THE CARBON CYCLE
6.1A: The biogeochemical carbon cycle consists of carbon stores of different sizes (terrestrial, oceans and
atmosphere), with annual fluxes between stores of varying sizes (measured in Pg/Gt), rates and on
different timescales.
Carbon is found everywhere: oceans, rocks, soils, the atmosphere and in all forms of life. The carbon cycle
is the cycle by which carbon moves from one Earth sphere (atmosphere, hydrosphere, lithosphere and
biosphere) to another. It is a closed system but made up of interlinked subsystems which are open and
have inputs and outputs.
The carbon cycle is made of two main components:
Stores – which function as sources and sinks
Fluxes/Flows – which is the movement of carbon from one store to another
Stores
There are many carbon stores, which also vary in size:
1. Earth’s crust is the largest store, containing around 100 million gigatons of carbon
2. Oceans contain around 38,000 gigatons
3. Fossil fuels contain about 4,000 gigatons
4. Soils contain 1,500 gigatons
5. Atmosphere has around 750 gigatons
6. Plants/Vegetation and other living organisms contain around 560 gigatons
Fluxes
There are also many fluxes that allow carbon to move from one store to another, and also vary in size.
These are usually measured in either petagrams or gigatons of carbon per year:
1. Photosynthesis removes most of the carbon from the atmosphere, around 120 gigatons
2. Ocean uptake and ocean loss absorb (to form dissolved CO2) and release carbon to the
atmosphere, at around 92 and 90 respectively
3. Plant respiration, opposite to photosynthesis, releases carbon into the atmosphere, around 60
gigatons
4. Burning fossil fuels (artificial combustion) constitutes for 6 gigatons being released into the
atmosphere
5. Deforestation and land use change, as well as Volcanoes, both account for 0.1 gigatons of carbon
being released into the atmosphere
6.1B: Most of the Earth's carbon is geological, resulting from the formation of sedimentary carbonate
rocks (limestone) in oceans and biologically derived carbon in shale, coal and other rocks.
Most of the world’s carbon is geological in rocks, with the Earth’s crust being the largest carbon store
containing 100 million gigatons of carbon, fossil fuels containing 4,000 and soils containing 1,500.
Formation of Sedimentary Carbonate
1. Sediment is deposited in layers in low-energy environment e.g. lakes or sea bed
, 2. Further layers are deposited and sediment undergoes diagenesis, which is process of physical and
chemical changes that occur during the conversion of sediment to sedimentary rock
3. Lower layers become compressed and chemical reactions cement particles together, under high
pressure due to weight of layers above
4. Conversion of loose, unconsolidated sediment into solid rock is known as lithification
Limestone
1. Marine organisms make skeletons and shells by extracting calcium carbonate from ocean
2. Dead organisms sink to ocean floor and accumulate in layers
3. Weight of layers cause compaction, squeezing out water
4. Dissolved calcium carbonate cements layers together to form limestone rock (lithification)
Or
5. Water may can evaporate, if in shallow sea or in lake, and leave behind the calcium carbonate
deposit from dead organisms
Metamorphism (change in rocks’ appearance and structure due to intense heat and pressure) can cause
uplift of calcium carbonate deposits in formation of mountains e.g. Himalayan rock is rich in calcium
carbonate derived from crustaceans, corals and plankton – when tectonic activity forced uplift of rock,
mountain range formed and is exposed to weathering and erosion that can release carbon back into the
ocean.
Formation of biologically derived Carbon
Formation of shale:
1. Rivers carry clay, silt and organic materials
2. When water reaches a lake or shallow sea, the water slows down and the sediment is deposited as
a delta
3. Weight of layers cause compaction, squeezing out water
4. Over time, layers fuse together (cementation) and lithification occurs to form shale
Formation of Coal:
1. Giant plants died in swamps millions of years ago, even before the dinosaurs
2. Over millions of years, plants were buried under water, dirt and sediment
3. Heat and pressure transform dead plants into coal
Formation of Oil and Gas:
1. Plants and animals die and sink to bottom of sea
2. Over time, more sediment creates pressure, compressing dead organisms into oil
3. Oil moves up through porous rock and eventually forms a reservoir as it reaches an impermeable
layer, with oil sinking to bottom and gas rising to top of reservoir
4. Tectonic activity and movement of rock into anticlines can form reservoir layers which is
concentrated with oil and gas (otherwise oil and gas are distributed in small amounts over long
distances)
6.1C: Geological processes release carbon into the atmosphere through volcanic out-gassing at ocean
ridges/subduction zones, and the chemical weathering of rocks.
Volcanic Out-gassing
