Ocean biological carbon pump

In the sunlit upper ocean, phytoplankton use photosynthesis to convert inorganic carbon into organic matter. As this production supports food webs, zooplankton and other organisms consume it, respire some carbon back into dissolved form, and produce fecal material. Dead organisms, discarded tissues and other particles can sink as marine snow. This downward movement of organic carbon from surface waters is often called export production. Most of the material is decomposed or consumed on its descent, returning carbon to the water as dissolved inorganic or organic forms; only a fraction reaches the deep ocean or seabed.

Gravity-driven sinking is only one part of the pump. Animals that feed near the surface and migrate to depth each day can carry carbon downward in their bodies and waste. Turbulence and large-scale circulation also move dissolved and suspended carbon below the surface. These pathways operate together, and their relative importance differs between ecosystems. Their combined effect is a biological transfer of carbon from the surface ocean toward deeper waters, not a permanent removal from the Earth system.

Carbon that reaches depth may remain separated from the atmosphere until ocean circulation brings it back toward the surface. The duration depends on the depth, water circulation and the form in which carbon is stored. Some educational treatments use roughly 500 metres as a practical threshold for sequestration on timescales of hundreds of years, but this is a convention rather than a universal boundary. Recent research has also examined continuous vertical sequestration: carbon can be stored for long periods at intermediate depths, so estimates based only on flux crossing a fixed deep-ocean level may omit some storage. The term sequestration therefore needs an explicit timescale and accounting method.

The biological pump helps regulate the exchange of carbon between the atmosphere and ocean, but it works alongside physical air–sea gas exchange and the solubility pump. Its climate effect depends not just on how much carbon phytoplankton fix, but on how efficiently carbon is exported and how long it remains out of contact with the atmosphere. NASA's EXPORTS field campaigns combine ship measurements, autonomous instruments and satellite observations to study these pathways and improve their representation in ocean and climate models. Changes in plankton communities, food webs, circulation and ocean conditions can alter the pump, making regional observations important.