The carbon cycle: a simple explanation

Carbon dioxide from the atmosphere dissolves in the surface waters. On entering the ocean, carbon dioxide undergoes rapid chemical reactions with the water and only a small fraction remains as carbon dioxide. The carbon dioxide and the associated chemical forms are collectively known as dissolved inorganic carbon or DIC. This chemical partitioning of DIC ('buffering') affects the air–sea transfer of carbon dioxide, as only the unreacted carbon dioxide fraction in the sea water takes part in ocean–atmosphere interaction.

The dissolved inorganic carbon (DIC) is transported by ocean currents. Near the poles, cold dense waters sink towards the bottom of the ocean and subsequently spread through the ocean basins. These waters return to the surface hundreds of years later. As more carbon dioxide can dissolve in cold water than in warm, these cold dense waters sinking at high latitudes are rich in carbon and act to move large quantities of carbon from the surface to deep waters. This mechanism is known as the 'solubility pump'.

As well as being transported around the ocean, dissolved inorganic carbon is also used by ocean biology. In the surface waters, drifting microscopic oceanic plants known as phytoplankton grow. As with land based plants, phytoplankton take in carbon dioxide during growth and convert it to complex organic forms. The phytoplankton are eaten by drifting oceanic animals known as zooplankton, which themselves are preyed upon by other zooplankton, fish or even whales. During these biological processes, some of the carbon taken in during growth of the phytoplankton is broken down from the organic forms of the biology back to inorganic forms (DIC). If between the carbon uptake by phytoplankton and the subsequent return of the carbon to DIC, the biological material has been transported to depth, for example by the sinking of large biologically formed particles, there is a net transfer of carbon from the surface to depth. This process is termed the 'biological pump'. The carbon can also sink as skeletal structures of the biology which is known as the 'carbonate pump'.