How the deep ocean carbon cycle works

01 Carbon first crosses the air-sea boundary

The ocean and atmosphere continually exchange carbon dioxide at the sea surface. The direction and size of that exchange depend on concentration differences, temperature, winds, and the chemistry of seawater. Some carbon also arrives through rivers and coastal processes.

02 The biological pump turns dissolved carbon into particles

Phytoplankton use dissolved carbon during photosynthesis. When organisms grow, die, or are consumed, some of that carbon becomes organic matter that can sink as particles. Most is recycled in the upper ocean, but a fraction travels deeper.

03 The carbonate system changes the chemical form

Dissolved carbon dioxide participates in seawater chemistry, forming bicarbonate and carbonate ions. Organisms that build shells or skeletons move carbon into solid material, while dissolution and respiration return carbon to dissolved forms. The carbonate system is a set of linked reactions, not a single tank.

04 Depth brings recycling and delay

Sinking organic matter is consumed by microbes and animals. This remineralization releases dissolved inorganic carbon and nutrients back into the water, often at depths where exchange with the atmosphere is slower. The result is a vertical gradient of age, chemistry, and available oxygen.

05 Circulation carries the memory of the surface

Water masses formed or modified near the surface can move into the ocean interior and carry dissolved carbon with them. Deep currents eventually return water toward the surface, where mixing and upwelling can expose that carbon to new biology or to the atmosphere.

06 Burial is the long end of the cycle

A small share of sinking material reaches seafloor sediments and may be buried for much longer than the water-mass journey. Burial, dissolution, and later geological processes connect the rapid biological cycle to the slower carbon cycle of rocks. The deep ocean is therefore a moving reservoir, not a permanent vault.