Oceanic carbon cycle

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. 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. 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. The chemical equations below show the reactions that CO2 undergoes after it enters the ocean and transforms into its aqueous form. First, carbon dioxide reacts with water to form carbonic acid. concentration in the 1990s (from the GLODAP climatology) : Carbonic acid rapidly dissociates into free hydrogen ion (technically, hydronium) and bicarbonate. :{{Chem2|H2CO3 -> H^{+} + HCO3^{-} }} The free hydrogen ion meets carbonate, already present in the water from the dissolution of CaCO3, and reacts to form more bicarbonate ion. :{{Chem2|H^{+} + CO3^{2-} -> HCO3^{-} }} The dissolved species in the equations above, mostly bicarbonate, make up the carbonate alkalinity system, the dominant contributor to seawater alkalinity. :{{Chem2|Ca^{2+} + 2HCO3^{-} ↔ CaCO3 + CO2 + H2O}} Coccolithophores, a nearly ubiquitous group of phytoplankton that produce shells of calcium carbonate, are the dominant contributors to the carbonate pump. 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). 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. 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. They are consumed by microbes within hours or days of production and reside in the surface oceans, Semilabile molecules, much more difficult to consume, are able to reach depths of hundreds of meters below the surface before being metabolized. Refractory DOM largely comprises highly conjugated molecules like Polycyclic aromatic hydrocarbons or lignin. 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. {{NumBlk|:|\underset{carbon~dioxide}{6CO2} + \underset{water}{6H2O} ->[\overset{}\text{light energy}] \underset{carbohydrate}{C6H12O6} + \underset{oxygen}{6O2}|}}{{NumBlk|:|\underset{carbohydrate}{C6H12O6} + \underset{oxygen}{6O2} -> \underset{carbon~dioxide}{6CO2} + \underset{water}{6H2O} + heat|}} ==Inputs==

Inputs to the marine carbon cycle are numerous, but the primary contributions, on a net basis, come from the atmosphere and rivers. Carbon dioxide is absorbed from the atmosphere at the ocean's surface at an exchange rate which varies locally and with time but on average, the oceans have a net absorption of around 2.9 Pg (equivalent to 2.9 billion metric tonnes) of carbon from atmospheric CO2 per year. Because the solubility of carbon dioxide increases when temperature decreases, cold areas can contain more CO2 and still be in equilibrium with the atmosphere; In contrast, rising sea surface temperatures decrease the capacity of the oceans to take in carbon dioxide. The North Atlantic and Nordic oceans have the highest carbon uptake per unit area in the world, and in the North Atlantic deep convection transports approximately 197 Tg per year of non-refractory carbon to depth. The rate of CO2 absorption by the ocean has been increasing with time as atmospheric CO2 concentrations have increased due to anthropogenic emissions. However, the ocean carbon sink may be more sensitive to climate change than previously thought, and ocean warming and circulation changes due to climate change could result in the ocean absorbing less CO2 from the atmosphere in future than expected. Carbon dioxide exchange rates between ocean and atmosphere 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. This exchange rate can be approximated by Henry's law and can be calculated as S = kP, where the solubility (S) of the carbon dioxide gas is proportional to the amount of gas in the atmosphere, or its partial pressure. Rivers Rivers can also transport organic carbon to the ocean through weathering or erosion of aluminosilicate (equation 7) and carbonate rocks (equation 8) on land, :{{chem2|2 NaAlSi3O8 + 2 H2CO3 + 9 H2O -> 2 Na+ + 2 HCO3^{-} + 4 H4SiO4 + Al2Si2O5(OH)4}} :{{chem2|CaCO3 + H2CO3 -> Ca^{2+} + 2 HCO3^{-} }} or by the decomposition of life (equation 5, e.g. plant and soil material). The total carbon transport of rivers represents approximately 0.02% of the total carbon in the atmosphere. Though it seems small, over long time scales (1000 to 10,000 years) the carbon that enters rivers (and therefore does not enter the atmosphere) serves as a stabilizing feedback for greenhouse warming. == Outputs ==

The key outputs of the marine carbon system are particulate organic matter (POC) and calcium carbonate (PIC) preservation as well as reverse weathering. 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. Ocean floor surface sediments account for 1.75×1015 kg of carbon in the global carbon cycle. 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. 90% of organic carbon burial occurs in deposits of deltas and continental shelves and upper slopes; this is due partly to short exposure time because of a shorter distance to the seafloor and the composition of the organic matter that is already deposited in those environments. Organic carbon burial is also sensitive to climate patterns: the accumulation rate of organic carbon was 50% larger during the glacial maximum compared to interglacials. Degradation of POC also results in microbial methane production which is the main gas hydrate on the continental margins. Lignin and pollen are inherently resistant to degradation, and some studies show that inorganic matrices may also protect organic matter. Preservation rates of organic matter depend on other interdependent variables that vary nonlinearly in time and space. Although organic matter breakdown occurs rapidly in the presence of oxygen, microbes utilizing a variety of chemical species (via redox gradients) can degrade organic matter in anoxic sediments. This occurs because of preferential decomposition of labile molecules over refractile molecules. Burial can only take place if organic carbon arrives to the sea floor, making continental shelves and coastal margins the main storage of organic carbon from terrestrial and oceanic primary production. 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. Particulate organic carbon is buried in oceanic sediments, creating a pathway between a rapidly available carbon pool in the ocean to its storage for geological timescales. Once carbon is sequestered in the seafloor, it is considered blue carbon. Burial rates can be calculated as the difference between the rate at which organic matter sinks and the rate at which it decomposes. Calcium carbonate preservation The precipitation of calcium carbonate is important as it results in a loss of alkalinity as well as a release of CO2 (Equation 4), and therefore a change in the rate of preservation of calcium carbonate can alter the partial pressure of CO2 in Earth's atmosphere. 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. == Human impacts ==

Oceans take up around 25 – 31% of anthropogenic CO2. Because the Revelle factor increases with increasing CO2, a smaller fraction of the anthropogenic flux will be taken up by the ocean in the future. Current annual increase in atmospheric CO2 is approximately 4–5 gigatons of carbon, about 2–3ppm CO2 per year. 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. Overfishing and the plastic pollution of the oceans contribute to the degraded state of the world's biggest carbon sink. Ocean acidification Full article: Ocean acidification The pH of the oceans is declining due to uptake of atmospheric CO2. The rise in dissolved carbon dioxide reduces the availability of the carbonate ion, reducing CaCO3 saturation state, thus making it thermodynamically harder to make CaCO3 shell. Carbonate ions preferentially bind to hydrogen ions to form bicarbonate, and perhaps the most conspicuous, a structure built by organisms – the coral reefs. Due to the scale of the ocean and the fast response times of heterotrophic communities to increases in primary production, it is difficult to determine whether limiting-nutrient fertilization results in an increase in carbon export. Dams and reservoirs There are over 16 million dams in the world that alter carbon transport from rivers to oceans. Using data from the Global Reservoirs and Dams database, which contains approximately 7000 reservoirs that hold 77% of the total volume of water held back by dams (8000 km3), it is estimated that the delivery of carbon to the ocean has decreased by 13% since 1970 and is projected to reach 19% by 2030. The excess carbon contained in the reservoirs may emit an additional ~0.184 Gt of carbon to the atmosphere per year and an additional ~0.2 GtC will be buried in sediment. Prior to 2000, the Mississippi, the Niger, and the Ganges River basins account for 25 – 31% of all reservoir carbon burial. After 2000, the Paraná (home to 70 dams) and the Zambezi (home to the largest reservoir) River basins exceeded the burial by the Mississippi. 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. == See also ==