The Neoproterozoic
Jarðsaga 1
-Saga Lífs og Lands –
Ólafur Ingólfsson
The Neoproterozoic Era
A time period when a variety of global environmental
changes led to the development of the fungi and
animal kingdoms...
• The Varangian Ice Age,
ca. 850-600 MY:
There is evidence from all
continents except Antarctica
of a major Ice Age during late
Precambrium. During this ice
age there were at least four
major glaciations, of which the
latest (~650-600 MY) was
extremely widespread.
The origin of
Fungi
While fungi are not uncommon fossils, their fossils have not received
a great deal of attention compared
to other groups of fossils. Their
fossils tend to be microscopic;
very few large fungal bodies, such
as mushrooms, have ever been
found as fossils.
Based on the fossil
record, fungi are
presumed to have been
present since M-L
Proterozoic, 1.000-900
MY. As the sister group
of animals and part of
the eukaryotic crown
group that radiated
about a billion years ago,
the fungi constitute an
independent group equal
in rank to that of plants
and animals.
Rodina (Russian: Homeland) Supercontinent
The oldest known supercontinent: Rodinia. It was formed about 1100 MY
ago and started braking apart ~750 MY ago. Though its exact size and
configuration are not known, it appears that North America formed the
core of this supercontinent. At that time, the east coast of North
America was adjacent to western South America and the west coast of
North America lay next to Australia and Antarctica.
The split-up of
Rodina
Rodinia split into 2 halves ~ 750 MY
ago, opening the Panthalassic Ocean. N
America rotated towards the South
Pole.
• The northern half of Rodinia,
(Antarctica, Australia, India, Arabia,
China), rotated across the North Pole.
• Between the two halves of Rodinia, a
third continent - the Congo craton was caught in the middle as the two
halves of Rodinia came crashing down
on it.
• By the end of the Precambrian, ~ 550
MY ago, the three continents collided
to form a new supercontinent called
Pannotia. The mountain-building event
associated with this collision is called
the Pan-African orogeny.
•
Rodinia was surrounded
by a single ocean, called
the Iapetus Ocean.
Towards the end of the
Proterozoic, Rodinia
fragmented, giving rise to
the late Vendian
continents of Pannotia,
Siberia, and North China.
From Pannotia in turn
came the diverse
continents of Laurentia,
Gondwana, and Baltica.
Later these in turn came
together to form Pangea...
Mjög góð vefsíða, þar
sem hægt er að skoða
legu meginlanda frá
frumlífsöld (700 MY)
og fram til nútíma:
http://www.uwgb.edu/dutchs/platetec/plhist94.htm#700my
The Rodina Supercontinent
http://www.scotese.com/climate.htm
Glacial
deposits from
the Varangian
Ice Age
They occur as far apart as in
Africa (Namibia), Finland,
Australia, Svalbard, N
America. Even areas which at
the time were close to the
Equator (Australia) were
extensively glaciated. It is
difficult to understand how
the glaciation could have
been so widespread.
Sites where ~750-600 MY old
tills occur
Glacial Deposits (Tillites) overlain by Carbonates
carbonates
glacial deposits
Skeleton Coast, Namibia
A 600 MY old tillite in Namibia
A 600 MY old tillite on
Svalbard
The Snowball Earth
Snowball Earth Model:
Runaway Icehouse Effect
Paul Hoffman and Daniel
Schrag, 1998
In most extreme scenario,
almost entire Earth frozen
Over
Up to 4 major snowball events
between 750 and 600 Ma
The Snowball Earth
• Between ~750 and 600 m.y. ago, Earth
froze completely (or almost) about four
times
• Global freezing alternated with extremely
rapid sea-level rise and global warming
• Evidence:
– Glacial deposits on all continents, even at
low latitudes
– Glacial deposits immediately succeeded
by thick deposits of carbonate rocks
The Snowball Earth
The Snowball Earth
Before the Snowball (about 770 MY)
• Breakup of a single landmass leaves small continents scattered
near the equator.
• Formerly landlocked areas are now closer to oceanic sources of
moisture. Increased rainfall scrubs heat-trapping carbon dioxide
out of the air, eroding continental rocks more quickly.
• Global temperatures fall, and large ice packs form in the polar
oceans. White ice reflects more solar energy than darker
seawater, driving temperatures even lower.
• Feedback cycle triggers strong cooling effect.
Into the Ice House
• Average global temperatures plummet to -50 oC shortly after the
runaway freeze begins.
• Oceans freeze ice to average depth of more than 1 km, limited only by
heat slowly released from Earth's interior.
• Most microscopic marine organisms die, but a few cling to life around
volcanic hot springs.
• Cold, dry air arrests the growth of land glaciers, creating vast
deserts of windblown sand.
• With no rainfall, carbon dioxide emitted from volcanoes is not
removed from the atmosphere.
• As carbon dioxide accumulates, Earth warms and sea ice slowly thins.
Snowball to Slushball
• Concentrations of carbon dioxide in the atmosphere increase 1,000x
due to 10 million years of normal volcanic activity
• Ongoing greenhouse warming effect pushes temperatures to the
melting point at the equator
• As Earth heats up, moisture from sea ice sublimating near the
equator refreezes at higher elevations and feeds the growth of land
glaciers.
• Open water in the tropics absorbs more solar energy and initiates a
faster rise in global temperatures
From Freeze to Fry
• As tropical oceans thaw, seawater evaporates and works along with
carbon dioxide to produce even more intense greenhouse conditions.
• Surface temperatures soar to more than 50 oC driving an intense
cycle of evaporation and rainfall.
• Torrents of carbonic acid rain erode the rock debris left in the wake
of the retreating glaciers.
• Swollen rivers wash bicarbonate and other ions into the oceans,
where they form carbonate sediment.
The Snowball Earth
Possible reasons:
• Fainter early sun
• Biological changes
- Global ice cover
- Weathering and erosion shut down
- Volcanoes continue to erupt CO2
- At 10% CO2, abrupt warming begins
- Go from –50 C to +50 C in 10,000
years?
• Implications for life?
Iron gives clues as to how the
environments changed...
Snowball Earth
1.
3.
2.
4.
http://www.sciam.com/article.cfm?articleID=00027B74-C59A-1C75-9B81809EC588EF21&pageNumber=1&catID=2
Did the
Varangian
Ice Age
cause
explosion
of life?
ANIMALS descended
from the first eukaryotes,
which appeared ~2.000
MY ago. By the time of
the Varangian Ice Age,
more than one billion
years later, eukaryotes
had not developed beyond
unicellular protozoa and
filamentous algae. All 11
animal phyla ever to
inhabit the Earth emerged
within a narrow window of
time in the aftermath of
the glaciation. The
prolonged genetic isolation
and selective pressure
intrinsic to a long ice age
might be responsible for
this explosion of new lifeforms...
The Varangian Ice Age
set the pattern for global
temperature through
geological time
There have been two basic
modes of global climate: a
steaming ”greenhouse” and cool
to cold ”icehouse”. The overall
control lies in the
configuration of the continents
and the global distribution of
energy through the Oceanic
and Atmospheric systems...
Possibilities/questions posed by timing of earliest
complex metazoan (vefdýr, fjölfrumungar) and
Snowball Earth
1.
Metazoans evolved from unicellular eukaryotes REALLY fast
(580 to 575 million years provides only 5 million years for this to
occur)
2. Metazoans evolved before or during Snowball Earth
(perhaps just no record up until 575 Ma) ?
If so, how did they survive on ice-covered Earth ?
3. Perhaps Snowball Earth wasn’t as severe as originally asserted by
Hoffman and Schrag (to permit survival of life) ?
Perhaps a fair amount of open ocean at equator ?
Result: Yeepee! Snowball Fight !
Many geologists in favour of Snowball Earth concept
BUT many against !
Momentous evolutionary changes as
the Proterozoic Eon neared its end
Organisms for the first time evolve to be larger than
of microscopic size, displaying a wide variety of forms
Causes:
• Origin of some kind of thermohaline Oceanic circulation?
• Chemical changes in the Oceans (oscillations in carbon isotope
ratios, oxygenation of the Oceans)?
• A build-up of atmospheric oxygen to to a critical level?
• The great Varangian glaciation caused led to the evolution of new
species adapted to new climatic conditions?
• Maybe the evolutionary radiation was triggered by initial evolution
of a key adaptive features, such as muscle or nerve cells.
Latest Proterozoic
(650 to 544 Million Years Ago)
Oldest Metazoa (complex multicellular organisms)
well established
“Ediacaran” or “Vendian” fauna
The Precambrian Edicara (Venedian)
Fauna
Venedian Fauna
Sites
What is the Edicara Fauna all about?
100 million years before the end of
Proterozoic a group of marine animals,
so-called Edicaran fauna, appeared.
They were named after a locality in
South Australia - Edicara, where they
were first discovered.
Edicara/Venedian Fauna...
For a long time it was thought that this fauna
consisted of invertebrates like jelly fishes,
Annelide worms, arthropods and many forms
that aren't numbered among any known groups.
It was also assumed that these organisms were
ancestry to Cambrian and even some
contemporary animals.
Some more Venedian fossils
What are we seeing preserved?
Not easy to interpret;
alternative explanations
for many fossils...
One alternative interpretation
These animals were "highly flattened fronds, sheets
and circlets composed of numerous slender
segments quilted together" that lay on the bottom
of the ocean. Their plan of the body was different
from any fossil or contemporary group of animals.
"They may constitute a separate and failed
experiment in animal life, or they may represent a
full range of diploblastic (two-layered) organization,
of which the modern phylum Cnidaria (corals, jelly
fishes and their allies) remains as a small and much
altered remnant" (Stephen J. Gould 1994).
But what do they represent?
They represent the oldest forms of complex,
multi-celluar animals to live on Earth.
When the Edicara Biota appeared, it is certain that some
kind of threshold had been crossed. Animals had arrived
on the scene...
Are they monsters, hopeful ones that were seminal to
to what followed, or doomed experiments committed to
oblivion?
Some intresting observations on the
Venedian fauna...
• No
animal of the Edicara fauna has shell, mineralized
skeleton or armor – these appeared in the Cambrian.
• No animal of the Edicara fauna has developed eyes –
these developed in the Cambrian.
• No evidence of predation has been found in the
Edicara fauna.
• Was the Venedian the last geological period
of innocence; before wide-spread predation
where animals either are eaten or eat other
animals or both?
Why did the Venedian fauna become extinct?
Summing up the Precambrian
References, web resources etc
Stanley, Earth System History, chapter 12 (Mjög gott yfirlit, LESIÐ!!)
Fortey, Life. Vintage Books, New York, 346 p. (Besta bók á markaðnum
um þróun lífsins. Jólagjöfin í ár!)
Snowball Earth. Feature Article in January 2000 issue of Scientific American
http://www-eps.harvard.edu/people/faculty/hoffman/snowball_paper.html
(NB: Hér finnið þið grein Hoffman & Schrag “The Snowball Earth”).
http://www.uwgb.edu/dutchs/platetec/plhist94.htm#700my
http://www.palaeos.com/Earth/Geography/palaeogeography.htm
http://www.pbs.org/wgbh/evolution/library/index.html
http://www.ucmp.berkeley.edu/index.html
http://www.science501.com/PTProteroz.html
http://www.scotese.com/Rodinia3.htm