Carbon Stores

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Introduction to the Carbon Cycle

The carbon cycle plays a key role in balancing the Earth's systems. Carbon is stored in the atmosphere, on land, and in oceans, and is moved between these by various processes.

The carbon cycle

There are no inputs or outputs in the carbon cycle. It is a closed system with three parts:
- Stores - Where carbon is held.
- Fluxes - The flows of carbon between different stores. Fluxes differ in size.
- Processes - The physical ways in which carbon fluxes happen.

Carbon stores

Carbon stores are measured in petagrams of carbon (PgC).
- The hydrosphere (38,000 PgC) - dissolved carbon dioxide in bodies of water.
- The lithosphere (100,000 PgC)- rocks like calcium carbonate and fossil fuels.
- The biosphere (2,000 PgC) - animals and plants (dead and living).
- The atmosphere (750 PgC) - gases like carbon dioxide.

Biochemical Carbon Stores

Carbon is found across our world in different forms: as a liquid (i.e. dissolved in bodies of water), as a gas (e.g. carbon dioxide) and as a solid (i.e. calcium carbonate).

Terrestrial

Carbon is found within every living organism on the planet. On the land, every plant and animal is an example of a carbon store.
When plant and animal matter decays, it can be converted into other carbon stores by going into the soil.
Other terrestrial carbon is stored within the mantle and can be converted into atmospheric carbon via the process of ‘out-gassing’ by volcanoes, where they let out carbon dioxide during eruptions.

The role of organisms

Photosynthesis - plants remove carbon dioxide from the atmosphere to produce energy for plant growth.
Respiration - all living organisms release carbon dioxide as they breathe and grow.
Decomposition - when living organisms die, they are broken down, releasing carbon dioxide into the soil.
Combustion - the burning of biomass or fossil fuels, both by man and in natural wildfires, releases carbon dioxide back into the atmosphere.

Oceans

Aquatic plants and animals (organic matter) fall to the ocean bed after death.
Due to compression and cementation, they form into sedimentary rock.
During these processes, crude oil and natural gas can form.

Atmosphere

Carbon dioxide is stored as a gas in the atmosphere.
Carbon can leave the atmosphere when mixing with water vapour. When precipitation happens this falls as carbonic acid (commonly referred to as acid rain).

Fluxes varying in size

The carbon cycle can happen at different scales and timeframes.
The quickest cycle is completed in seconds during photosynthesis.
But, it can take years for dead organic material to be returned.
Some organic material will be buried in the sea and form into sedimentary rock, which is an extremely slow process.

Geological Carbon Stores

In the formation of sedimentary rock, carbon can be found in several different forms including limestone, crude oil, coal and natural gas.

Formation of limestone

Phytoplankton have carbon-based shells. Other marine organisms also have shells or skeletons that are formed from calcium carbonate.
When these aquatic organisms die, they sink to the bottom of the ocean.
As more sediment falls on top of them, they are compacted. When the sediment has reached around 100 metres in depth, the pressure and chemical reactions cause cementation to take place. This leads to the formation of limestone rock.

Formation of crude oil

Crude oil starts with the settling of fine-grained sediments and biologically degraded materials. There needs to be a minimum of 2% organic carbon.
A series of anaerobic reactions (reactions happening without oxygen) happen to turn the majority of the organic carbon into a liquid (crude oil).
Due to its light density, crude oil may migrate upwards through layers of permeable or porous rock. But a layer of impermeable rock will result in the crude oil being trapped.

Formation of coal

Coal is formed on land, unlike limestone and sedimentary rock which are formed in the oceans.
When land-based plants die and enter into swamps, they slowly settle and compact to form peat and coal.
Coal takes millions of years to form, depending on the temperatures and pressures.

Formation of natural gas

Methane (CH₄) is an example of natural gas and is created as a by-product during the formation of coal and crude oil.
Natural gas is trapped within the same sedimentary layers that coal and crude oil are found in.