The carbon cycle is the movement of carbon between the different reservoirs on the earth. The key carbon cycles operate at the terrestrial, atmospheric and oceanic levels. Carbon is stored within the atmosphere in land and in oceans, and is moved by various processes. Carbon stores are where the carbon is stored, and fluxes are the movement/transfer of the carbon between stores. Processes are the physical ways in which fluxes happen.
Global carbon cycle. Image: NASA/Globe Program, CC BY 2.0
The carbon cycle and climate change
The carbon cycle was first described at the end of the eighteenth century by Antoine Lavoisier and Joseph Priestly. Since then, the balance of the carbon cycle has been affected by the increasing use of energy. Burning fossil fuels releases carbon dioxide (CO2) into the atmosphere and adds to the greenhouse gases that trap heat, warming the Earth. With more carbon being emitted than stored, the carbon cycle is pushed out of balance. In 2015, 195 countries adopted the first legally binding global climate deal at the Paris Climate Conference (COP21).
What forms of carbon are in the carbon cycle?
There are three forms of carbon found in the carbon cycle.
Inorganic - found in rocks as bicarbonates and carbonates
Organic - found in plant material and living organisms
Gaseous - found as carbon dioxide (CO2) and methane (CH4) in the atmosphere
A diagram of the carbon cycle focusing on where carbon dioxide and methane are found in the cycle. https://eos.org/features/the-future-of-the-carbon-cycle-in-a-changing-climate
Fast carbon cycle
Carbon cycles have different scales and timeframes. Photosynthesis can happen in seconds, and others take years for the dead organic matter to be returned. The cycling of carbon between the atmosphere and terrestrial or marine ecosystems as well as soils is known as the fast carbon cycle. This cycle involves relatively short-term biogeochemical processes between the environment and living organisms in the biosphere.
Terrestrial ecosystem
The movement of carbon through the living organisms into the atmosphere takes place through respiration with carbon dioxide (CO2) and methane (CH4) released from the decomposition of plants and animals. When the animal and plant matter decay, the carbon stored gets converted into another carbon store by going into the soil.
Marine ecosystem
Carbon is stored in the ocean as dissolved CO2 in the water and carbon compounds in marine organisms. The input to the ocean store is through absorption via gas exchange with the atmosphere. Carbon can leave the atmosphere mixing with water vapour. When precipitation happens, this falls as carbonic acid, commonly known as acid rain.
Slow carbon cycle
The cycling of carbon between surface bedrock and atmospheric or oceanic stores is known as the slow carbon cycle. Marine organisms, such as shellfish and phytoplankton build their shells by combining calcium with carbon. Much of the carbon is stored in a carbon sink on the floor of shallow oceans as accumulating sediments from aquatic plants and animals (organic matter) fall to the sea bed after death. These organisms become compressed and become carbon-rich sedimentary rock. The carbon can take around 100 to 200 million years to move between rock, soil, ocean and atmosphere.
Chemical weathering of rocks
Chemical weathering is the wearing away of rock by chemical reactions, causing the material to dissolve through solution, hydrolysis and oxidation. Carbon dioxide in the atmosphere reacts with the moisture to form weak carbonic acid, which then falls as acidic rain. When acidic rain hits carbon-rich rocks (e.g. limestone), it can dissolve material and form calcium carbonate. These dissolved materials are transported down rivers and deposited into the sea, forming sedimentary rock.
Volcanic out-gassing
Volcanic out-gassing happens when there is volcanic activity at two types of plate boundaries. These are constructive plate boundaries, when two plates move apart and destructive subduction zones, when an oceanic plate subducts under a continental plate. The extreme heat from the tectonic processes leads to sedimentary rock undergoing chemical changes causing carbon dioxide to be released into the atmosphere. Out-gassing is common in geothermal locations like New Zealand.
What is a carbon store?
A carbon store is where the carbon is stored within the cycle. It is measured using gigatonnes of carbon (GtC). A carbon sink is a store that takes in more carbon than it emits, while a carbon source is a store that emits more carbon than it stores.
Where are carbon stores located?
There are six different main spheres on the Earth where carbon is stored:
Atmosphere: as gases like carbon dioxide and methane.
Biosphere: in all living and dead organisms.
Cryosphere: in the frozen ground of the tundra and arctic regions containing plant material.
Pedosphere: in soil. This contains organic carbon and the remains of dead animals and plants.
Lithosphere: as rocks like calcium carbonate and fossil fuels.
- Hydrosphere: as dissolved carbon dioxide in bodies of water.
Examples of carbon stores
Carbon exists in different forms depending on the store. Most of the Earth's carbon is geological, and the largest store is in rocks.
Size: 66,000 - 100,000 GtC. This process takes thousands if not millions of years.
Size: 38,000 GtC. Carbon is constantly sequestered through the marine organisms, lost as an output to the lithosphere or gained as an input from rivers and erosion.
Size: 4,000 GtC. The exploitation of fossil fuels by humans has led to rapid depletion.
Size: 1,500 GtC. Deforestation, agriculture and land use are affecting this store.
Size: 750 GtC. Human activity has caused CO2 levels in the atmosphere to increase by around 40% since the industrial revolution, causing unprecedented change to the global climate.
Size: 560 GtC. Due to climate change and deforestation, carbon storage in forests is declining annually in some parts of the world.
Geological carbon stores
Most of the Earth's carbon is in geological carbon stores. Fossil fuels such as coal, oil and gas store carbon for millions of years. Below are examples of these geological carbon stores and how they form.
Formations of coal
Coal forms on land. When land-based plants die and enter swamps, they slowly settle and compact to form peat and coal. Coal takes millions of years to form, depending on temperature and pressures.
Formations of natural gas
Natural gas is trapped in the same sedimentary layers in which coal and crude oil are found. Methane is an example of natural gas created as a by-product during the formation of coal and crude oil.
Formation of crude oil
The formation of crude oil begins with the settling of fine-grained sediments and biologically degraded materials. There needs to be at least 2% organic carbon. A series of anaerobic reactions happen that turns most organic carbon into a liquid, crude oil. Because of its light density, crude oil can migrate up through the layers of permeable or porous rocks. However, when there is a layer of impermeable rock, the crude oil ends up being trapped.
Formation of limestone
The formation of limestone starts with marine organisms having carbon-based shells formed from calcium carbonate. When these organisms die, they sink to the bottom of the ocean and as more sediment falls, they become compacted. When the sediment reaches 100 meters in depth, the pressure and chemical reactions cause cementation which leads to the formation of limestone rock.
Carbon Cycle - Key takeaways
The carbon cycle is the movement of carbon between different reservoirs on the earth.
Carbon stores are where the carbon is stored; fluxes refer to the movement or transfer of the carbon between stores, and processes refer to the physical way the fluxes happen.
There are three types of carbon found in the carbon cycle, inorganic, organic and gaseous.
The fast carbon cycle involves short term biogeochemical processes such as photosynthesis which can take seconds. The slow carbon cycle requires sediment to accumulate and compress to become carbon-rich sedimentary rocks. This can take millions of years.
There are six different spheres in which carbon is stored, atmosphere, biosphere, cryosphere, pedosphere, lithosphere and hydrosphere.
The largest Earth's carbon store is geological in marine sediments and sedimentary rocks.
There are various forms of geological carbon stores such as crude oil, natural gas, coal and limestone.
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