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Geology LibreTexts

5.4: The Carbon Cycle

The Carbon Cycle

Carbon is the building block of life, which is constantly cycled throughout the Earth in physical, biological, and chemical processes. Carbon can exist in many forms, and although these are always changing, there exists a stable equilibrium where a constant amount of carbon is cycling. This carbon is stored in the atmosphere, plants, animals, rocks, and water. When organisms die or rocks decay, carbon is released into the atmosphere and the cycle continues. Volcanic eruptions can also expel large amounts of carbon in the form of CO2. There has also been a large increase in anthropogenic contributions of carbon to the atmosphere with use of fossil fuels and cement production, which have had significant impacts on the cycle. 

The CO2 in the atmosphere can dissolve into the ocean or other bodies of water at the surface, it then goes through another cycle. Once in solution, CO2 reacts with water to form carbonic acid. This acid can further dissociate to form bicarbonate and carbonate in the ocean. This is a buffering process that prevents the oceans from acidifying rapidly due to the addition of atmospheric CO2 in the oceans. Ocean acidification is a term that describes the increase in COthat is being absorbed by the oceans and the resulting decrease in pH. Since the oceans make up a huge part of the earth, most of the earth’s carbon is stored in the ocean.

As far as biological processes go, the cycle starts when carbon is dissolved into the water. In shallow waters, phytoplankton take up carbon dioxide and produce much of the oxygen which we breathe, as well as transform carbon dioxide and transport it when the algae sink or are consumed. Small organisms such as coccolithophores, which take up calcium and carbonate to form their shells, can distribute carbon throughout the ocean depths when the organism dies.

Physically, carbon is cycled within oceans between various layers by upwelling and downwelling. When carbon dioxide dissolves in cold waters, this denser water can downwell and join the ocean conveyer belt on a journey that may last more than 500 years before it reaches the surface again.


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