Oceanic carbon cycle
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] 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.
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.
[8] In recent decades, the ocean has acted as a sink for anthropogenic CO2, absorbing around a quarter of the CO2 produced by humans through the burning of fossil fuels and land use changes.
[11] The slowed rate of global warming occurring from 2000–2010[12] may be attributed to an observed increase in upper ocean heat content.
[16] 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.
In the former, dissolved inorganic carbon is biologically converted into organic matter by photosynthesis (equation 5) and other forms of autotrophy[16] that then sinks and is, in part or whole, digested by heterotrophs.
[22][23] 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.
[24] They are consumed by microbes within hours or days of production and reside in the surface oceans,[23] where they contribute a majority of the labile carbon flux.
[26] Refractory DOM largely comprises highly conjugated molecules like Polycyclic aromatic hydrocarbons or lignin.
[23] Marine dissolved organic matter (DOM) can store as much carbon as the current atmospheric CO2 supply,[28] but industrial processes are altering the balance of this cycle.
[29] Inputs to the marine carbon cycle are numerous, but the primary contributions, on a net basis, come from the atmosphere and rivers.
[37] Ocean-atmospheric exchanges rates of CO2 depend on the concentration of carbon dioxide already present in both the atmosphere and the ocean, temperature, salinity, and wind speed.
[43] Oceanic carbon can exit the system in the form of detritus that sinks and is buried in the seafloor without being fully decomposed or dissolved.
[44] At most, 4% of the particulate organic carbon from the euphotic zone in the Pacific Ocean, where light-powered primary production occurs, is buried in marine sediments.
Historically, sediments with the highest organic carbon contents were frequently found in areas with high surface water productivity or those with low bottom-water oxygen concentrations.
[49][50] Degradation of POC also results in microbial methane production which is the main gas hydrate on the continental margins.
[51] Lignin and pollen are inherently resistant to degradation, and some studies show that inorganic matrices may also protect organic matter.
[52] Preservation rates of organic matter depend on other interdependent variables that vary nonlinearly in time and space.
[54] Organic carbon burial is an input of energy for underground biological environments and can regulate oxygen in the atmosphere at long time-scales (> 10,000 years).
Fjords, or cliffs created by glacial erosion, have also been identified as areas of significant carbon burial, with rates one hundred times greater than the ocean average.
[56] Rocks formed in the ocean seafloor are recycled through plate tectonics back to the surface and weathered or subducted into the mantle, the carbon outgassed by volcanoes.
[61][62] This induces climate change that drives carbon concentration and carbon-climate feedback processes that modifies ocean circulation and the physical and chemical properties of seawater, which alters CO2 uptake.
[63][64] Overfishing and the plastic pollution of the oceans contribute to the degraded state of the world's biggest carbon sink.
[68] Most surface water will remain supersaturated with respect to CaCO3 (both calcite and aragonite) for some time on current emissions trajectories,[68] but the organisms that require carbonate will likely be replaced in many areas.
[68] Coral reefs are under pressure from overfishing, nitrate pollution, and warming waters; ocean acidification will add additional stress on these important structures.
Of current geoengineering interest is the possibility of accelerating the biological pump to increase export of carbon from the surface ocean.
This increased export could theoretically remove excess carbon dioxide from the atmosphere for storage in the deep ocean.
[74] Other large contributors to carbon burial caused by damming occur on the Danube, the Amazon, the Yangtze, the Mekong, the Yenisei, and the Tocantins Rivers.
