worlds current
reserves of fossil fuels in a very short period of time.
The greenhouse effect
Carbon exists mainly in the form of carbon dioxide (CO2).
If the sun protector was placed on the inside of the windscreen, above the
dashboard, the visible light will still pass through the glass and the
invisible infrared radiation will be trapped within the car, heating the
interior. By placing the sun protector on the outside of the window both
the visible light and the infrared radiation will be reflected before it
enters the cabin.
Part 5: Carbon in the atmosphere and hydrosphere
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Appendix
Part 5: Carbon in the atmosphere and hydrosphere
17
18
Planet Earth and its environment
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Exercise – Part 5
Name: _________________________________
The following activity illustrates the correlation between past carbon
dioxide levels and temperature variations.
Aim:
To recognise a correlation in plotted data between carbon dioxide
concentrations and average global temperatures.
Method:
1
Plot the two datasets from the table on the same graph. Graph paper
is in the Appendix. Use a different colour for each dataset but a
common horizontal axis. You should end up with two lines on the
one graph. It may be useful to plot the change in temperature in red
and the change in carbon dioxide concentrations in blue.
2
Use the left hand vertical axis to plot the carbon dioxide levels.
3
Use the right hand vertical axis to plot the change in temperature
(from the current mean).
4
Use the horizontal axis to plot Years ago (x1000 yr).
5
Remember to label your axis correctly and include the correct units.
You may also find it useful to consult the Resource book for graphing
procedures.
You should submit your graph with the return pages.
Part 5: Carbon in the atmosphere and hydrosphere
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Data
20
Concentration of CO2 in atmosphere (ppm)
Years ago (X1000 yr)
277
0
200
20
230
30
177
40
220
52
195
60
247
70
220
80
210
85
238
95
222
100
245
108
280
120
260
130
298
135
190
145
200
160
Planet Earth and its environment
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Change in temperature from current mean
(°C)
Years ago (X1000 yr)
0.00
0
+1.40
8
–10.00
20
–6.25
40
–7.70
50
–3.75
52
–8.75
60
–2.50
80
–6.00
85
–2.50
100
–7.50
110
0.00
130
+3.00
135
–9.00
145
–8.75
160
Part 5: Carbon in the atmosphere and hydrosphere
21
Conclusion:
1
In analysing your results look for any similarity in trends such as
common decreases or increases, between the datasets over time.
Can you say certainly that the carbon dioxide levels influenced the
temperature changes? Explain your answer.
______________________________________________________
______________________________________________________
______________________________________________________
2
From this information alone is it just as valid to say that the changes
in temperature caused the changes in carbon dioxide?
Discuss this point.
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
3
With the information contained in the graphs, and the information
outlined above about the greenhouse effect, write your own thoughts
in a concluding statement.
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
4
From a broader perspective, what impact do you think increased
global temperatures would have on world climates?
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
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Earth and Environmental Science
Preliminary Course
Stage 6
Planet Earth and its environment
Part 6: Climatic variation
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Eon
Era
Period
Quaternary
Epoch
Pleistocene
Pliocene
Holocene
(last 10 000
years)
Change
of scale
10
Miocene
20
Cenozoic
30
Tertiary
Oligocene
40
Millions of years before present (Ma BP)
Eocene
50
60
Palaeocene
Phanerozoic
mass
extinction
70
Change
of scale
Cretaceous
Mesozoic
100
Jurassic
200
Triassic
Permian
Carboniferous
300
Palaeozoic
400
500
mass
extinction
Devonian
Silurian
Ordovician
Cambrian
Ediacaran
600
Change
of scale
Precambrian
1000
Proterozoic
age of BIFs
2000
3000
4000
Archaean
Hadean
oldest stromatolites
oldest evidence indicating life
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Contents
Introduction ............................................................................... 3
Geological time line ................................................................... 4
Quaternary temperature changes .......................................................6
The 60 million year climate........................................................ 8
Evidence for cooling.............................................................................8
Past climate ........................................................................................10
What affects climate?.........................................................................11
A look at CO2 ......................................................................................14
Oxygen isotopes.................................................................................15
Ice core records..................................................................................22
Sea level changes..............................................................................23
Marine sediments ...............................................................................26
Lake sediments ..................................................................................27
Appendix ................................................................................. 29
Suggested answers................................................................. 33
Exercises–Part 6 ..................................................................... 35
Part 6: Climatic variation
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Introduction
In Part five you learned how carbon can exist in different forms and the
interrelationship between these forms is described by the carbon cycle.
You also examined the way that carbon compounds may influence the
Earth’s temperature by enhancing the greenhouse effect.
In this part you will study climatic variations over time and the evidence
used in determining these variations. In particular you will gather,
process and analyse information regarding climatic variations over the
past sixty million years.
At the end of Part 6, you be given opportunities to learn to:
•
identify that evidence from marine and lake sediments, ice cores and
sea level change suggests average global temperatures have
decreased over the last 60 million years
At the end of Part 6, you be given opportunities to:
•
gather, process and analyse information from secondary sources on
the varying climate of the Earth since the end of the Cretaceous,
sixty million years ago.
Extract from Earth and environmental science Stage 6 Syllabus © Board of
Studies NSW, amended November 2002.
The original and most up–to–date version of this document can be found on the
Board’s website at:
http://www.boardofstudies.nsw.edu.au/syllabus_hsc/syllabus2000_liste.html#e .
Part 6: Climatic variation
3
Geological time line
This is a good activity that will help you gain a better understanding of
relative time frames and will help consolidate some of the areas covered in
previous parts. As you find out more information throughout this part of
your work you can add this to your time line.
Materials:
•
Ruler
•
Pen
•
5 metres of adding machine tape (or a roll of toilet paper or even a
roll of wide ribbon)
Procedure:
1
Calculate the scale you will use.
For example: If the 5 metres (500 centimetres) of tape represents
roughly 5000 million years, then calculate how many years 1 mm
represents and use this as your scale.
5 m = 5000 million years
_______ mm = 5000 million years
1 mm = _______________ years
2
Mark the eras listed below on the tape.
Era
4
Time period (millions of years ago)
Cenozoic
0–65
Mesozoic
65–250
Palaeozoic
250–542
Proterozoic
542–4700 (?)
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Using the scale you have just devised, mark the geological eras
across your tape at their boundaries as shown in the diagram below.
adding machine tape
era
Cenozoic
Mesozoic
boundary
You can use adding machine tape to produce your time line.
3
Using the scale you have created, measure and record the following
on your geological time line.
Event
Age (millions of years ago)
oldest known rocks
3800
stromatolites (formed by photosynthesising
bacteria)
3450
last sedimentary deposits of uraninite
2300
rise of the redbed sedimentation
2000
last main sedimentary deposits of banded iron
formations (BIF) from continual depositional
sequence
1700
first eukaryote filamentous algae fossils
1500
primitive small shelled animals
550
the greatest loss of species in a mass extinction
(96 % of all known species wiped out)–said by
some to be the time the Earth almost died
250
rise of dinosaurs
200
first birds
150
extinction of 65% of species including the
dinosaurs
65
first human–like primates
(Homo habilis–Africa)
1.7
Part 6: Climatic variation
5
Conclusion:
From the information above make a statement about the amount of free
oxygen available at different times in the Earth’s history and the
implications this would have on weathering, the composition of rocks
(mineralogy) and the evolution of life.
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
_________________________________________________________
Quaternary temperature changes
Temperature records in the distant past are less well known than more
modern record. The further back in time the less reliable the precision of
placement of the indicator of climate against a numerical timescale.
For example the current Holocene epoch would be almost an
undetectable blip on the geological record 50 million years ago despite
its 10 000 year duration.
The Quaternary refers to the latest period of geological time and
extends from two million years ago to the present day. It occurs within
the Cenozoic era. (Refer to your time line.)
The Quaternary is characterised by an alternation of warm and cold
climatic periods. These periods occur relatively regularly. We are
currently in a warming phase or what is correctly known as an
interglacial period, even though we have a substantial amount of land
ice. Global temperatures during this period of climatic variation range
between the present–day temperatures and 5 to 10∞C cooler.
There is a large degree of uncertainty surrounding the reasons why these
fluctuations occur. However, there is sufficient evidence to suggest that
these fluctuations have occurred.
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The formation of the Quaternary ice was a relatively slow process.
For ice to form and stabilise, the temperature of the ocean as a whole
needs to decrease sufficiently to maintain ice at its surface. This cooling
process takes a relatively large amount of time. However, for ice to melt,
only the top layers need to be heated, and so the melting of ice occurs
over a relatively short period of compared to its formation time. This has
resulted in an asymmetrical or uneven pattern of cooling and heating,
which is evident in the Quaternary.
Apart from the past two million years (the Quaternary) scientists have
compiled detailed evidence for climatic changes over the past sixty
million years.
Earlier in the Earth’s history major glaciations (large ice sheets) affected
larger areas including land closer to the Equator, than did the Quaternary
ice. Questions of how this more extensive ice formed, and how it
eventually retreated, remain largely unanswered.
Perhaps you have some thoughts on this matter.
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7
The 60 million year climate
The Cretaceous period makes up the upper one third of the Mesozoic
era. It expanded over a period of approximately 76 million years,
beginning at approximately 141 million years ago and extending through
until 65 million years ago. Almost since the close of the Cretaceous the
Earth’s climate has been on a bit of a roller coaster but with a general
temperature direction of down. That is not to say though that there has
not been extended periods of warming.
Climate variation
The causes of temperature changes over extended periods are often
confirmed by complex computer models with the data suggested by
modelling confirmed by on the ground observations. If the data from the
ground doesn’t confirm the computer model predictions it is necessary to
reconfigure the computer model. The gathering of on the ground data is
painstaking work.
The precise timing and ordering of geologically historical events is often
less precise than people would like to admit. The recent interglacial or
warm period of about 10 000 years duration known as the Holocene
would probably be an unnoticed blip on the geological record if it had
occurred even 20 million years ago. For that reason to say that the
Earth’s climate has been cooling for the past 60 million years is
somewhat of a confusing statement. Distinct periods of time were in fact
extended warming trend periods whereas other periods were of a cooling
trend within the past 60 million years.
Evidence for cooling
There is some evidence of climate fluctuations trending toward a
decrease on global average temperature over the past 60 million years
though this fluctuation was by no means a linear decrease in temperature.
One of the problems facing people who study climate over this interval is
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the direct lack of records. All the information must be accumulated by
interpretation of data from indirect means. Many of the climate
predictions for the past are based on computer modelling.
The trend in the Earth’s climate appears to have been a long term
decrease from a high temperature period at the Late Palaeocene thermal
maximum (LPTM) about 55 million years ago until today. At the LPTM
the temperature of the Earth’s surface was about 7° C on average higher
than it is today. At this time a large mass extinction (much above
average extinction rates) of animal and plant life also occurred.
Climate changes either warmer or colder can result in mass extinctions of
organisms. Estimates for the proportion of species that became extinct at
the Paleocene/Eocene boundary are generally around the 55% mark.
The cause of the LPTM is uncertain though there is one theory that
suggests a rapid release of the gas methane from storage in deposits
called methane clathrates, a type of methane iceblock that is stabilised by
high pressures and low temperatures, may have been the cause.
Methane clathrates exist at water depths between 300–500m and are
extensive on continental shelves today. The sudden release of methane
over a period of some hundreds of years from these large reservoirs of
methane could have potentially altered climate and produced a warm
period. Methane as a greenhouse enhancing gas is around 26 times more
effective than CO2.
The evidence for the decrease in global average temperatures throughout
the 60 million years is varied and patchy. It is mostly based on the
following:
•
Sediment changes. Sediments deposited in warm climates are
different than they are in cold climates. A change in the sediments
deposited at a certain latitude may therefore indicate a change in the
temperature of the time of deposition of that sediment. The presence
of certain types of sediments such as varved shales or tillites indicate
glacial conditions. The presence of evaporates indicates warm
conditions. The expansion and contraction of the continental shelf is
represented in fossil bearing sediments that indicate sea–level rise
and fall. Sea level rises in times of warming as ice–sheets melt and
then falls as land ice–sheets form. Sediment changes in lakes
represent the waxing and waning of lake levels and the fossils within
the sediments indicate climate changes in the surrounding areas.
•
Isotopic ratios in sediments. Certain isotopes like those of oxygen
preserved in calcium carbonate from shells of animals or in
limestone can indicate the climate at the time of their deposition.
•
Fossil types found in the sediments. Organism types are different in
warm climates to those in cold climates. You know today that the
types of organisms living in the tropics are different and have
Part 6: Climatic variation
9
different adaptations to those living in cooler climates.
Similar differences existed in the past.
•
Ice cores. These can provide a relatively recent look at the
fluctuations in temperature over the past 2 million years or shorter
time frames. Ice is no use in times older than that because the ice
sheets melt in times of periodic temperature rise. Once melted the
information the ice contains about climate is lost. Ice cores are
critical to modern climate studies because they actually contain
samples of past atmospheres trapped in small bubbles.
Scientists using precise equipment like mass spectrometers can
analyse the composition of the small gas samples and get a picture of
past atmospheres. The assumption is that the atmosphere was well
mixed and the bubbles are representative of past atmospheres.
Past climate
The broad climate over the past 60 million years can be described as
follows:
10
•
57 to 52 mya in the Eocene, the Earth was much warmer than it is
today. Tropical conditions were experienced into the mid–latitudes
and polar regions were temperate by today’s standard.
The temperature gradient or difference between the Equator and the
poles was less than it is today. The evidence for this warm period
includes things such as the isotopic ratio of O18/O16 in carbonate
shells of foraminifera, the fossils of trees in the Arctic and Antarctic
where none grow today and reptile fossils at much higher latitudes
than can be found today. If it gets too cold reptiles that are cold
blooded cannot survive.
•
52 to 36 mya in the Eocene was characterised by a global cooling
trend that has been repeated twice since. During the interval from 52
to 36 mya, ice caps developed in East Antarctica, the water
temperature at or near the surface dropped to between 5 and 8° C
and the air temperature at mid latitudes fell significantly on the order
of 10° C based on the vegetation types found as fossils at such
latitudes.
•
20 to 16 mya in the Miocene, was characterised by a warming
interval when temperatures rose.
•
16 to 6 mya in the Miocene was a second major cooling period that
was more pronounced than the periods that occurred earlier.
During that interval southeastern Greenland was completely covered
with glaciers and the glaciation extended south from the North Pole
into Scandinavia. Ice sheet formation in Antarctica accelerated.
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•
5 to 3 mya in the Pliocene was a warming period and fossils on the
land and in the oceans indicate that Earth was much warmer than it
is today. Fossils suggest that warm–weather plants grew in Northern
Europe, trees grew in Iceland, Greenland, and Canada as far north as
80 degrees North of the Equator and the European and North
American continents were free of ice.
•
3 mya to present in the Pliocene, Pleistocene and recent Holocene a
cooling period developed. That cooling continues with minor
fluctuations that are controlled by numerous factors including
atmospheric composition and differences in solar behaviour.
These shorter term deviations in the climate controlling conditions
are superimposed on the longer term trend toward cooling.
Why the 60 my cooling trend?
The most expected answer to this question by people is that the global
climate is controlled by the level of CO2 in the atmosphere. More CO2
means a warmer world doesn’t it? The truth of that statement is still not
firmly established over the time frame of the past 60 million years though
some scientist have argued strongly the link is there.
The real problem is that the CO2 levels for the past 60 million years
aren’t all that clear about a direct link between high atmospheric CO2 and
high temperatures. The truth is that some evidence exists for elevated
CO2 levels in the atmosphere during times of cooling. The picture long
term over millions of years simply isn’t clear and the precision
difficulties in matching data to exact ages doesn’t help the situation.
What affects climate?
The factors that could and probably do change the levels of temperature
on the surface of the Earth, over the time frames that a 60 million year
study can detect as trends, are:
•
The output of energy from the Sun. Evidence from astronomy
suggests that the luminosity and therefore energy output from the
Sun has increased over the past 60 million years. That should
suggest a warming. Contrary to that is the cooling trend described
above. There is some evidence that the low energy output periods of
the Sun correlate with periods of cooling though this is open to
interpretation. Another superimposed problem in describing whether
of not the cooling effect over the past 60 million years has any
relation to the energy output from the Sun is the issue of constancy
Part 6: Climatic variation
11
of solar energy output. Studies to investigate whether it was
constant are inconclusive at the present time.
•
Plate tectonic motions. These cause changes in the elevations and
position of the continents. These processes are long term and
therefore have the potential to affect climate long term.
Plate motions lead to cycles of ocean basin growth and contraction
and involve continental splitting, seafloor spreading, subduction, and
plate collisions.
Effects of plate tectonics on climate
Through the course of the continuous plate tectonic cycle continents
collide and split apart, mountains are uplifted and eroded, and ocean
basins are born, grow and close again. Changes in the size, position and
shape of landmasses over time can cause changes in factors such as the
ocean circulation and weathering rates of rocks that potentially affect
long term climate.
Weathering rates at lower latitudes (tropics) can accelerate removing
atmospheric CO2 while rapid seafloor spreading volcanism can cause
release of vast amounts of CO2.
The presence of a continental landmass at the poles means that the heat
energy distribution to the poles by ocean currents is more limited.
Changing topography may assist to explain long term cooling trends.
The Himalayas, the greatest mountain chain on the continents were born
almost at the same time as the long climatic cooling trend began.
Mountains and high plateaus such as Tibet and western North America
may have effected global temperature to some degree but the question is
whether these changes in topography alone can explain the longevity of
the cooling trend. They fail to suggest why climate reversal in the last
60 million years occurs periodically because the rate of mountain uplift
seems to have been consistent.
Plateau uplift can have a significant impact on climate due to factors such
as the diversion of Northern Hemisphere westerly winds. Plateaus also
increase the Earth’s albedo or ability to reflect radiation energy back into
space as more land areas are covered in reflective snow at altitude.
The ocean is critical in the redistribution of the solar energy from the Sun
that is dominantly received at the Equator. This is accomplished by large
ocean currents. As continents drift they may have opened or closed a
passage for the flow of energy redistributing ocean currents.
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It is proposed that the cooling of Antarctica was at least in part due to the
opening of the Southern Ocean when Australia and Antarctica separated
and the resulting development of the circum–polar (around Antarctica)
ocean current that prevents the redistribution of energy from the Equator
to the Antarctic continent. This cooling has caused the growth of ice
sheets and sea ice, and increased the temperature gradient between the
Equator and the South Pole. The separation of Australia from Antarctica
to form the Southern Ocean began somewhere around 55 mya and was
finally achieved about 38 mya when the last bonds between Tasmania
and Antarctica were finally broken. After that cooling really began.
Atmospheric greenhouse gases
Changes in the concentration of carbon dioxide in the atmosphere are the
immediate thought of most people trying to explain the overall pattern of
climatic change. This is partly due too the extensive publicity of recent
times. The question is though can this be justified. It may surprise you
to find out that opinion on the long term effect of elevated or depressed
CO2 levels in the atmosphere is not as clearly defined as you might
expect.
Carbon dioxide does have the capacity to influence the mean global
temperature through the enhanced greenhouse effect. There would be few
scientists today that would question the validity of that statement.
The average surface temperature of the Earth is 15° C. Without the
enhanced greenhouse effect it would be considerably lower, in fact below
zero. But it must be remembered that CO2 is only one of a host of
greenhouse gases in the atmosphere. Water vapor, carbon dioxide,
methane, and other minor gases in the Earth's atmosphere are also
greenhouse gases.
The potential role of methane in producing the extensive warm period
around 55 mya has already been mentioned. Methane when it reacts with
oxygen produces CO2. The effect of methane has apparently been
dramatic. Greenhouse gases can therefore cause dramatic changes in
Earth history and climate.
Part 6: Climatic variation
13
A look at CO2
The average amount of carbon dioxide in the atmosphere is affected by
four factors that increase or decrease the amount of atmospheric CO2
.These factors, called carbon fluxes are:
•
metamorphic degassing that releases CO2 trapped in minerals
subducted or buried and subjected to increased heat and pressure.
This degassing may occur at mid–ocean ridges as part of the seafloor
spreading regime though this cycle may be on a billion year plus
timeframe.
•
weathering of organic carbon bearing rocks such as shales and
limestones which release CO2 into the atmosphere that was
previously stored
•
weathering of silicate rocks that absorbs CO2,
•
burial of organic carbon that returns CO2 to storage as organic
carbon in shales or limestones for a long term. The deposition of the
Tertiary brown coals of the world may have been a significant
contributor to the reduction in global CO2. The 45 million year old
Loy Yang brown coal seam in Victoria’s La Trobe Valley for
example is extensive and up to 250 m thick. Extensive brown coals
were deposited in South Australia, New Zealand and North America
and South America during the Tertiary, many coinciding with, or
contributing to, cooling events.
Age of Victorian Tertiary coals
The Yallourn seam is approximately 7 million years old
The Morwell 1 seam is approximately 10 million years old
The Morwell 2 seam is approximately 25 million years old
The Traralgon South/Loy Yang seam is approximately 45 million years old
Plate motions affect carbon fluxes tending to elevate or lower the
atmospheric CO2 level. For example it has been suggested that the
LPTM 55 million years ago, was caused by elevated atmospheric carbon
dioxide resulting from the methane release and that the decrease in
elevated atmospheric carbon dioxide from that time has been a major
cause of the cooling trend that has existed since over the past
52 million years.
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One mechanism proposed as a cause of this decrease in carbon dioxide is
that mountain uplift such as the Himalayas lead to increased weathering
of fresh silicate.
Refer back to your time line that you constructed earlier in this part.
Mark the Cretaceous period and the other times mentioned in the passages
above on your geological time line. Work out a scheme to mark in times of
warming and cooling over the past 60 million years.
Evidence for the temperature from shortly after the end of the Cretaceous
(65 million years ago) includes the results of studies of:
•
oxygen isotopes
•
ice core records
•
sea level changes
•
marine sediments
•
lake sediments.
Oxygen isotopes
What is an isotope?
An isotope is a different type of atom of the one element. That means
that different atoms of the same element have the same number of
positively charged protons in the nucleus giving them the same atomic
number on the periodic table . Both atoms also have the same number
of negatively charged electrons orbiting around the nucleus giving them
the same degree of reactivity. However, the isotopes differ in the number
of neutrons they contain in the nucleus.
Neutrons do not have any charge and really only add mass and therefore
weight to the atom. They do affect the ratio of atoms of a particular type
which react and participate in chemical reactions. Molecules that are
made up of isotopes that are heavier tend to be less likely to be
evaporated. That means that in times of low temperature the more likely
water molecules to evaporate are rich in O16. That means that the
remaining water is enriched in O18.
The next diagram shows how this isotope separation occurs as you boil
water.
Part 6: Climatic variation
15
H 16
2 O
16 O
H2
H 1
2 6
O
H 16
2 O
H 1
2 6
O
H216O
H216O
H216O
H218O
16
18
H218O H2 O H2 O
16O
H
18
2
H2 O
H218O
H216O
You may find it useful to refer to the Resource book or a periodic table to
refresh your memory of some basic chemistry.
You may recall the following from some of your studies of science in
previous years:
atomic number = the number of protons
atomic mass = the number of protons + the number of neutrons
The following diagram represents the oxygen–16 isotope.
e–
e–
8 protons
8 electrons
e–
e–
p+
+
n p n
n
n
p+
p+ n
p+
n
p+
p+
n
p+
e–
n
e–
nucleus
e–
8 neutrons
e–
16
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Answer the following questions using the information above.
1
a) What is the atomic number of oxygen–16?
__________________________________________________
b) How did you obtain this answer?
_________________________________________________
_________________________________________________
2
a) What is the atomic mass of oxygen–16?
_________________________________________________
b) How did you obtain this answer?
_________________________________________________
_________________________________________________
Oxygen isotopes are used to determine past temperature conditions.
The ratio of oxygen–16 (O16) isotope to oxygen–18 (O18) is used to
provide a sort of past historical thermometer.
3
By referring to the information given above draw an atomic diagram of
the oxygen–18 isotope. Refer to the atomic diagram of the oxygen–16
isotope above.
Check your answers.
Part 6: Climatic variation
17
Using the evidence of isotopes
How do scientists use isotopes in determining past climatic conditions?
Oxygen isotopes are locked up in the calcium carbonate (CaCO3) of
shells in microfossils. Temperature of the environment can be
determined by determining the isotope ratio O16/O18 that is locked up in
the shells. Oxygen–16 (O16) has been found in a higher ratio in the
carbonate shells of microfossils that existed in warmer environments,
whereas oxygen–18 (O18) has been found in a higher ratio in the shells of
microfossils that were thought to exist during much colder conditions.
The reason for these differences is discussed below. If latitude data is
known this can indicate an ice age for shallow water fossils.
The microfossils that have proven to be most useful in this type of study
are the bottom dwelling foraminiferas.
Foraminiferas. (Photo credit: NASA)
The photograph above shows the particular type of foraminiferas
commonly though not exclusively, used to determine the temperature of
the past. These were photographed using a scanning electron
microscope. This photograph is magnified approximately 120 times the
original size.
The oxygen isotope O16/O18 can be measured using a special piece of
equipment called a mass spectrometer.
During an ice age, the seawater is heavier due to an increased
evaporation of water enriched in oxygen–16 (O16) that ends up being
precipitated and can end up as long term ice sheets, such as the ones in
Greenland and Antarctica. Therefore, the oxygen which is absorbed in
the calcium carbonate (CaCO3) of these ocean dwelling foraminifera will
be the heavier oxygen–18 (O18) because the ocean waters are enriched in
the heavier isotope and the composition of the water affects the oxygen
isotope ratio of the shells of the animals that live in it.
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precipitation
(O16 enriched H2O)
evaporation
(O16 enriched H2O)
ice sheet
snow and ice
(O16 enriched H2O)
ocean
(O18 enriched H2O)
land
The different concentrations of oxygen–16 and oxygen–18 in different water
bodies. Note that the ocean waters are relatively well mixed on a time frame of
about 4000 years and certainly to depths of hundreds of metres in a decade or
so.
By mapping the relative abundance of both the O18 rich and the O18
depleted foraminifera through time, a good picture can be obtained of the
temperature conditions under which these organisms existed.
The difficulty is in determining the precise age of the foraminifera.
In this activity you will be using the data gathered from oxygen isotopes to
analyse variations in global temperatures over the last sixty million years.
Method:
The following data has been collected from oxygen isotopic data taken
from the shells of foraminifers.
1
Graph the data shown in the table below. (Graph paper can be found
in the Appendix.)
2
Ensure that you:
•
use a new page and turn your page on its side (landscape)
•
place the Years before present data along the horizontal axis
•
place Temperature along the vertical axis
•
include the correct units.
Part 6: Climatic variation
19
Oxygen isotropic data from foraminifers
20
Average global water temperature
(°C)
Years before present (x1000)
18.0
62 400
19.5
62 000
15.5
58 000
17.5
57 500
15.0
57 200
12.5
54 500
13.5
52 000
12.5
50 500
13.5
49 500
11.5
46 800
11.0
44 200
10.5
41 500
6.8
40 800
7.0
36 400
8.0
35 000
6.9
34 000
8.0
32 500
6.8
31 000
7.0
28 600
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Average global water temperature
(°C)
Years before present (x1000)
6.8
24 000
9.0
19 000
7.5
15 600
9.0
13 000
5.5
10 500
3.5
7 500
6.0
6 500
3.5
2 700
0.5
2 500
3.8
200
1.0
100
Conclusion:
1
Make a statement, based on your graph, on the overall trend of the
temperature variations.
_____________________________________________________
_____________________________________________________
_____________________________________________________
2
Do you think this trend will continue? What information do you
base the answer to this question on?
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
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21
Ice core records
Sensitive records about past climatic conditions can be obtained from ice
cores that have been drilled to depths of up to 2 km. Cores have been
taken from ice sheets in the Antarctic, Canadian Arctic, Greenland and
various sites in Peru and Tibet.
The deeper the ice is, within an ice core, the further back in time
scientists can analyse. Think about when your desk at home becomes
very messy. The most recent pieces of paper and books that you were
working on would be closest to the top. The books and notes that you
first began using would be on the bottom. Similarly, the oldest
precipitation would be present as ice at the bottom of the ice core sample
and the more recent ice accumulations would be nearer to the top and
closer to the surface. Often two metres of ice represents about 100 years
although it can represent a longer or shorter time period.
Ice cores also record the relative abundance of oxygen isotopes.
As discussed above, water containing oxygen–18 is more inclined to
exist as a liquid and is less likely to be evaporated off as water vapour,
than water containing oxygen–16. The warmer the climate the greater
proportion there will be of water vapour containing oxygen–18 in the
atmosphere. Under cooler conditions, water containing oxygen–18 will
tend to remain in the liquid state. The clouds that carry these isotopes as
water vapour are pushed towards the poles, where precipitation results in
them being stored as ice.
Which is the heaviest isotope, oxygen–16 (O16) or oxygen–18 (O18)?
_________________________________________________________
Check your answer.
Taking ice core samples, therefore, allows scientists to analyse the
different isotopic ratios of oxygen and then draw conclusions about the
temperature of their environment at the time they were deposited.
These ice cores are so precise in their layering, that they can indicate
temperature variations over centuries and even decades.
The ice cores also preserve and record carbon dioxide levels in the
atmosphere at the time of freezing. Carbon dioxide gas is trapped in air
bubbles in the ice during deposition whenever the temperatures fall low
enough for ice to form and accumulate.
Why do you think it becomes increasingly difficult to analyse carbon
dioxide bubbles as the depth of the ice samples increase?
_________________________________________________________
Check your answer.
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Sample sites have to be selected carefully as the weight of overlying ice
can sometimes squash these air bubbles to the point where they cannot be
sampled. Dust is also preserved, indicating the degree of dryness for
particular time periods. The drier the environment the more dust there
will be in the atmosphere. This dust can travel long distances eg dust
from central Australia is found in the ice of glaciers in the mountains of
New Zealand.
Other information can also be gathered from ice cores, such as: sea salts,
pollen, volcanic debris, cosmic particles (particles from space) and in
more recent times, isotopes from nuclear testing, can also be obtained
from ice cores. Where these events are a unique time marker they
provide a datum or marker level in the core.
Sea level changes
During the last two million years large quantities of ice have
accumulated on the Earth’s surface during cooler periods. Most of the
water that precipitated as snow evaporated from the oceans. The more
water that is locked up in ice, the less run–off from rivers and the less
water available to replenish the oceans. The result is a drop in the level
of the oceans.
There have been four broad time intervals during the past two million
years, each is considered to be an ‘ice age’. During these glacial periods
the Earth’s ocean levels dropped considerably, and during the periods in
between glacial periods, referred to as interglacial periods, the ice
melted and the ocean levels once again rose.
The following activity shows the sea level changes over the past twenty
thousand years. This data was gathered from studying the distribution of a
particular species of coral that only exists within two metres of the ocean
surface and therefore is a good indicator of sea level.
1
Graph the following data and answer the questions that follow. The
graph paper is in the Appendix at the back of this part.
2
Place Sea level (m) up along the vertical axis with 0 metres (present
level) at the top and–140 metres (below present sea level) at the
base.
3
Place Age (x 1000 yr) along the horizontal axis, beginning with 0
(present day) on the left–hand side ranging to 20 on the right–hand
side.
Part 6: Climatic variation
23
4
Sea level (metres below present)
Age (x1000 years before present)
0
0
–20
8
–25
9
–30
10
–42
11.5
–60
12
–66
13
–72
14
–92
14.5
–99
15
–110
18
–115
19
Comment on the general trend shown by your graph.
______________________________________________________
______________________________________________________
______________________________________________________
5
Are the results you obtained in the previous activity on temperature
variation over time related in any way to the results you obtained in
this activity? Explain.
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
______________________________________________________
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The total volume of ice on Earth today is approximately 25 million cubic
kilometres. The volume during the height of the ice ages has been
estimated to be approximately 70 million cubic kilometres. This would
have resulted in the oceans being lowered by approximately 130 m from
its present day level.
1
What do you think would be the implications of ocean waters becoming
warmer due to increased surface temperatures?
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
2
If the warming trend over the past century were to continue, sea
level would increase by an estimated 15 cm by the year 2100 due to
the increase in global temperature.
In the last part of your work you were asked to consider the effects
industrialisation has had on the atmosphere. How does this information
relate to changes in sea levels?
_____________________________________________________
_____________________________________________________
_____________________________________________________
_____________________________________________________
3
Change in the sea level and the causes and effects of such change is
controversial and a complex issue. Increased temperature at the
surface of the sea means an increase in evaporation over the oceans,
leading to an increase in precipitation.
Read again carefully the last paragraph, immediately above this
exercise, then answer the following questions.
a) Present an explanation identifying how an increase in global
temperature could result in an increase in sea level.
_________________________________________________
_________________________________________________
_________________________________________________
_________________________________________________
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25
b) Now present an explanation describing how an increase in
global temperature may result in a decrease in sea level.
You may find this question challenging.
__________________________________________________
__________________________________________________
__________________________________________________
__________________________________________________
__________________________________________________
Check your answers.
Marine sediments
Marine sediments (sediments deposited in oceans) collected from ocean
floors can be obtained from deep sea drilling programs. From these
sediments, microfossils of minute organisms such as coccoliths and
foraminifera can be extracted.
The type of fossils retrieved indicates the type of environment from
which it came. If the conditions at the surface (when the organism was
living) were tropical then only a tropical species of this organism would
exist. The same would apply if the surface temperatures were temperate.
Then a different species of the micro organism would exist.
By obtaining a date on the sediments and matching the type of organism
found in particular sediments, a history of temperature variations can
be created.
Turn to the end of this part and complete the exercise.
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Planet Earth and its environment
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Lake sediments
Climatic conditions can be reflected in the different mineralogy and
chemistry of the sediment deposited in the lakes. In layers that contain
fossils, differences in animal/plant compositions can indicate different
environments and therefore temperatures in which they were deposited.
Salts and evaporites give an indication of warmer temperatures and
lower lake levels. These deposits can be observed today in many places
throughout the northern part of South Australia. Fossils of particular
animals and plants surrounding lakes can also give a good indication of
the type of temperature and climatic conditions that existed when these
organisms were alive.
Geochemical analysis (the study of chemistry in rocks and fossils) of
microfossil remains, similar to those used on the foraminiferas in
determining oceanic sequences (outlined above), can be used in the
analysis of lake sediments. Different ratios of oxygen isotopes (O16/O18)
used in the construction of their calcareous shells, give an indication of
the temperature conditions at the lake’s surface at the time of their
formation.
This process is very similar to the process used in oceans, however due to
the absence of foraminifera in lakes, scientists have had to analyse a
different type of microorganism–the lake dwelling ostracod.
(Refer to your notes above on oxygen isotopes.)
Part 6: Climatic variation
27
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Planet Earth and its environment
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Appendix
Graph: Oxygen isotopes
Part 6: Climatic variation
29
30
Planet Earth and its environment
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Graph: Sea level changes
Part 6: Climatic variation
31
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Planet Earth and its environment
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Suggested answers
Oxygen isotopes
1
a) The atomic number of oxygen is eight.
b) The atomic number is equivalent to the number of protons in the
nucleus.
2
a) The atomic mass is equal to 16.
b) The atomic mass is calculated by adding the number of protons
and neutrons in the nucleus of an atom.
e–
e–
8 protons
8 electrons
e–
+
n p n
+ p+
n p
n n
n
+
+
n p
p
p+
n
n
p+
e–
p+
n
e–
e–
nucleus
e–
10 neutrons
e–
Ice core records
Oxygen–18, since it has two more neutrons in the nucleus than
oxygen–16.
Sample sites have to be selected carefully as the weight of overlying ice
can sometimes squash these air bubbles to the point where they cannot
be sampled.
Part 6: Climatic variation
33
Sea level changes
1
If the water is warmed, as has been the case over the past century,
the sea level will rise due to an expansion of the water.
Other contributing factors would be the shrinking of ice caps,
retreating of glaciers and the melting of ice sheets.
2
Industrialisation since the acceleration of the industrial revolution at
the end of the nineteenth century, has been responsible for the
release of massive amounts of carbon dioxide into the atmosphere.
This is the result of the burning of fossil fuels. This increase in
carbon dioxide enhances the greenhouse effect resulting in an
increase in surface temperature. This increased surface temperature
will in turn heat the oceans.
3
a) If temperatures increase then ice sheets, glaciers and even the
polar caps may either be reduced or if the temperature is high
enough may even disappear. The water contained in bodies of
ice will then run into the oceans via rivers and streams, raising
the sea level.
b) If temperatures increase then so will the amount of evaporation.
Temperatures nearer the equator are the warmest therefore
evaporation in these lower latitudes will be the greatest.
The atmosphere will precipitate this water in the colder regions
near the poles on land where the water will remain locked up as
ice. As a result the oceans will experience a subsequent drop in
sea level, whilst the land ice caps will increase in volume.
This would have no effect on ocean ice in a similar way to ice
melting in your drink doesn’t cause your drink to flow over.
If all the sea ice in the world were to melt then the effect on sea
level would be negligible.
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Planet Earth and its environment
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Exercise – Part 6
Name: _________________________________
Identifying past climatic conditions
Read the following information and then answer the questions that
follow.
The following foraminifera (microfossils) were obtained from the same
coastal area on the east coast of Australia at three different sites.
•
Site A is 2 metres above the present sea level.
•
Site B is 10 metres above the present sea level.
•
Site C is 18 metres above the present sea level.
Five different types of foraminifera fossils were recovered from the three
sites. The distribution and abundance of these fossils are shown below.
Foram type
Site A
Site B
Site C
Foram 1
10%
0%
0%
Foram 2
20%
0%
0%
Foram 3
40%
30%
0%
Foram 4
15%
50%
30%
Foram 5
15%
20%
70%
These foraminiferas lived in a shallow marine environment, therefore
they were deposited when the sea level was higher than present.
An explanation could be that over time either the sea level has changed
due to a change in the climate. Or, the land on the coast where these
fossils were recovered has been uplifted.
Part 6: Climatic variation
35
The following diagram shows the relative abundance of the foraminifera
fossils plotted against sea water temperatures.
30∞C
Sea water temperature
25∞C
20∞C
15∞C
10∞C
5∞C
0∞C
Foram 1 Foram 2 Foram 3 Foram 4 Foram 5
Relative abundance of foraminifera.
1
At approximately what temperature was the sea when foraminiferas
were deposited at each of the three sites? An answer is required for
each of the three sites.
Site A
Site B
Site C
temperature
2
Comment on the relationship between temperature and the present
day height above sea level of the three sites.
______________________________________________________
______________________________________________________
3
What extra evidence would you look for to support that sea level
changes are linked to changes in the temperature of the water?
______________________________________________________
______________________________________________________
4
If an ice age occurred and the temperature of the waters around this
location were between 8–12∞, what type and proportion of
foraminifera fossils would be deposited. Comment on the resultant
sea level.
______________________________________________________
______________________________________________________
36
Planet Earth and its environment
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Name: _______________________
Location: ______________________
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1
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2
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_____________________________________________________
_____________________________________________________
3
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_____________________________________________________
_____________________________________________________
4
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approximate length of time spent on the module.)
_____________________________________________________
_____________________________________________________
5
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internet, scientific equipment, chemicals, people that can provide
information and help with understanding science.
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_____________________________________________________
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