Carbon cycle


Content – Other fuels


Carbon is a non-metallic chemical element with the atomic number 6 on the periodic table and with an atomic weight of 12,011.

Carbon atoms can be bonded together many ways providing different physical properties, such as graphite, diamond, and amorphous carbon.


Illustration of carbon atom


All forms (allotropes) of carbon are under normal conditions solids that are chemically resistant and require high temperature to react even with oxygen.

All living organisms are built from carbon compounds. It is the fundamental building block of all life and an important component of many chemical processes.

Along with the nitrogen cycle and the water cycle, the carbon cycle comprises a sequence of events that are key to making the Earth capable of sustaining life, it describes the movement of carbon as it is recycled and reused throughout the ecosystems of the Earth.

Carbon in various forms is exchanged between different carbon reservoirs, all being elements of the carbon cycle.

The global carbon cycle is usually divided into the following major interconnected reservoirs of carbon:

  • The atmosphere.
  • The terrestrial biosphere (plants and soil).
  • The oceans, including dissolved inorganic carbon and living and non-living marine biota.
  • The sediments, including fossil hydrocarbons (oil and gas), fresh water systems and non-living organic material, such as soil carbon.
  • The interior of the Earth, carbon from the Earth’s mantle and crust

The carbon exchanges between reservoirs occur as the result of various chemical, physical, geological, and biological processes.Illustration of the carbon cycle including emissions from automobiles, humans and the oceans and CO2 absorption by green plants and the oceans

The total amount of carbon in one form or the other is constant and therefore if carbon is removed from one reservoir it means that the carbon content of another reservoir is increased.

Atmosphere – Carbon in the atmosphere exists in two main forms, that is carbon dioxide and methane.

Carbon dioxide extracted from the atmosphere through photosynthesis, thus entering the terrestrial and oceanic biospheres. Carbon dioxide also dissolves directly from the atmosphere into water (oceans, lakes, etc.), as well as dissolving in precipitation as raindrops fall through the atmosphere. When dissolved in water, carbon dioxide reacts with water molecules and forms carbonic acid, which contributes to ocean acidity.

Terrestrial biosphere – Includes the organic carbon in all land-living organisms, both alive and dead, as well as carbon stored in soils.

Most carbon in the terrestrial biosphere is organic carbon, while about a third of soil carbon is stored in inorganic forms, such as calcium carbonate.

Organic carbon is a major component of all organisms living on earth.

Carbon is released from the terrestrial biosphere in several ways and on different time scales. The combustion or respiration of organic carbon releases it rapidly into the atmosphere. It can be exported into the oceans through rivers or remain in soils in the form of inert carbon. Carbon stored in soil can remain there for thousands of years before being washed into rivers by erosion or released into the atmosphere through soil respiration.

Oceans – The oceans are the reservoir containing the greatest quantity of cycled carbon and are second only to the interior of the Earth. The surface layers of the oceans holds large amounts of dissolved organic carbon that is exchanged rapidly with the atmosphere.

The concentration of dissolved carbon in the deeper layers is about 15% higher than that of the surface layer. The storage in the deeper layers is for much longer periods of time than the surface layer. Carbon is exchanged between these two layers.

Carbon enters the oceans through the dissolution of atmospheric carbon dioxide, which is converted into carbonate. It can also enter the oceans through rivers as dissolved organic carbon. It is converted by organisms into organic carbon through photosynthesis and can either be exchanged throughout the food chain or precipitated into the ocean’s deeper, more carbon rich layers as dead soft tissue or in shells as calcium carbonate. It circulates in this layer for long periods of time before either being deposited as sediment or, eventually, returned to the surface waters through thermohaline circulation.

The interior of the Earth – The geologic component of the carbon cycle operates slowly in comparison to the other parts of the carbon cycle.

Most of the carbon is stored inertly in the mantle and crust. Much of the carbon stored in the mantle has been there since the earth formed and some was deposited in the form of organic carbon from the biosphere.

Geological processes play an important role in the carbon cycle. This includes the formation of sedimentary rocks and recycling via plate tectonics, weathering and volcanic eruption.

Rocks are over time broken down into small particles and dissolved materials. These materials combined with particles that result from decomposition and surface erosion are carried to the ocean where the sediments will accumulate and bury older sediments below. The layering of sediment causes pressure to build and eventually turn the deeper sediments into rock, such as shale.

Within the ocean water, dissolved materials are used by marine life to make calcium carbonate (CaCO3) as skeletons and shells, which in turn forms into limestone when these organisms die and falls to the seafloor.

About 80% of the carbon stored in the interior of the earth is limestone and derivatives thereof. The remaining 20% is stored as kerogens (possible source rock for hydrocarbons) formed through the sedimentation and burial of terrestrial organisms under high heat and pressure. Organic carbon stored in the geosphere can remain there for millions of years.

Carbon can be relaesed in several ways. Carbon dioxide is released during the metamorphosis of carbonate rocks when they are subducted into the mantle. This carbon dioxide can be released into the atmosphere and ocean through volcanoes and hotspots. Carbon can also be removed through extraction of kerogens (hydrocarbons).

The “slow carbon cycle”
Scientists belive that through a series of chemical reactions and tectonic activity, carbon takes between 100-200 million years to move between rocks, soil, ocean, and atmosphere. On average, 10–100 million metric tons of carbon move through the slow carbon cycle every year, whereas the fast carbon cycle moves 10000 – 100000 million metric tons per year. The emission of carbon from human activity is estimated to around 1000 million metric tons per year.

The “Fast Carbon Cycle”
The time it takes carbon to move through the fast carbon cycle is measured in a lifespan. The fast carbon cycle is largely the movement of carbon through life forms on Earth, or the biosphere. Between 10000 to 100000 million metric tons of carbon move through the fast carbon cycle every year.

Approximate quantities of carbon reservoirs

Reservoir Quantity – Gigatons
(109 tonnes)
Atmosphere 720
Oceans (total) 38 500
Ocean – Deep layers 37 000
Ocean – Upper layers 1 000
Lithosphere 79 000 000
Earth Crust – Sedimentary rock and kerogen 75 000 000
Earth Crust – Hydrocarbons 4 000
Terrestrial biosphere (total) 2 000
Living biomass (plants) 800
Dead biomass (soil) 1 200