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Summary a level geography notes on the carbon cycle
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THE CARBON CYCLE AND ENERGY SECURITYWhat is the biogeochemical carbon cycle?
Key definitions:
• Biogeochemical cycle: The flow of chemical elements and compounds between living
organisms and the physical environment.
• Carbon cycle: The biogeochemical cycle by which carbon moves between the atmosphere,
biosphere, oceans and sediments.
• Stores: Also known as stocks, are the amount of carbon held within the cycle.
• Fluxes: Movement of carbon between stores.
• Processes: The physical mechanisms which drive the flux of material between stores, for
example photosynthesis.
• Pg: The units used to measure carbon; one pentagram (Pg) also known as gigatonne (Gt) is
equal to a trillion kilograms or one billion tonnes.
• Sequester: To store carbon naturally by processes such as photosynthesis.
• Carbonate pump: Operates in oceans to circulate and store carbon.
Carbon stores in Pg
• Fossil fuels 4,000
• Oceans 38,000
• Plants 560
• Soils including permafrost (permanently frozen ground) 1,500
• Atmosphere 750
• Earth’s Crust 100,000,000
Human fluxes in Pg per year to and from the atmosphere store
• Burning fossil fuels releases 6 Pg a year
• Deforestation and land use change release 0.9 PG a year
•
Natural fluxes in Pg per year to and from the atmosphere store
• Soil respiration releases 60 Pg a year
• Photosynthesis sequesters 120 Pg a year
• Volcanoes release 0.1 Pg a year
,• Ocean Loss releases 90 Pg per year
• Ocean uptake removes 92 Pg per year
• Rivers remove 0.8 Pg a year
• 0.1 Pg is buried to sediments
CONCLUSION:
This shows that oceans operate as sinks (absorbing 2 Pg per year) and human activity has
unbalanced the carbon cycle (adding 6.9 Pg per year to the atmosphere store).
Other important information about the carbon cycle:
The carbon cycle is a closed system that has inputs, flows and outputs. As it is closed system,
carbon is not transferred beyond its boundaries. Carbon stores within the cycle function as sources
and sinks.
Most of the earth’s carbon is stored in the earth’s crust.
Geological carbon results from the formation of carbonate sedimentary rocks like limestone in the
oceans, and biologically derived carbon in rocks like coal and shale. Slow geological processes
release carbon into the atmosphere through chemical weathering of rocks and volcanic outgassing
at ocean ridges/subduction zones.
Calcium carbonate from marine organisms accumulated in strata approximately 300 million years
ago.
The strata (layers) were compressed during a process known as sedimentation.
Peat (partially decomposed vegetation) also accumulated approximately 300 million years ago and
under increasing pressure due to burial under more and more layers of sediment, various type of
coal are formed.
How do geological processes store carbon in the earth’s crust and
release carbon into the atmosphere?
There are two key processes: chemical weathering and outgassing.
• Water reacts with atmospheric carbon dioxide to form carbonic acid.
• Rainwater is therefore weakly acidic and reacts with some surface minerals in silicate rocks,
slowly dissolving them into their component ions. For example, calcium silicate CaSiO3 reacts
with rainwater to yield a calcium ion, Ca2+, a bicarbonate ion, HCO3-, and dissolved silica.
(Silicates and are a major component of igneous rocks.)
• River runoff carries these soluble products to the ocean, where shell-building organisms use
them to form their shells and skeletons, a process called carbonate precipitation.
• Some of this calcium carbonate CaCO3 accumulates on the floor of oceans.
• Deposition and burial turns this calcite sediment into sedimentary rocks such as limestone.
• Sedimentary rocks in oceanic crust converge with less dense continental crust.
• Sedimentary rocks in oceanic crust are subducted beneath continental margins forming carbon
containing-magma.
• Partial melting of sedimentary rocks liberates carbon dioxide which is returned to the
atmosphere.
Outgassing:
• Pockets of carbon dioxide exist in the earth’s crust; disturbances by volcanic eruptions or
earthquake activity may allow pulses or more diffuse fluxes into the atmosphere
Outgassing occurs at:
• Active or passive volcanic zones associated with tectonic plates boundaries, including hotspot
volcanoes, subduction zones and spreading ocean ridges
• Places with no current volcanic activity such as the springs and geysers in Yellowstone
National Park, USA
, • Direct emissions from fractures in the earth’s crust
• Volcanoes are responsible for a flux of approximately 0.1 Pg per year, this is a very small flux
in comparison to the emissions from burning fossil fuels, which is 60X volcanoes
Positive and negative feedback:
• Positive feedback amplifies change in a system whereas negative feedback dampens
change in a system. If there is an increase in volcanic activity negative feedback may
rebalance the carbon cycle although this takes a few hundred thousand years.
• Negative feedback: volcanic activity causes a rise in carbon dioxide emissions and hence
temperature, subsequently leading to more rainfall. Therefore, there is more chemical
weathering of rocks and more carbon is stored in rocks.
What is the carbonate pump?
• Thermohaline circulation is a global system of surface and deep-water ocean currents driven
by differences in temperature and salinity.
• Some marine organisms use calcium carbonate to make hard outer shells and inner skeletons.
• When these organisms die and sink, most shells dissolve before reaching the sea floor and
sediments are moved into the deep ocean by thermohaline circulation.
• Shells that do not dissolve before reaching the sea floor build up and form limestone
sediments.
• This process is carbonate pump.
• The carbonate pump is crucial as it pulls carbon out of the atmosphere and into the ocean
store.
• The world’s oceans are carbon sinks (absorbed 50 percent of the carbon produced by burning
focal fuels thus far).
• Nevertheless, climate change may disrupt the thermohaline circulation which maintains the
pump - as more carbon dioxide dissolves in ocean water, acidification occurs which reduces
the ability of the oceans to absorb carbon and act as a sink.
The physical pump is based on the oceanic circulation of water including upwelling, downwelling
and thermohaline circulation. It also involves the movement of carbon dioxide from the atmosphere
to the ocean by diffusion.
It is important to know that geological processes take place over a long period of time
(millions of years).
How do biological processes sequester carbon on land and in the
oceans on shorter timescales?
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