The oceanic carbon cycle (or marine carbon cycle) is composed of processes that exchange carbon between various pools within the ocean as well as between the atmosphere, Earth interior, and the seafloor. The carbon cycle is a result of many interacting forces across multiple time and space scales that circulates carbon around the planet, ensuring that carbon is available globally. The Oceanic carbon cycle is a central process to the global carbon cycle and contains both inorganic carbon (carbon not associated with a living thing, such as carbon dioxide) and organic carbon (carbon that is, or has been, incorporated into a living thing). Part of the marine carbon cycle transforms carbon between non-living and living matter.

Three main processes (or pumps) that make up the marine carbon cycle bring atmospheric carbon dioxide (CO₂) into the ocean interior and distribute it through the oceans. These three pumps are: (1) the solubility pump, (2) the carbonate pump, and (3) the biological pump. The total active pool of carbon at the Earth's surface for durations of less than 10,000 years is roughly 40,000 gigatons C (Gt C, a gigaton is one billion tons, or the weight of approximately 6 million blue whales), and about 95% (~38,000 Gt C) is stored in the ocean, mostly as dissolved inorganic carbon.^[1][2] The speciation^{[clarification needed]} of dissolved inorganic carbon in the marine carbon cycle is a primary controller of acid-base chemistry in the oceans.

Earth's plants and algae (primary producers) are responsible for the largest annual carbon fluxes. Although the amount of carbon stored in marine biota (~3 Gt C) is very small compared with terrestrial vegetation (~610 GtC), the amount of carbon exchanged (the flux) by these groups is nearly equal – about 50 GtC each.^[1] Marine organisms link the carbon and oxygen cycles through processes such as photosynthesis.^[1] The marine carbon cycle is also biologically tied to the nitrogen and phosphorus cycles by a near-constant stoichiometric ratio C:N:P of 106:16:1, also known as the Redfield Ketchum Richards (RKR) ratio,^[3] which states that organisms tend to take up nitrogen and phosphorus incorporating new organic carbon. Likewise, organic matter decomposed by bacteria releases phosphorus and nitrogen.

Based on the publications of NASA, World Meteorological Association, IPCC, and International Council for the Exploration of the Sea, as well as scientists from NOAA, Woods Hole Oceanographic Institution, Scripps Institution of Oceanography, CSIRO, and Oak Ridge National Laboratory, the human impacts on the marine carbon cycle are significant.^[4][5][6][7] Before the Industrial Revolution, the ocean was a net source of CO₂ to the atmosphere whereas now the majority of the carbon that enters the ocean comes from atmospheric carbon dioxide (CO₂).^[8] The burning of fossil fuels and production of cement have changed the balance of carbon dioxide between the atmosphere and oceans,^[6] causing acidification of the oceans.^[8][9] Climate change, a result of excess CO₂ in the atmosphere, has increased the temperature of the ocean and atmosphere (global warming).^[10] The slowed rate of global warming occurring from 2000–2010^[11] may be attributed to an observed increase in upper ocean heat content.^[12][13]

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Carbon dioxide In the atmosphere Ocean acidification Removal Satellite measurements
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Metabolic pathways Photosynthesis Chemosynthesis Calvin cycle Reverse Krebs cycle Carbon fixation C3 C4
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Marine carbon

Carbon compounds can be distinguished as either organic or inorganic, and dissolved or particulate, depending on their composition. Organic carbon forms the backbone of key component of organic compounds such as – proteins, lipids, carbohydrates, and nucleic acids. Inorganic carbon is found primarily in simple compounds such as carbon dioxide, carbonic acid, bicarbonate, and carbonate (CO₂, H₂CO₃, HCO₃⁻, CO₃²⁻ respectively).

Marine carbon is further separated into particulate and dissolved phases. These pools are operationally defined by physical separation – dissolved carbon passes through a 0.2 μm filter, and particulate carbon does not.

Inorganic carbon

There are two main types of inorganic carbon that are found in the oceans. Dissolved inorganic carbon (DIC) is made up of bicarbonate (HCO₃⁻), carbonate (CO₃²⁻) and carbon dioxide (including both dissolved CO₂ and carbonic acid H₂CO₃). DIC can be converted to particulate inorganic carbon (PIC) through precipitation of CaCO₃ (biologically or abiotically). DIC can also be converted to particulate organic carbon (POC) through photosynthesis and chemoautotrophy (i.e. primary production). DIC increases with depth as organic carbon particles sink and are respired. Free oxygen decreases as DIC increases because oxygen is consumed during aerobic respiration.

Particulate inorganic carbon (PIC) is the other form of inorganic carbon found in the ocean. Most PIC is the CaCO₃ that makes up shells of various marine organisms, but can also form in whiting events. Marine fish also excrete calcium carbonate during osmoregulation.^[14]

Some of the inorganic carbon species in the ocean, such as bicarbonate and carbonate, are major contributors to alkalinity, a natural ocean buffer that prevents drastic changes in acidity (or pH). The marine carbon cycle also affects the reaction and dissolution rates of some chemical compounds, regulates the amount of carbon dioxide in the atmosphere and Earth's temperature.^[15]

Organic carbon

Like inorganic carbon, there are two main forms of organic carbon found in the ocean (dissolved and particulate). Dissolved organic carbon (DOC) is defined operationally as any organic molecule that can pass through a 0.2 μm filter. DOC can be converted into particulate organic carbon through heterotrophy and it can also be converted back to dissolved inorganic carbon (DIC) through respiration.

Those organic carbon molecules being captured on a filter are defined as particulate organic carbon (POC). POC is composed of organisms (dead or alive), their fecal matter, and detritus. POC can be converted to DOC through disaggregation of molecules and by exudation by phytoplankton, for example. POC is generally converted to DIC through heterotrophy and respiration.

Marine carbon pumps

Solubility pump

Full article: Solubility pump

The oceans store the largest pool of reactive carbon on the planet as DIC, which is introduced as a result of the dissolution of atmospheric carbon dioxide into seawater – the solubility pump.^[15] Aqueous CO₂, carbonic acid, bicarbonate ion, and carbonate ion concentrations comprise dissolved inorganic carbon (DIC). DIC circulates throughout the whole ocean by Thermohaline circulation, which facilitates the tremendous DIC storage capacity of the ocean.^[16] The chemical equations below show the reactions that CO₂ undergoes after it enters the ocean and transforms into its aqueous form.

${\displaystyle {\ce {CO2(aq) + H2O -> H2CO3}}}$

(1)

Carbonic acid rapidly dissociates into free hydrogen ion (technically, hydronium) and bicarbonate.

${\displaystyle {\ce {H2CO3 -> H+ + HCO3^-}}}$

(2)

The free hydrogen ion meets carbonate, already present in the water from the dissolution of CaCO₃, and reacts to form more bicarbonate ion.

${\displaystyle {\ce {H+ + CO3^2- -> HCO3^-}}}$

(3)

The dissolved species in the equations above, mostly bicarbonate, make up the carbonate alkalinity system, the dominant contributor to seawater alkalinity.^[9]

Carbonate pump

The carbonate pump, sometimes called the carbonate counter pump, starts with marine organisms at the ocean's surface producing particulate inorganic carbon (PIC) in the form of calcium carbonate (calcite or aragonite, CaCO₃). This CaCO₃ is what forms hard body parts like shells.^[15] The formation of these shells increases atmospheric CO₂ due to the production of CaCO₃^[9] in the following reaction with simplified stoichiometry:^[17]

${\displaystyle {\ce {Ca^2+ + 2HCO3^- <=> CaCO3 + CO2 + H2O}}}$[18]

(4)

Coccolithophores, a nearly ubiquitous group of phytoplankton that produce shells of calcium carbonate, are the dominant contributors to the carbonate pump.^[15] Due to their abundance, coccolithophores have significant implications on carbonate chemistry, in the surface waters they inhabit and in the ocean below: they provide a large mechanism for the downward transport of CaCO₃.^[19] The air-sea CO₂ flux induced by a marine biological community can be determined by the rain ratio - the proportion of carbon from calcium carbonate compared to that from organic carbon in particulate matter sinking to the ocean floor, (PIC/POC).^[18] The carbonate pump acts as a negative feedback on CO₂ taken into the ocean by the solubility pump. It occurs with lesser magnitude than the solubility pump.

Biological pump

Full article: Biological pump

Particulate organic carbon, created through biological production, can be exported from the upper ocean in a flux commonly termed the biological pump, or respired (equation 6) back into inorganic carbon. In the former, dissolved inorganic carbon is biologically converted into organic matter by photosynthesis (equation 5) and other forms of autotrophy^[15] that then sinks and is, in part or whole, digested by heterotrophs.^[20] Particulate organic carbon can be classified, based on how easily organisms can break them down for food, as labile, semilabile, or refractory. Photosynthesis by phytoplankton is the primary source for labile and semilabile molecules, and is the indirect source for most refractory molecules.^[21][22] Labile molecules are present at low concentrations outside of cells (in the picomolar range) and have half-lives of only minutes when free in the ocean.^[23] They are consumed by microbes within hours or days of production and reside in the surface oceans,^[22] where they contribute a majority of the labile carbon flux.^[24] Semilabile molecules, much more difficult to consume, are able to reach depths of hundreds of meters below the surface before being metabolized.^[25] Refractory DOM largely comprises highly conjugated molecules like Polycyclic aromatic hydrocarbons or lignin.^[21] Refractory DOM can reach depths greater than 1000 m and circulates through the oceans over thousands of years.^[26][22][27] Over the course of a year, approximately 20 gigatons of photosynthetically-fixed labile and semilabile carbon is taken up by heterotrophs, whereas fewer than 0.2 gigatons of refractory carbon is consumed.^[22] Marine dissolved organic matter (DOM) can store as much carbon as the current atmospheric CO₂ supply,^[27] but industrial processes are altering the balance of this cycle.^[28]

${\displaystyle {\ce {{\underset {carbon~dioxide}{6CO2}}+{\underset {water}{6H2O}}->{\underset {carbohydrate}{C6H12O6}}+{\underset {oxygen}{6O2}}}}}$

(5)