View source for Oceanic carbon cycle

{{Short description|Ocean/atmosphere carbon exchange process}}
[[File:OceanCarbonCycle.jpg|thumb|{{center|Oceanic carbon cycle ([[IPCC]])}}|200x200px]]

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 [[Seabed|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 [[Total inorganic carbon|inorganic]] carbon (carbon not associated with a living thing, such as carbon dioxide) and [[Organic matter|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]] (CO<sub>2</sub>) 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 whale]]s), and about 95% (~38,000 Gt C) is stored in the ocean, mostly as [[dissolved inorganic carbon]].<ref name=":0">{{Cite book|title=Biogeochemistry : an analysis of global change|last1=Schlesinger |first1=William H.|date=2013|publisher=Academic Press|last2=Bernhardt|first2=Emily S.|isbn=9780123858740|edition=3rd|location=Waltham, Mass.|oclc=827935936}}</ref><ref>{{Cite journal|last1=Falkowski|first1=P.|last2=Scholes|first2=R. J.|last3=Boyle|first3=E.|last4=Canadell|first4=J.|last5=Canfield|first5=D.|last6=Elser|first6=J.|last7=Gruber|first7=N.|last8=Hibbard|first8=K.|last9=Högberg|first9=P.|date=2000-10-13|title=The Global Carbon Cycle: A Test of Our Knowledge of Earth as a System|journal=Science|language=en|volume=290|issue=5490|pages=291–296|doi=10.1126/science.290.5490.291|issn=0036-8075|pmid=11030643|bibcode=2000Sci...290..291F}}</ref> The speciation{{Clarify|reason=|date=November 2020}} of dissolved inorganic carbon in the marine carbon cycle is a primary controller of [[Acid-base reaction|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 [[Life|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.<ref name=":0" /> Marine organisms link the carbon and oxygen cycles through processes such as [[photosynthesis]].<ref name=":0" /> The marine carbon cycle is also biologically tied to the [[Nitrogen cycle|nitrogen]] and [[Phosphorus cycle|phosphorus]] cycles by a near-constant [[Stoichiometry|stoichiometric]] ratio C:N:P of 106:16:1, also known as the [[Redfield ratio|Redfield Ketchum Richards (RKR) ratio]],<ref>{{Cite journal|last=Redfield|first=Alfred C.|date=1958|journal=American Scientist|volume=46|issue=3|pages=230A–221|jstor=27827150|title=The Biological Control of Chemical Factors in the Environment|pmid=24545739}}</ref> which states that organisms tend to take up nitrogen and phosphorus incorporating new organic carbon. Likewise, organic matter [[Microbiology of decomposition|decomposed by bacteria]] releases phosphorus and nitrogen.

Based on the publications of [[NASA]], World Meteorological Association, [[Intergovernmental Panel on Climate Change|IPCC]], and [[International Council for the Exploration of the Sea]], as well as scientists from [[National Oceanic and Atmospheric Administration|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.<ref>{{Cite web|url=https://earthobservatory.nasa.gov/Features/CarbonCycle/page5.php|title=The Carbon Cycle: Feature Articles|last=Holli|first=Riebeek|date=2011-06-16|website=earthobservatory.nasa.gov|language=en|access-date=2017-11-30}}</ref><ref>{{Cite news|url=https://public-old.wmo.int/en/media/news/new-report-published-%E2%80%9Cclimate-carbon-and-coral-reefs|archive-url=https://web.archive.org/web/20231218182438/https://public-old.wmo.int/en/media/news/new-report-published-%E2%80%9Cclimate-carbon-and-coral-reefs|url-status=dead|archive-date=December 18, 2023|title=New report published on "Climate, Carbon and Coral Reefs|date=2015-11-05|work=World Meteorological Organization|access-date=2017-11-30|language=en}}</ref><ref name=":8">{{Cite web|url=https://www.ipcc.ch/report/ar5/wg1/|title=Fifth Assessment Report – Climate Change 2013|website=www.ipcc.ch|access-date=2017-11-30}}</ref><ref>{{Cite web|url=https://www.pmel.noaa.gov/pubs/outstand/sabi2683/sabi2683.shtml|title=Sabine et al. – The Oceanic Sink for Anthropogenic CO2|website=www.pmel.noaa.gov|access-date=2017-11-30}}</ref> Before the Industrial Revolution, the ocean was a net source of CO<sub>2</sub> to the atmosphere whereas now the majority of the carbon that enters the ocean comes from atmospheric carbon dioxide (CO<sub>2</sub>).<ref name="Royal">{{cite book|url=http://oceanrep.geomar.de/7878/1/965_Raven_2005_OceanAcidificationDueToIncreasing_Monogr_pubid13120.pdf|title=Ocean acidification due to increasing atmospheric carbon dioxide|date=2005|publisher=The Royal Society|isbn=0-85403-617-2|location=London|access-date=November 17, 2017}}</ref> 

In recent decades, the ocean has acted as a sink for anthropogenic CO<sub>2</sub>, absorbing around a quarter of the CO<sub>2</sub> produced by humans through the burning of [[Fossil fuel|fossil fuels]] and land use changes.<ref>{{Cite journal |last1=Friedlingstein |first1=Pierre |last2=O'Sullivan |first2=Michael |last3=Jones |first3=Matthew W. |last4=Andrew |first4=Robbie M. |last5=Bakker |first5=Dorothee C. E. |last6=Hauck |first6=Judith |last7=Landschützer |first7=Peter |last8=Le Quéré |first8=Corinne |last9=Luijkx |first9=Ingrid T. |last10=Peters |first10=Glen P. |last11=Peters |first11=Wouter |last12=Pongratz |first12=Julia |last13=Schwingshackl |first13=Clemens |last14=Sitch |first14=Stephen |last15=Canadell |first15=Josep G. |date=2023-12-05 |title=Global Carbon Budget 2023 |url=https://essd.copernicus.org/articles/15/5301/2023/ |journal=Earth System Science Data |language=English |volume=15 |issue=12 |pages=5301–5369 |doi=10.5194/essd-15-5301-2023 |doi-access=free |issn=1866-3508|hdl=20.500.11850/665569 |hdl-access=free }}</ref> By doing so, the ocean has acted as a buffer, somewhat slowing the rise in atmospheric CO<sub>2</sub> levels. However, this absorption of anthropogenic CO<sub>2</sub> has also caused [[Ocean acidification|acidification of the oceans]].<ref name="Royal" /><ref name="Zeebe" /> [[Climate change]], a result of this excess CO<sub>2</sub> in the atmosphere, has increased the temperature of the ocean and atmosphere.<ref>{{Cite web|url=https://www.ipcc.ch/report/ar5/wg1/|title=Fifth Assessment Report – Climate Change 2013|website=www.ipcc.ch|access-date=2017-11-26}}</ref> The slowed rate of global warming occurring from 2000–2010<ref name="Knight">{{cite journal|last1=Knight|first1=J|date=2009|title=Global oceans: Do global temperature trends over the last decade falsify climate predictions?|journal=Bulletin of the American Meteorological Society|volume=90|page=S56–S57}}</ref> may be attributed to an observed increase in upper [[ocean heat content]].<ref>{{Cite web|url=https://www.nodc.noaa.gov/OC5/3M_HEAT_CONTENT/|title=Global ocean heat and salt content|publisher=US Department of Commerce, NOAA National Centers for Environmental Information|website=www.nodc.noaa.gov|language=en|access-date=2017-11-26}}</ref><ref name="Guemas">{{cite journal|last1=Guemas|first1=V|last2=Doblas-Reyes|first2=F|last3=Andreu-Burillo|first3=I|last4=Asif|first4=M|date=2013|title=Retrospective prediction of the global warming slowdown in the past decade|journal=Nature Climate Change|volume=3|issue=7|pages=649–653|doi=10.1038/nclimate1863|bibcode=2013NatCC...3..649G|url=https://zenodo.org/record/3425796|access-date=2019-12-10|archive-date=2022-11-25|archive-url=https://web.archive.org/web/20221125232107/https://zenodo.org/record/3425796|url-status=dead}}</ref>

{{Carbon cycle|By regions}}
{{TOC limit|3}}
== Marine carbon ==
[[File:Marine carbon cycle.png|thumb|upright=1.8|Carbon is separated into four distinct pools based on whether it is organic/inorganic and whether it is dissolved/particulate. The processes associated with each arrow describe the transformation associated with the transfer of carbon from one reservoir to another.]]

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 – [[protein]]s, [[lipid]]s, [[carbohydrate]]s, and [[nucleic acid]]s. Inorganic carbon is found primarily in simple compounds such as carbon dioxide, carbonic acid, bicarbonate, and carbonate (CO<sub>2</sub>, H<sub>2</sub>CO<sub>3</sub>, HCO<sub>3</sub><sup>−</sup>, CO<sub>3</sub><sup>2−</sup> 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. [[Total inorganic carbon|Dissolved inorganic carbon]] (DIC) is made up of bicarbonate (HCO<sub>3</sub><sup>−</sup>), carbonate (CO<sub>3</sub><sup>2−</sup>) and carbon dioxide (including both dissolved CO<sub>2</sub> and carbonic acid H<sub>2</sub>CO<sub>3</sub>). DIC can be converted to [[particulate inorganic carbon]] (PIC) through precipitation of CaCO<sub>3</sub> (biologically or abiotically). DIC can also be converted to particulate organic carbon (POC) through photosynthesis and [[Chemotroph|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 CaCO<sub>3</sub> that makes up shells of various marine organisms, but can also form in [[whiting event]]s. Marine fish also excrete calcium carbonate during [[osmoregulation]].<ref>{{Cite journal|last1=Wilson|first1=R. W.|last2=Millero|first2=F. J.|last3=Taylor|first3=J. R.|last4=Walsh|first4=P. J.|last5=Christensen|first5=V.|last6=Jennings|first6=S.|last7=Grosell|first7=M.|date=2009-01-16|title=Contribution of Fish to the Marine Inorganic Carbon Cycle|journal=Science|language=en|volume=323|issue=5912|pages=359–362|doi=10.1126/science.1157972|issn=0036-8075|pmid=19150840|bibcode=2009Sci...323..359W|s2cid=36321414}}</ref>

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.<ref name=":1">{{Cite book|title=Chemical Oceanography and the Marine Carbon Cycle|last=Emerson|first=Steven|publisher=Cambridge University Press|year=2008|isbn=978-0-521-83313-4|location=United Kingdom}}</ref>

=== 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&nbsp;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 [[Exudate|exudation]] by [[phytoplankton]], for example. POC is generally converted to DIC through heterotrophy and respiration.

== Marine carbon pumps ==
=== Solubility pump ===
''Full article: [[Solubility pump]]''
[[File:Dissociation of carbon and henry's law in the oceanic carbon pump nov 2017.jpg|thumb|right|300px|Dissociation of carbon dioxide following Henry's Law]]
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.<ref name=":1" /> Aqueous CO<sub>2</sub>, [[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.<ref name="GlobalCarbonCycle">{{Cite journal | last1 = Falkowski | first1 = P. | last2 = Scholes | first2 = R. J. | last3 = Boyle | first3 = E. | last4 = Canadell | first4 = J. | last5 = Canfield | first5 = D. | last6 = Elser | first6 = J. | last7 = Gruber | first7 = N. | last8 = Hibbard | first8 = K. | last9 = Högberg | first9 = P. | last10 = Linder | first10 = S. | last11 = MacKenzie | first11 = F. T. | last12 = Moore III| first12 = B. | last13 = Pedersen | first13 = T. | last14 = Rosenthal | first14 = Y. | last15 = Seitzinger | first15 = S. | last16 = Smetacek | first16 = V. | last17 = Steffen | first17 = W. | title = The Global Carbon Cycle: A Test of Our Knowledge of Earth as a System | doi = 10.1126/science.290.5490.291 | journal = Science | volume = 290 | issue = 5490 | pages = 291–296 | year = 2000 | pmid =  11030643| bibcode = 2000Sci...290..291F }}</ref> The chemical equations below show the reactions that CO<sub>2</sub> undergoes after it enters the ocean and transforms into its aqueous form.

[[File:Annual mean sea surface dissolved inorganic carbon for the 1990s (GLODAP).png|thumb|right|300px|Sea surface [[Total inorganic carbon|dissolved inorganic carbon]]

First, carbon dioxide reacts with water to form carbonic acid. concentration in the 1990s (from the [[Global Ocean Data Analysis Project|GLODAP]] [[climatology]])]]{{NumBlk|:|<chem>CO2(aq) + H2O -> H2CO3</chem>|{{EquationRef|1}}}}Carbonic acid rapidly dissociates into free [[hydrogen ion]] (technically, [[hydronium]]) and bicarbonate.{{NumBlk|:|<chem>H2CO3 -> H+ +  HCO3^-</chem>|{{EquationRef|2}}}}The free hydrogen ion meets carbonate, already present in the water from the dissolution of [[Calcium carbonate|CaCO<sub>3</sub>]], and reacts to form more bicarbonate ion.{{NumBlk|:|<chem>H+ + CO3^2- -> HCO3^-</chem>|{{EquationRef|3}}}}The dissolved species in the equations above, mostly bicarbonate, make up the carbonate alkalinity system, the dominant contributor to seawater alkalinity.<ref name="Zeebe" />

=== 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]], CaCO<sub>3</sub>). This CaCO<sub>3</sub> is what forms hard body parts like [[Seashell|shells]].<ref name=":1" /> The formation of these shells increases atmospheric CO<sub>2</sub> due to the production of CaCO<sub>3</sub><ref name="Zeebe" /> in the following reaction with simplified stoichiometry:<ref>{{Cite web|url=https://aslo.org/page/limnology-%26-oceanography-e-books|title=ASLO : Limnology & Oceanography: e-Books|website=aslo.org|access-date=2017-11-28|archive-date=2017-12-07|archive-url=https://web.archive.org/web/20171207084449/https://aslo.org/page/limnology-%26-oceanography-e-books|url-status=dead}}</ref>{{NumBlk|:|<chem>Ca^2+ + 2HCO3^- <=> CaCO3 + CO2 + H2O</chem><ref name=":5">{{Cite journal|last1=Smith|first1=S. V.|last2=Key|first2=G. S.|date=1975-05-01|title=Carbon dioxide and metabolism in marine environments1|journal=Limnology and Oceanography|language=en|volume=20|issue=3|pages=493–495|doi=10.4319/lo.1975.20.3.0493|issn=1939-5590|bibcode=1975LimOc..20..493S|doi-access=free}}</ref>|{{EquationRef|4}}}}[[Coccolithophore]]s, a nearly ubiquitous group of phytoplankton that produce shells of calcium carbonate, are the dominant contributors to the carbonate pump.<ref name=":1" /> 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 CaCO<sub>3</sub>.<ref>{{Cite book|title=Coccolithophores|last1=Rost|first1=Björn|last2=Riebesell|first2=Ulf|chapter=Coccolithophores and the biological pump: Responses to environmental changes |date=2004|publisher=Springer, Berlin, Heidelberg|isbn=9783642060168|pages=99–125|language=en|doi=10.1007/978-3-662-06278-4_5|citeseerx = 10.1.1.455.2864}}</ref> The air-sea CO<sub>2</sub> flux induced by a marine biological [[Community (ecology)|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).<ref name=":5" /> The carbonate pump acts as a negative feedback on CO<sub>2</sub> 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 [[autotroph]]y<ref name=":1" /> that then sinks and is, in part or whole, digested by heterotrophs.<ref name="Kim">{{cite journal|last1=Kim|first1=S|last2=Kramer|first2=R|last3=Hatcher|first3=P|date=2003|title=Graphical method for analysis of ultrahigh-resolution broadband mass spectra of natural organic matter, the van Krevelen diagram|journal=Analytical Chemistry|volume=75|issue=20|pages=5336–5344|doi=10.1021/AC034415P|pmid=14710810}}</ref> Particulate organic carbon can be classified, based on how easily organisms can break them down for food, as [[Lability|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.<ref name=":2">{{Cite journal|last1=Brophy|first1=Jennifer E.|last2=Carlson|first2=David J.|title=Production of biologically refractory dissolved organic carbon by natural seawater microbial populations|journal=Deep Sea Research Part A. Oceanographic Research Papers|volume=36|issue=4|pages=497–507|doi=10.1016/0198-0149(89)90002-2|year=1989|bibcode=1989DSRA...36..497B}}</ref><ref name="Moran">{{cite journal|last1=Moran|first1=M|last2=Kujawinski|first2=E|last3=Stubbins|first3=A|last4=Fatland|first4=R|last5=Aluwihare|first5=L|last6=Buchan|first6=A|last7=Crump|first7=B|last8=Dorrestein|first8=P|last9=Dyhrman|first9=S|date=2016|title=Deciphering ocean carbon in a changing world|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=113|issue=12|pages=3143–3151|doi=10.1073/pnas.1514645113|pmid=26951682|last10=Hess|first10=N|last11=Howe|first11=B|last12=Longnecker|first12=K|last13=Medeiros|first13=P|last14=Niggemann|first14=J|last15=Obernosterer|first15=I|last16=Repeta|first16=D|last17=Waldbauer|first17=J|pmc=4812754|bibcode=2016PNAS..113.3143M|doi-access=free}}</ref> Labile molecules are present at low concentrations outside of cells (in the [[Molar concentration|picomolar]] range) and have half-lives of only minutes when free in the ocean.<ref name="Azam">{{cite journal|last1=Azam|first1=F|last2=Malfatti|first2=F|date=2007|title=Microbial structuring of marine ecosystems|journal=Nature Reviews Microbiology|volume=5|issue=10|pages=782–791|doi=10.1038/nrmicro1747|pmid=17853906|s2cid=10055219}}</ref> They are consumed by microbes within hours or days of production and reside in the surface oceans,<ref name="Moran" /> where they contribute a majority of the labile carbon flux.<ref name="Moran2013">{{cite journal|last1=Moran|first1=X|last2=Ducklow|first2=H|last3=Erickson|first3=M|date=2013|title=Carbon fluxes through estuarine bacteria reflect coupling with phytoplankton|journal=Marine Ecology Progress Series|volume=489|pages=75–85|doi=10.3354/meps10428|bibcode=2013MEPS..489...75M|doi-access=free}}</ref> Semilabile molecules, much more difficult to consume, are able to reach depths of hundreds of meters below the surface before being metabolized.<ref name="Hansell1998">{{cite journal|last1=Hansell|first1=D|last2=Carlson|first2=C|date=1998|title=Net community production of dissolved organic carbon|journal=Global Biogeochemical Cycles|volume=12|issue=3|pages=443–453|doi=10.1029/98gb01928|bibcode=1998GBioC..12..443H|doi-access=free}}</ref> Refractory DOM largely comprises highly [[Aromaticity|conjugated]] molecules like [[Polycyclic aromatic hydrocarbon]]s or [[lignin]].<ref name=":2" /> Refractory DOM can reach depths greater than 1000 m and circulates through the oceans over thousands of years.<ref name="Follett">{{cite journal|last1=Follett|first1=C|last2=Repeta|first2=D|last3=Rothman|first3=D|last4=Xu|first4=L|last5=Santinelli|first5=C|date=2014|title=Hidden cycle of dissolved organic carbon in the deep ocean|journal=Proceedings of the National Academy of Sciences of the United States of America|volume=111|issue=47|pages=16706–16711|doi=10.1073/pnas.1407445111|pmid=25385632|pmc=4250131|bibcode=2014PNAS..11116706F|doi-access=free}}</ref><ref name="Moran" /><ref name="Hansell">{{cite journal|last1=Hansell|first1=D|date=2013|title=Recalcitrant dissolved organic carbon fractions|journal=Annual Review of Marine Science|volume=5|issue=1|pages=421–445|doi=10.1146/annurev-marine-120710-100757|pmid=22881353}}</ref> Over the course of a year, approximately 20 gigatons of photosynthetically-fixed labile and semilabile carbon is taken up by [[heterotroph]]s, whereas fewer than 0.2 gigatons of refractory carbon is consumed.<ref name="Moran" /> Marine [[dissolved organic matter]] (DOM) can store as much carbon as the current atmospheric CO<sub>2</sub> supply,<ref name="Hansell" /> but industrial processes are altering the balance of this cycle.<ref name="Doney">{{cite journal|last1=Doney|first1=Scott|author-link1=Scott Doney|last2=Ruckelshaus|first2=Mary|last3=Duffy|first3=Emmett|last4=Barry|first4=James|last5=Chan|first5=Francis|last6=English|first6=Chad|last7=Galindo|first7=Heather|last8=Grebmeier|first8=Jacqueline|author-link8=Jacqueline M. Grebmeier|last9=Hollowed|first9=Anne|date=2012|title=Climate change impacts on marine ecosystems|journal=Annual Review of Marine Science|volume=4|issue=1|pages=11–37|doi=10.1146/annurev-marine-041911-111611|pmid=22457967|last10=Knowlton|first10=Nancy|last11=Polovina|first11=Jeffrey|last12=Rabalais|first12=Nancy|last13=Sydeman|first13=William|last14=Talley|first14=Lynne|bibcode=2012ARMS....4...11D|s2cid=35349779}}</ref>

{{NumBlk|:|<chem>\underset{carbon~dioxide}{6CO2} + \underset{water}{6H2O}  ->[light~energy]  \underset{carbohydrate}{C6H12O6} + \underset{oxygen}{6O2}</chem>|{{EquationRef|5}}}}{{NumBlk|:|<chem>\underset{carbohydrate}{C6H12O6} + \underset{oxygen}{6O2} -> \underset{carbon~dioxide}{6CO2} + \underset{water}{6H2O} + heat</chem>|{{EquationRef|6}}}}

==Inputs==
Inputs to the marine carbon cycle are numerous, but the primary contributions, on a net basis, come from the atmosphere and rivers.<ref name=":0" /> [[Hydrothermal vent]]s generally supply carbon equal to the amount they consume.<ref name=":1" />

=== Atmosphere ===
{{See also|Carbon sequestration#Sequestration in oceans}}
[[File:CO2 flux IPCC.jpg|thumb|upright=1.8| {{center|Global air-sea CO2 flux – IPCC 2007}}]]

[[File:Carbon cycle processes in high-latitude shelf seas.jpg|thumb|upright=1.8| {{center|Carbon cycle processes in high-latitude [[shelf sea]]s{{hsp}}<ref name=Capelle2020>{{cite journal |doi = 10.1016/j.pocean.2020.102319|title = Effect of terrestrial organic matter on ocean acidification and CO2 flux in an Arctic shelf sea|year = 2020|last1 = Capelle|first1 = David W.|last2 = Kuzyk|first2 = Zou Zou A.|last3 = Papakyriakou|first3 = Tim|last4 = Guéguen|first4 = Céline|last5 = Miller|first5 = Lisa A.|last6 = MacDonald|first6 = Robie W.|journal = Progress in Oceanography|volume = 185|page = 102319|bibcode = 2020PrOce.18502319C|doi-access = free|hdl = 1993/34767|hdl-access = free}} [[File:CC-BY icon.svg|50px]] Material was copied from this source, which is available under a [https://creativecommons.org/licenses/by/4.0/ Creative Commons Attribution 4.0 International License].</ref>}}]]

Before the [[Industrial Revolution]], the ocean was a source of CO<sub>2</sub> to the [[atmosphere]]<ref name="Royal" /> balancing the impact of rock weathering and terrestrial particulate organic carbon; now it has become a sink for the excess atmospheric CO<sub>2</sub>.<ref name="SINKS">{{cite journal|last1=Raven|first1=J.A.|last2=Falkowskli|first2=P.G.|title=Oceanic sinks for atmospheric CO2 |journal=Global Biogeochemical Cycles|date=2009|volume=23|issue=1|pages=GB1005|doi=10.1029/2008gb003349|bibcode=2009GBioC..23.1005G|hdl=1912/3415|citeseerx=10.1.1.715.9875|s2cid=17471174 |url=http://www.vliz.be/imisdocs/publications/291174.pdf}}</ref> Carbon dioxide is absorbed from the atmosphere at the ocean's surface at an exchange rate which varies locally and with time<ref name="Takahashi2002">{{cite journal|last1=Takahashi|first1=T|last2=Sutherland|first2=S|last3=Sweeney|first3=C|last4=Poisson|first4=A|last5=Metzl|first5=N|title= Global sea–air CO2 flux based on climatological surface ocean pCO2, and seasonal biological and temperature effects|date=2002|volume=49|issue=9–10|pages=1601–1622|doi=10.1016/S0967-0645(02)00003-6|journal=Deep Sea Research Part II: Topical Studies in Oceanography|bibcode=2002DSRII..49.1601T}}</ref> 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 CO<sub>2</sub> per year.<ref>{{Cite journal |last1=Friedlingstein |first1=Pierre |last2=O'Sullivan |first2=Michael |last3=Jones |first3=Matthew W. |last4=Andrew |first4=Robbie M. |last5=Bakker |first5=Dorothee C. E. |last6=Hauck |first6=Judith |last7=Landschützer |first7=Peter |last8=Le Quéré |first8=Corinne |last9=Luijkx |first9=Ingrid T. |last10=Peters |first10=Glen P. |last11=Peters |first11=Wouter |last12=Pongratz |first12=Julia |last13=Schwingshackl |first13=Clemens |last14=Sitch |first14=Stephen |last15=Canadell |first15=Josep G. |date=2023-12-05 |title=Global Carbon Budget 2023 |url=https://essd.copernicus.org/articles/15/5301/2023/ |journal=Earth System Science Data |language=English |volume=15 |issue=12 |pages=5301–5369 |doi=10.5194/essd-15-5301-2023 |doi-access=free |issn=1866-3508|hdl=20.500.11850/665569 |hdl-access=free }}</ref> Because the solubility of carbon dioxide increases when temperature decreases, cold areas can contain more CO<sub>2</sub> and still be in equilibrium with the atmosphere; In contrast, rising [[sea surface temperature]]s decrease the capacity of the oceans to take in carbon dioxide.<ref name="Revelle" /><ref name="Zeebe">{{cite book|last1=Zeebe|first1=R|last2=Wolf-Gladrow|first2=D|title=CO2 in seawater: Equilibrium, Kinetics, Isotopes|date=2001|publisher=Elsevier Science|pages=360}}</ref> The North [[Atlantic Ocean|Atlantic]] and [[Nordic Seas|Nordic]] oceans have the highest carbon uptake per unit area in the world,<ref name="Takahashi">{{cite journal|display-authors=3 |last1=Takahashi |first1=T |last2=Sutherland |first2=S |last3=Wanninkhof |first3=R |last4=Sweeney |first4=C |last5=Feely |first5=RA |last6=Chipman |first6=DW |last7=Hales |first7=Burke |last8=Friedrich |first8=Gernot |last9=Chavez |first9=Francisco |last10=Sabine |first10=Christopher |last11=Watson |first11=Andrew |last12=Bakker |first12=Dorothee CE |last13=Schuster |first13=Ute |last14=Metzl |first14=Nicolas |last15=Yoshikawa-Inoue |first15=Hisayuki |last16=Ishii |first16=Masao |last17=Midorikawa |first17=Takashi |last18=Nojiri |first18=Yukihiro |last19=de Baar |first19=Hein J.W. |title=Climatological mean and decadal change in surface ocean pCO2, and net sea-air CO2 flux over the global oceans|journal=Deep Sea Research Part II: Topical Studies in Oceanography|date=2009|volume=56|issue=8–10|pages=554–577|doi=10.1016/j.dsr2.2008.12.009|bibcode=2009DSRII..56..554T}}</ref> and in the North Atlantic deep convection transports approximately 197 Tg per year of non-refractory carbon to depth.<ref name="Fontal">{{cite journal|last1=Fontela|first1=M|last2=Garcia-Ibanez|first2=M|last3=Hansell|first3=D|last4=Mercier|first4=H|last5=Perez|first5=F|title=Dissolved organic carbon in the North Atlantic meridional overturning circulation|journal=Nature|date=2016|volume=6|page=26931|doi=10.1038/srep26931|pmid=27240625|pmc=4886255|bibcode=2016NatSR...626931F}}</ref>

The rate of CO<sub>2</sub> absorption by the ocean has been increasing with time as atmospheric CO<sub>2</sub> 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 CO<sub>2</sub> from the atmosphere in future than expected.<ref>{{Cite journal |last1=Gruber |first1=Nicolas |last2=Bakker |first2=Dorothee C. E. |last3=DeVries |first3=Tim |last4=Gregor |first4=Luke |last5=Hauck |first5=Judith |last6=Landschützer |first6=Peter |last7=McKinley |first7=Galen A. |last8=Müller |first8=Jens Daniel |date=24 January 2023 |title=Trends and variability in the ocean carbon sink |url=https://www.nature.com/articles/s43017-022-00381-x |journal=Nature Reviews Earth & Environment |language=en |volume=4 |issue=2 |pages=119–134 |doi=10.1038/s43017-022-00381-x |bibcode=2023NRvEE...4..119G |issn=2662-138X|hdl=20.500.11850/595538 |hdl-access=free }}</ref>

====Carbon dioxide exchange rates between ocean and atmosphere====
{{See also|Carbon dioxide in Earth's atmosphere}}
Ocean-atmospheric exchanges rates of CO<sub>2</sub> depend on the concentration of carbon dioxide already present in both the atmosphere and the ocean, temperature, salinity, and wind speed.<ref name="CO2calc">{{cite book|last1=Robbins|first1=L.L.|last2=Hansen|first2=M.E.|last3=Kleypas|first3=J.A.|last4=Meylan|first4=S.C.|title=CO2calc—A user-friendly seawater carbon calculator for Windows, Mac OS X, and iOS (iPhone)|date=2010|publisher=U.S. Geological Survey Open-File Report 2010-1280|pages=16}}</ref> 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]].<ref name=":0" />

====Revelle factor====
Since the oceanic intake of carbon dioxide is limited, CO<sub>2</sub> influx can also be described by the [[Revelle factor]].<ref name="Revelle">{{cite journal|last1=Revelle|first1=R|last2=Suess|first2=H|title=Carbon dioxide exchange between atmosphere and ocean and the question of an increase of atmospheric CO2 during the past decades|journal=Tellus|date=1957|volume=9|issue=1|pages=18–27|doi=10.1111/j.2153-3490.1957.tb01849.x|bibcode=1957Tell....9...18R}}</ref><ref name="Zeebe"/> The Revelle Factor is a ratio of the change of carbon dioxide to the change in dissolved inorganic carbon, which serves as an indicator of carbon dioxide dissolution in the mixed layer considering the solubility pump. The Revelle Factor is an expression to characterize the [[Thermal efficiency|thermodynamic efficiency]] of the DIC pool to absorb CO<sub>2</sub> into bicarbonate. The lower the Revelle factor, the higher the capacity for ocean water to take in carbon dioxide. While Revelle calculated a factor of around 10 in his day, in a 2004 study data showed a Revelle factor ranging from approximately 9 in low-latitude tropical regions to 15 in the southern ocean near Antarctica.<ref name="RF">{{cite journal|last1=Sabine|first1=C.L.|last2=Feely|first2=R.A.|last3=Gruber|first3=N|last4=Key|first4=R.M.|last5=Lee|first5=K|title= The oceanic sink for anthropogenic CO2|date=2004|volume=305|issue=5682|pages=367–371|doi=10.1126/science.1097403|pmid=15256665|journal=Science|bibcode=2004Sci...305..367S|hdl=10261/52596|s2cid=5607281|url=http://oceanrep.geomar.de/46251/1/1193.full.pdf|hdl-access=free}}</ref>

=== Rivers ===

[[River]]s can also transport organic carbon to the ocean through [[weathering]] or erosion of [[aluminosilicate]] (equation 7) and carbonate rocks (equation 8) on land,

{{NumBlk|:|<chem>2 NaAlSi3O8 + 2 H2CO3 + 9 H2O  ->  2 Na+ + 2 HCO3^- + 4 H4SiO4 + Al2Si2O5(OH)4</chem>|{{EquationRef|7}}}}

{{NumBlk|:|<chem>CaCO3 + H2CO3  ->  Ca^2+ + 2 HCO3^-</chem>|{{EquationRef|8}}}}

or by the decomposition of life (equation 5, e.g. plant and soil material).<ref name=":0" /> Rivers contribute roughly equal amounts (~0.4 GtC/yr) of DIC and DOC to the oceans.<ref name=":0" /> It is estimated that approximately 0.8 GtC (DIC + DOC) is transported annually from the rivers to the ocean.<ref name=":0" /> The rivers that flow into [[Chesapeake Bay]] ([[Susquehanna River|Susquehanna]], [[Potomac River|Potomac]], and [[James River|James]] rivers) input approximately 0.004 Gt (6.5 x 10<sup>10</sup> moles) DIC per year.<ref name="Waldbusser">{{cite journal|last1=Waldbusser|first1=G|last2=Powell|first2=E|last3=Mann|first3=R|title=Ecosystem effects of shell aggregations and cycling in coastal waters: an example of Chesapeake Bay oyster reefs|journal=Ecology|date=2013|volume=94|issue=4|pages=895–903|doi=10.1890/12-1179.1|bibcode=2013Ecol...94..895W|url=https://scholarworks.wm.edu/cgi/viewcontent.cgi?article=2673&context=vimsarticles}}</ref> The total carbon transport of rivers represents approximately 0.02% of the total carbon in the atmosphere.<ref name="NATerosion">{{cite journal|last1=Galy|first1=Valier|last2=Peucker-Ehrenbrink|first2=Bernhard|last3=Eglinton|first3=Timothy|title=Global carbon export from the terrestrial biosphere controlled by erosion|journal=Nature|date=2015|volume=521|issue=7551|pages=204–207|doi=10.1038/nature14400|pmid=25971513|bibcode=2015Natur.521..204G|s2cid=205243485}}</ref> 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 effect|greenhouse]] warming.<ref>{{Cite journal|last=Velbel|first=Michael Anthony|date=1993-12-01|title=Temperature dependence of silicate weathering in nature: How strong a negative feedback on long-term accumulation of atmospheric CO2 and global greenhouse warming?|journal=Geology|language=en|volume=21|issue=12|pages=1059–1062|doi=10.1130/0091-7613(1993)021<1059:TDOSWI>2.3.CO;2|issn=0091-7613|bibcode=1993Geo....21.1059V|s2cid=747129}}</ref>

== Outputs ==
[[File:Fate of Buried Organic Matter.jpg|thumb|right|400px|Fate of buried organic matter]]
The key outputs of the marine carbon system are particulate organic matter (POC) and calcium carbonate (PIC) preservation as well as [[reverse weathering]].<ref name=":0" /> While there are regions with local loss of CO<sub>2</sub> to the atmosphere and hydrothermal processes, a net loss in the cycle does not occur.<ref name=":1" />

=== Organic matter preservation ===
Sedimentation is a long-term sink for carbon in the ocean, as well as the largest loss of carbon from the oceanic system.<ref name="Paleo" /> Deep [[marine sediment]]s and [[geologic formation]]s are important since they provide a thorough record of life on Earth and an important source of fossil fuel.<ref name="Paleo">{{cite journal|last1=Emerson|first1=S|last2=Hedges|first2=J|date=October 1988|title=Processes Controlling the Organic Carbon Content of Open Ocean Sediments|journal=Paleoceanography|volume=3|issue=5|pages=621–634|doi=10.1029/pa003i005p00621|bibcode=1988PalOc...3..621E}}</ref> 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 [[sediment]]s account for 1.75x10<sup>15</sup> kg of carbon in the global carbon cycle.<ref name="IPCC">{{cite book |last1= Ciais|first1= Philippe|last2= Sabine |first2=Christopher | title= Climate Change 2013: the physical science basis. Contribution of Working Group I to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change.|url= http://pubman.mpdl.mpg.de/pubman/item/escidoc:2058766/component/escidoc:2058768/WG1AR5_Chapter06_FINAL.pdf |publisher= Cambridge University Press|date= 2014 |pages= 465–470 }}</ref> At most, 4% of the particulate organic carbon from the [[Photic zone|euphotic]] zone in the Pacific Ocean, where light-powered [[primary production]] occurs, is buried in marine sediments.<ref name="Paleo" /> It is then implied that since there is a higher input of organic matter to the ocean than what is being buried, a large portion of it is used up or consumed within.

==== Fate of sinking organic carbon ====
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.<ref>{{Cite book|last1=Fleming|first1=R.H.|last2=Revelle|first2=R.|date=1939|chapter=Physical processes in the oceans|pages=48–141|editor=Trask, P.D.|title=Recent Marine Sediments|location=Tulsa|publisher=[[American Association of Petroleum Geologists]]}}</ref> 90% of organic carbon burial occurs in deposits of [[River delta|deltas]] and [[Continental shelf|continental shelves]] and upper slopes;<ref>{{Cite journal|last=Berner|first=Robert A.|date=1989-01-01|title=Biogeochemical cycles of carbon and sulfur and their effect on atmospheric oxygen over phanerozoic time|journal=Palaeogeography, Palaeoclimatology, Palaeoecology|series=The Long Term Stability of the Earth System|volume=75|issue=1|pages=97–122|doi=10.1016/0031-0182(89)90186-7|bibcode=1989PPP....75...97B}}</ref> 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.<ref>{{Cite journal|last=Henrichs|first=Susan|date=1992|title=Early diagenesis of organic matter in marine sediments: progress and perplexity|journal=Marine Chemistry|volume=39|issue=1–3|pages=119–149|doi=10.1016/0304-4203(92)90098-U|bibcode=1992MarCh..39..119H }}</ref> Organic carbon burial is also sensitive to climate patterns: the accumulation rate of organic carbon was 50% larger during the [[Last Glacial Maximum|glacial maximum]] compared to [[interglacial]]s.<ref name=Globalpulses />

====Degradation====
{{biogeochemical cycle sidebar|carbon}}

POC is decomposed by a series of microbe-driven processes, such as [[methanogenesis]] and sulfate reduction, before burial in the seafloor.<ref name="Claypool">{{cite book|last1=Claypool|first1=G.E.|last2=Kaplan|first2=I.R.|date=1974|title=Natural Gases in Marine Sediments|publisher=Plenum Press|pages=99–139}}</ref><ref name="Rutherford">{{cite journal|last1=D'Hondt|first1=S|last2=Rutherford|first2=S|last3=Spivack|first3=A.J.|date=2002|title=Metabolic activity of subsurface life in deep-sea sediments|journal=Science|volume=295|issue=5562|pages=2067–2070|doi=10.1126/science.1064878|pmid=11896277|bibcode=2002Sci...295.2067D|s2cid=26979705}}</ref> Degradation of POC also results in microbial methane production which is the main gas hydrate on the continental margins.<ref>{{Cite book|last1=Kvenvolden|first1=K.A.|last2=Lorenson|first2=T.D.|date=2001|title=Natural Gas Hydrates: Occurrence, Distribution, and Detection|pages=3–18 |isbn=978-0-875-90982-0|editor1=Charles K. Paull |editor2=William P. Dillon|publisher=American Geophysical Union|series=Geophysical Monograph Series |volume=124}}</ref> Lignin and pollen are inherently resistant to [[Biodegradation|degradation]], and some studies show that inorganic matrices may also protect organic matter.<ref>{{Cite journal|last1=Huguet|first1=Carme|last2=de Lange|first2=Gert J.|last3=Gustafsson|first3=Örjan|last4=Middelburg|first4=Jack J.|last5=Sinninghe Damsté|first5=Jaap S.|last6=Schouten|first6=Stefan|date=2008-12-15|title=Selective preservation of soil organic matter in oxidized marine sediments (Madeira Abyssal Plain)|journal=Geochimica et Cosmochimica Acta|volume=72|issue=24|pages=6061–6068|doi=10.1016/j.gca.2008.09.021|bibcode=2008GeCoA..72.6061H}}</ref> Preservation rates of organic matter depend on other interdependent variables that vary nonlinearly in time and space.<ref name=":6">{{Cite journal|last1=Hedges|first1=John I.|last2=Hu|first2=Feng Sheng|last3=Devol|first3=Allan H.|last4=Hartnett|first4=Hilairy E.|author-link4=Hilairy Hartnett|last5=Tsamakis|first5=Elizabeth|last6=Keil|first6=Richard G.|date=1999|title=Sedimentary organic matter preservation: A test for selective degradation under oxic conditions|url=https://asu.pure.elsevier.com/en/publications/sedimentary-organic-matter-preservation-a-test-for-selective-degr|journal=American Journal of Science|language=en|volume=299|issue=7–9|pages=529|issn=0002-9599|bibcode=1999AmJS..299..529H|doi=10.2475/ajs.299.7-9.529}}</ref> 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 waters|anoxic]] sediments.<ref name=":6" /> The burial depth at which degradation halts depends upon the sedimentation rate, the relative abundance of organic matter in the sediment, the type of organic matter being buried, and innumerable other variables.<ref name=":6" /> While decomposition of organic matter can occur in anoxic sediments when bacteria use oxidants other than oxygen ([[nitrate]], [[sulfate]], [[Ferric|Fe<sup>3+</sup>]]), decomposition tends to end short of complete [[Mineralization (soil science)|mineralization]].<ref name=":7">{{Cite journal|last1=Kristensen|first1=Erik|last2=Ahmed|first2=Saiyed I.|last3=Devol|first3=Allan H.|date=1995-12-01|title=Aerobic and anaerobic decomposition of organic matter in marine sediment: Which is fastest?|journal=Limnology and Oceanography|language=en|volume=40|issue=8|pages=1430–1437|doi=10.4319/lo.1995.40.8.1430|issn=1939-5590|bibcode=1995LimOc..40.1430K|doi-access=free}}</ref> This occurs because of preferential decomposition of labile molecules over refractile molecules.<ref name=":7" />

====Burial====
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).<ref name=Globalpulses>{{Cite journal|last1=Cartapanis|first1=Olivier|last2=Bianchi|first2=Daniele|last3=Jaccard|first3=Samuel|last4=Galbraith|first4=Eric|date=2016-01-21|title=Global pulses of organic carbon burial in deep-sea sediments during glacial maxima|volume=7|pages=10796|journal = Nature Communications|doi=10.1038/ncomms10796|pmid=26923945|pmc=4773493|bibcode=2016NatCo...710796C}}</ref> 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. [[Fjord]]s, 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.<ref name=fjords>{{Cite journal|last1=Smith|first1=Richard|last2=Bianchi|first2=Thomas|last3=Allison|first3=Mead|last4=Savage|first4=Candida|last5=Galy|first5=Valier|date=2015|title=High rates of organic carbon burial in fjord sediments globally|volume=8|issue=6|pages=450|journal = Nature Geoscience|doi=10.1038/ngeo2421|bibcode=2015NatGe...8..450S}}</ref> 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 CO<sub>2</sub> (Equation 4), and therefore a change in the rate of preservation of calcium carbonate can alter the partial pressure of CO<sub>2</sub> in Earth's atmosphere.<ref name=":1" /> CaCO<sub>3</sub> is [[Supersaturation|supersatured]] in the great majority of ocean surface waters and [[Undersaturation|undersaturated]] at depth,<ref name="Zeebe" /> meaning the shells are more likely to dissolve as they sink to ocean depths. CaCO<sub>3</sub> can also be dissolved through metabolic dissolution (i.e. can be used as food and excreted) and thus deep ocean sediments have very little calcium carbonate.<ref name=":1" /> The precipitation and burial of calcium carbonate in the ocean removes particulate inorganic carbon from the ocean and ultimately forms [[limestone]].<ref name=":1" /> On time scales greater than 500,000 years Earth's climate is moderated by the flux of carbon in and out of the [[lithosphere]].<ref>{{Cite journal|last1=Kasting|first1=J. F.|last2=Toon|first2=O. B.|last3=Pollack|first3=J. B.|date=1988-02-01|title=How climate evolved on the terrestrial planets|journal=Scientific American|volume=258|issue=2|pages=90–97|doi=10.1038/scientificamerican0288-90|pmid=11538470|issn=0036-8733|bibcode=1988SciAm.258b..90K}}</ref> Rocks formed in the ocean seafloor are recycled through [[plate tectonics]] back to the surface and weathered or [[Subduction|subducted]] into the [[Mantle (geology)|mantle]], the carbon [[Outgassing|outgassed]] by [[volcano]]es.<ref name=":0" />

== Human impacts ==
Oceans take up around 25 – 31% of anthropogenic CO<sub>2</sub>.<ref>{{Cite journal |last1=Gruber |first1=Nicolas |last2=Clement |first2=Dominic |last3=Carter |first3=Brendan R. |last4=Feely |first4=Richard A. |last5=van Heuven |first5=Steven |last6=Hoppema |first6=Mario |last7=Ishii |first7=Masao |last8=Key |first8=Robert M. |last9=Kozyr |first9=Alex |last10=Lauvset |first10=Siv K. |last11=Lo Monaco |first11=Claire |last12=Mathis |first12=Jeremy T. |last13=Murata |first13=Akihiko |last14=Olsen |first14=Are |last15=Perez |first15=Fiz F. |date=2019-03-15 |title=The oceanic sink for anthropogenic CO 2 from 1994 to 2007 |url=https://www.science.org/doi/10.1126/science.aau5153 |journal=Science |language=en |volume=363 |issue=6432 |pages=1193–1199 |doi=10.1126/science.aau5153 |pmid=30872519 |issn=0036-8075}}</ref><ref>{{Cite journal |last1=Gruber |first1=Nicolas |last2=Bakker |first2=Dorothee C. E. |last3=DeVries |first3=Tim |last4=Gregor |first4=Luke |last5=Hauck |first5=Judith |last6=Landschützer |first6=Peter |last7=McKinley |first7=Galen A. |last8=Müller |first8=Jens Daniel |date=24 January 2023 |title=Trends and variability in the ocean carbon sink |url=https://www.nature.com/articles/s43017-022-00381-x |journal=Nature Reviews Earth & Environment |language=en |volume=4 |issue=2 |pages=119–134 |doi=10.1038/s43017-022-00381-x |bibcode=2023NRvEE...4..119G |issn=2662-138X|hdl=20.500.11850/595538 |hdl-access=free }}</ref> Because the Revelle factor increases with increasing CO<sub>2</sub>, a smaller fraction of the anthropogenic flux will be taken up by the ocean in the future.<ref>{{Cite journal|last1=Revelle|first1=Roger|last2=Suess|first2=Hans E.|date=1957-02-01|title=Carbon Dioxide Exchange Between Atmosphere and Ocean and the Question of an Increase of Atmospheric CO2 during the Past Decades|journal=Tellus|language=en|volume=9|issue=1|pages=18–27|doi=10.1111/j.2153-3490.1957.tb01849.x|issn=2153-3490|bibcode=1957Tell....9...18R}}</ref> Current annual increase in atmospheric CO<sub>2</sub> is approximately 4–5 gigatons of carbon,<ref>{{Cite journal |last1=Friedlingstein |first1=Pierre |last2=O'Sullivan |first2=Michael |last3=Jones |first3=Matthew W. |last4=Andrew |first4=Robbie M. |last5=Bakker |first5=Dorothee C. E. |last6=Hauck |first6=Judith |last7=Landschützer |first7=Peter |last8=Le Quéré |first8=Corinne |last9=Luijkx |first9=Ingrid T. |last10=Peters |first10=Glen P. |last11=Peters |first11=Wouter |last12=Pongratz |first12=Julia |last13=Schwingshackl |first13=Clemens |last14=Sitch |first14=Stephen |last15=Canadell |first15=Josep G. |date=2023-12-05 |title=Global Carbon Budget 2023 |url=https://essd.copernicus.org/articles/15/5301/2023/#section3 |journal=Earth System Science Data |language=English |volume=15 |issue=12 |pages=5301–5369 |doi=10.5194/essd-15-5301-2023 |doi-access=free |issn=1866-3508|hdl=20.500.11850/665569 |hdl-access=free }}</ref> about 2–3ppm CO<sub>2</sub> per year.<ref>{{Cite web |last=Copernicus Climate Change Service |title=Greenhouse gas concentrations |url=https://climate.copernicus.eu/climate-indicators/greenhouse-gas-concentrations#:~:text=Based%20on%20additional%20data%20sources,contributed%20needs%20to%20be%20investigated. |access-date=2024-09-21 |website=climate.copernicus.eu}}</ref><ref>{{Cite web |last=Lindsey |first=Rebecca |date=2024-04-09 |editor-last=Dlugokencky |editor-first=Ed |title=Climate Change: Atmospheric Carbon Dioxide |url=https://www.climate.gov/news-features/understanding-climate/climate-change-atmospheric-carbon-dioxide |access-date=2024-09-21 |website=www.climate.gov |language=en}}</ref> This induces climate change that drives carbon concentration and carbon-climate feedback processes that modifies [[Thermohaline circulation|ocean circulation]] and the physical and chemical properties of [[seawater]], which alters CO<sub>2</sub> uptake.<ref name="Boer">{{cite journal|last1=Boer|first1=G|last2=Arora|first2=V|title=Feedbacks in emission-driven and concentration-driven global carbon budgets|journal=Journal of Climate|date=2013|volume=26|issue=10|pages=3326–3341|doi=10.1175/JCLI-D-12-00365.1|bibcode=2013JCli...26.3326B|doi-access=free}}</ref><ref name="Gregory">{{cite journal|last1=Gregory|first1=J|last2=Jones|first2=C|last3=Cadule|first3=P|last4=Friedlingstein|first4=P|title=Quantifying carbon cycle feedbacks|journal=Journal of Climate|date=2009|volume=22|issue=19|pages=5232–5250|doi=10.1175/2009JCLI2949.1|bibcode=2009JCli...22.5232G|s2cid=59385833|url=https://hal.archives-ouvertes.fr/hal-03197002/file/Gr199.pdf}}</ref> [[Overfishing]] and the [[plastic pollution]] of the oceans contribute to the degraded state of the world's biggest carbon sink.<ref>{{Cite news |last=Harvey |first=Fiona |author-link=Fiona Harvey |date=2019-12-04 |title=Tackling degraded oceans could mitigate climate crisis - report |url=https://www.theguardian.com/environment/2019/dec/04/tackling-ocean |access-date=2019-12-07 |work=The Guardian |language=en-GB |issn=0261-3077}}</ref><ref>{{Cite news |last=Harvey |first=Fiona |author-link=Fiona Harvey |date=2019-12-07 |title=Oceans losing oxygen at unprecedented rate, experts warn |url=https://www.theguardian.com/environment/2019/dec/07/oceans-losing-oxygen-at-unprecedented-rate-experts-warn |access-date=2019-12-07 |work=The Guardian |language=en-GB |issn=0261-3077}}</ref>

=== Ocean acidification ===
''Full article: [[Ocean acidification]]''

The pH of the oceans is declining due to uptake of atmospheric CO<sub>2</sub>.<ref>{{Cite journal|last1=Caldeira|first1=Ken|last2=Wickett|first2=Michael E.|date=2003-09-25|title=Oceanography: Anthropogenic carbon and ocean pH|journal=Nature|language=en|volume=425|issue=6956|pages=365|doi=10.1038/425365a|pmid=14508477|issn=1476-4687|bibcode=2003Natur.425..365C|s2cid=4417880|url=https://zenodo.org/record/1233227|doi-access=free}}</ref> The rise in dissolved carbon dioxide reduces the availability of the carbonate ion, reducing CaCO<sub>3</sub> saturation state, thus making it thermodynamically harder to make CaCO<sub>3</sub> shell.<ref name=":4">{{Cite book|title=Ocean acidification|date=2011|publisher=Oxford University Press|last1=Gattuso|first1=Jean-Pierre|last2=Hansson|first2=Lina|isbn=9780199591091|location=Oxford [England]|oclc=823163766}}</ref> Carbonate ions preferentially bind to hydrogen ions to form bicarbonate,<ref name="Zeebe" /> thus a reduction in carbonate ion availability increases the amount of unbound hydrogen ions, and decreases the amount of bicarbonate formed (Equations 1–3). pH is a measurement of hydrogen ion concentration, where a low pH means there are more unbound hydrogen ions. pH is therefore an indicator of carbonate [[Ion speciation|speciation]] (the ''format'' of carbon present) in the oceans and can be used to assess how ''healthy'' the ocean is.<ref name=":4" />

The list of organisms that may struggle due to ocean acidification include [[coccolithophore]]s and [[foraminifera]] (the base of the [[marine food chain]] in many areas), human food sources such as [[oyster]]s and [[mussel]]s,<ref>{{Cite journal|last=Barton|first=Alan|date=2015|title=Impacts of Coastal Acidification on the Pacific Northwest Shellfish Industry and Adaptation Strategies Implemented in Response|journal=Oceanography|volume=25|issue=2|pages=146–159|doi=10.5670/oceanog.2015.38|url=http://ir.library.oregonstate.edu/xmlui/bitstream/1957/56946/1/WaldbusserGeorgeCEOASImpactsCoastalAcidification.pdf|doi-access=free}}</ref> and perhaps the most conspicuous, a structure built by organisms – the coral reefs.<ref name=":4" /> Most surface water will remain supersaturated with respect to CaCO<sub>3</sub> (both calcite and aragonite) for some time on current emissions trajectories,<ref name=":4" /> but the organisms that require carbonate will likely be replaced in many areas.<ref name=":4" /> Coral reefs are under pressure from overfishing, nitrate pollution, and warming waters; ocean acidification will add additional stress on these important structures.<ref name=":4" />

=== Iron fertilization ===
''Full article: [[Iron fertilization|Iron Fertilization]]''

Iron fertilization is a facet of [[Climate engineering|geoengineering]], which purposefully manipulates the Earth's climate system, typically in aspects of the carbon cycle or radiative forcing. 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. Ongoing investigations regarding artificial fertilization exist.<ref name=":3">{{Cite journal|last1=Aumont|first1=O.|last2=Bopp|first2=L.|date=2006-06-01|title=Globalizing results from ocean in situ iron fertilization studies|journal=Global Biogeochemical Cycles|language=en|volume=20|issue=2|pages=GB2017|doi=10.1029/2005gb002591|issn=1944-9224|bibcode=2006GBioC..20.2017A|doi-access=free}}</ref> 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.<ref name=":3" /> However, the majority of the community does not believe this is a reasonable or viable approach.<ref>{{cite journal|last1=Chisholm|first1=S|last2=Falkowski|first2=P|last3=Cullen|first3=J|title=Dis-crediting ocean fertilization|journal=Science|date=2001|volume=294|issue=5541|pages=309–310|doi=10.1126/science.1065349|pmid=11598285|s2cid=130687109}}</ref>

=== Dams and reservoirs ===

There are over 16 million [[dam]]s in the world<ref name="Lehner">{{cite journal|last1=Lehner|first1=B|last2=Liermann|first2=C|last3=Revenga|first3=C|last4=Vorosmarty|first4=C|last5=Fekete|first5=B|last6=Crouzet|first6=P|last7=Doll|first7=P|last8=Endejan|first8=M|last9=Frenken|first9=K|last10=Magome|first10=J|last11=Nilsson|first11=C|last12=Robertson|first12=J|last13=Rodel|first13=R|last14=Sindorf|first14=N|last15=Wisser|first15=D|title=High-resolution mapping of the world's reservoirs and dams for sustainable river-flow management|journal=Frontiers in Ecology and the Environment|date=2011|volume=9|issue=9|pages=494–502|doi=10.1890/100125|url=http://urn.kb.se/resolve?urn=urn:nbn:se:umu:diva-49188|doi-access=free|bibcode=2011FrEE....9..494L}}</ref> that alter carbon transport from rivers to oceans.<ref name="Regnier">{{cite journal|display-authors=3|first1=Pierre |last1=Regnier |first2=Pierre |last2=Friedlingstein |first3=Philippe |last3=Ciais |first4=Fred T. |last4=Mackenzie |first5=Nicolas |last5=Gruber |first6=Ivan A. |last6=Janssens |first7=Goulven G. |last7=Laruelle |first8=Ronny |last8=Lauerwald |first9=Sebastiaan9 |last9=Luyssaert |first10=Andreas J. |last10=Andersson |first11=Sandra |last11=Arndt |first12=Carol |last12=Arnosti |first13=Alberto V. |last13=Borges |first14=Andrew W.|last14=Dale |first15=Angela |last15=Gallego-Sala |first16=Yves|last16=Goddéris |first17=Nicolas |last17=Goossens  |first18=Jens |last18=Hartmann |first19=Christoph |last19=Heinze |first20=Tatiana |last20=Ilyina |first21=Fortunat |last21=Joos |first22=Douglas E. |last22=LaRowe |first23=Jens |last23=Leifeld |first24=Filip J. R. |last24=Meysman |first25=Guy |last25=Munhoven |first26=Peter A.|last26=Raymond |first27=Renato|last27=Spahni |first28=Pradava |last28=Suntharalingam |first29=Martin |last29=Thullner|title=Anthropogenic perturbation of the carbon fluxes from land to ocean|journal=Nature Geoscience|date=2013|volume=6|issue=8|pages=597–607|doi=10.1038/ngeo1830|bibcode=2013NatGe...6..597R|url=http://orbi.ulg.ac.be/handle/2268/150126|hdl=10871/18939|s2cid=53418968 |hdl-access=free}}</ref> 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&nbsp;km<sup>3</sup>), 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.<ref name="Maavara">{{cite journal|last1=Maavara|first1=T|last2=Lauerwald|first2=R|last3=Regnier|first3=P|last4=Van Cappellen|first4=P|date=2016|title=Global perturbation of organic carbon cycling by river damming|journal=Nature|doi=10.1038/ncomms15347|pmid=28513580|pmc=5442313|volume=8|page=15347|bibcode=2017NatCo...815347M}}</ref> The excess carbon contained in the reservoirs may emit an additional ~0.184 Gt of carbon to the atmosphere per year<ref name="Barros">{{cite journal|last1=Barros|first1=N|last2=Cole|first2=J|last3=Tranvik|first3=L|last4=Prairie|first4=Y|last5=Bastviken|first5=D|last6=Huszar|first6=V|last7=del Giorgio|first7=P|last8=Roland|first8=F|title=Carbon emission from hydroelectric reservoirs linked to reservoir age and latitude|journal=Nature Geoscience|date=2011|volume=4|issue=9|pages=593–596|doi=10.1038/ngeo1211|bibcode=2011NatGe...4..593B|s2cid=52245758}}</ref> and an additional ~0.2 GtC will be buried in sediment.<ref name="Maavara" /> Prior to 2000, the [[Mississippi River|Mississippi]], the [[Niger River|Niger]], and the [[Ganges River]] basins account for 25 – 31% of all reservoir carbon burial.<ref name="Maavara" /> After 2000, the [[Paraná River|Paraná]] (home to 70 dams) and the [[Zambezi]] (home to the largest reservoir) River basins exceeded the burial by the Mississippi.<ref name="Maavara" /> Other large contributors to carbon burial caused by damming occur on the [[Danube]], the [[Amazon River|Amazon]], the [[Yangtze]], the [[Mekong]], the [[Yenisei River|Yenisei]], and the [[Tocantins River|Tocantins]] Rivers.<ref name="Maavara" />

== See also ==
{{portal|Oceans}}
* [[Phosphorus cycle]]

== References ==
{{reflist}}

==External links==
* [https://earth.nullschool.net/#current/ocean/surface/currents/overlay=spco2/winkel3 Current global map of the partial pressure of carbon dioxide at the ocean surface]
* [https://earth.nullschool.net/#current/ocean/surface/currents/overlay=fgco2/winkel3 Current global map of the sea-air carbon dioxide flux density]

{{biogeochemical cycle}}

[[Category:Carbon cycle]]
[[Category:Chemical oceanography]]