EAS 220
The Earth System
Spring 2008
Lecture 24
Lecture 24: Terrestrial Biosphere Dynamics & Hydrocarbon
Energy Resources
Differences in Primary Production between the Terrestrial and Marine Biosphere
Primary producers are almost entirely higher plants in terrestrial biosphere
Water is often the limiting factor
Temperature range is much larger
Nutrient levels are generally higher but still can be limiting
Silicate and iron are not, or at least rarely, limiting. Key nutrients in the terrestrial
environment, in addition to N, are phosphorus (P) and potassium (K)
Nitrogen cycles through biosphere and atmosphere: P and K derived from
weathering
Photosynthesis, Water & Temperature
Open stomata not only let CO2 in, but also let H2O out.
Photosynthetic rates are often limited by stomatal conductance - the rate at which
CO2 enters the leaf.
How open the stomata are depends on how much water the plants can afford to
lose.
In Hubbard Brooke Experimental Forest (New Hampshire) 25% of annual
precipitation is returned to the atmosphere via transpiration.
The Water Use Efficiency of a plant is defined as:
WUE = mmoles of CO2 fixed/moles H2O lost.
WUE varies from 0.86 to 1.50
Productivity generally increases with precipitation, but levels off around 3-4 m/yr
Largely because of the trade off between CO2 gain and H2O loss, experiments show
more photosynthesis at higher atmospheric CO2 levels.
Photosynthesis rates generally show a maximum around 30˚C.
Biomes
Since water and temperature are important controls on photosynthesis (primary
production), it should not be surprising that the distribution of ecosystems closely
matches climate distribution.
A biome is a region united by climate and a biological community adapted to those
conditions.
The same biome occurring in widely separated geographic areas will have similar
organisms and similar structure, but can have very different species (a point made
by Darwin).
Desert and Tundra Biomes
Characterized by extreme temperatures and low precipitation.
Low annual primary productivity (which can be seasonally high).
Low species richness.
Water logged conditions and low temperatures lead to low decomposition rates in
tundra and therefore build-up of organic carbon in soils (15% of global soil
carbon is in the tundra).
By contrast, deserts have organic-poor soils.
EAS 220
Lecture 24
Boreal Forest (Taiga) and Temperature Forest Biomes
Characterized by seasonal productivity, abundant moisture,
relatively high productivity and biomass.
Grassland Biome
Low to moderate rainfall.
Much lower biomass/area and lower productivity/area than forests.
Seasonal productivity
Grasslands:
Low to moderate rainfall
Low to moderate biomass & productivity
Temperate Occur in continental interiors with “continental climates”
Savana: occur in seasonally dry topics
Chaparral Biome
Also called Mediterranean or Scrubland
Low to moderate rainfall, low to moderate biomass & productivity
Areally smallest biome
Coastal areas with moderate winters and hot, dry summers.
Tropical Forest Biome
High productivity
SubtypeTropical Rain Forest has abundant year round rain & overall highest
productivity
SubtypeTropical Seasonal Forest has seasonal rainfall & productivity
Both have high annual productivity, both are species rich.
Soils are typically (but not always) very nutrient poor. Rapid decomposition leads
to less soil carbon than temperate biomes.
These biomes are very efficient at nutrient recycling.
Generalizations about Biome productivity
Forests have greater productivity and greater biomass than other biomes
Tropical rainforests are the most productive biome and have the largest biomass
(41%) of total. They are also species rich, both in plants and animals.
Carbon is cycled most quickly through the biomass in tundra and grassland biomes.
Because of slow decomposition, highest concentrations of soil carbon are found in
the taiga and tundra biomes.
These biomes constitute 13% of biomass, but 25% of soil organic C (some
estimates as high as 33%).
Terrestrial vs. Trophic & Biomass Relationships
No simple size relationship in terrestrial food webs
Indeed, primary herbivores (grazers) are often much smaller than primary
producers.
Grazers generally do not eat the entire plant.
As in marine trophic relationships, about 10% of the energy in one tropic level flows
to the next.
Terrestrial ecosystems have only 3 or 4 trophic levels, whereas marine ones may
have up to 7.
In terrestrial ecosystems, most of the biomass and energy is invariably in the primary
producers.
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Lecture 24
In marine ecosystems, most of the biomass is actually in the herbivores - the
zooplankton.
Ecosystem Dynamics
Terrestrial ecosystems are far less stable than marine ones, as a consequence of
Fire
Climate change/extreme weather (natural)
This lead to community succession
Rapid colonization, rapid growth ‘pioneer’ species slowly replaced by “climax
community”
Succession is sometimes quite regular and predictable.
Cultivated vs. “Native”
Net primary productivity of cultivated land is comparable to grasslands and much
less than forests.
Biomass in cultivated land is lower than forests and grasslands and comparable to
deserts and tundra.
Cultivation progressively reduces soil organic carbon, but this can be mitigated
By growing crops such as alfalfa and clover
Alternating use as pasture
Reducing tillage
Cultivation also progressively reduces natural nutrients in soils.
Hydrocarbons
Natural Gas:
Light hydrocarbons, consisting of 1 (methane) to 4 (butane) carbons and bonded
hydrogens with a mean C:H molecular ratio of 1:4 to 1:3.
Petroleum
Complex liquid mixture of acyclic (chains) and cyclic (rings) hydrocarbons of a
variety of lengths, with a mean C:H molecular ratio of ~1:2. Refined into:
Gasoline & other fuels: C-5 to -C9 hydrocarbons, such as octane (C8H18)
Lubricating oils, asphalt, and petrochemical feedstock: hydrocarbons C-10 and
above.
Coal
Complex organic solid having C:H ratio from 1:1 to 2:1, with minor amounts of O,
N and S.
Origin of Petroleum & Natural Gas
Petroleum and natural gas ultimately form from the remains of plankton that
accumulated beneath ancient seas.
Requires:
An area of high productivity in the surface water
Reducing conditions in the sediment (otherwise, the bacteria would eat all the
organic remains).
Bacteria consume the more reactive fraction, leading to the formation of kerogen
- a complex organic solid.
The effects of temperature and pressure, over time, convert kerogen to
petroleum, and at higher temperatures, to natural gas.
Petroleum is most likely to develop in slowly accumulated fine-grained sediments
such as shales.
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Lecture 24
Such “source rocks” generally are quite impermeable - making it difficult to pump
the petroleum or gas out.
Recoverable deposits occur where oil has migrated into a permeable reservoir rock.
Examples would include sandstones and reef limestones.
However, petroleum or gas could readily escape to the surface from such rock.
Thus an additional requirement is that the reservoir be overlain by and impermeable
seal or cap rock.
Finally, the petroleum should be concentrated by a structural trap.
Where is the Oil?
Over 50% of proven world oil reserves are in the Middle East.
S. America (particularly Venezuela) and the former USSR are next in importance.
Within these countries, most oil comes from a few large oil fields.
A single Saudi field produces 7% of the world’s oil.
When will the oil run out?
Strictly speaking, the answer is never, but this is not a good question.
A better question is when will oil become so expensive that most of us won’t be able
to afford it.
The answer is highly controversial, but one ominous sign is that we are pumping oil
much faster than we are finding it.
“Shell estimates that after 2015 supplies of easy-to-access oil and gas will no longer
keep up with demand”— Jeroen van der Veer, CEO, Royal Dutch Shell, January,
2008.
Origin of Coal
In contrast to petroleum, which is primarily of marine origin, coal originates as peat the organic remains of plants - deposited in terrestrial swamps and bogs.
Coal is widely distributed and generally abundant
Gas Hydrates - A new hydrocarbon source?
Gas hydrate, or clathrate, consists of a CH4 molecule locked in a cage of hydrogenbonded water molecules.
The methane essentially stablizes the water into ice at temperatures above the
freezing point.
Only stable, however, at elevated pressures.
Extensive deposits of gas hydrates exist on continental shelf and other areas. The
amount may exceed known natural gas reserves by three orders of magnitude.
However, no one has figured out how to safely extract it.
Comparing Carbon Energy Ratio of Fossil Fuels
Carbon emission per unit energy:
Natural gas:
13.6 kg Carbon/gigajoule
Petroleum Distillates:
Gasoline:
18.2 kg Carbon/gigajoule
Fuel Oil:
18.7 kg Carbon/gigajoule
Coal:
24.6 kg Carbon/gigajoule
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