Editing Carbon cycle (section)

===Ocean===
{{Main|Oceanic carbon cycle}}

The ocean can be conceptually divided into a [[surface layer]] within which water makes frequent (daily to annual) contact with the atmosphere, and a deep layer below the typical [[mixed layer]] depth of a few hundred meters or less, within which the time between consecutive contacts may be centuries. The [[dissolved inorganic carbon]] (DIC) in the surface layer is exchanged rapidly with the atmosphere, maintaining equilibrium. Partly because its concentration of DIC is about 15% higher<ref name=Sarmiento_and_Gruber_2006>{{cite book |last1=Sarmiento |first1=Jorge L. |last2=Gruber |first2=Nicolas |title=Ocean Biogeochemical Dynamics |date=2006 |publisher=Princeton University Press |isbn=978-0-691-01707-5 }}{{pn|date=October 2024}}</ref> but mainly due to its larger volume, the deep ocean contains far more carbon—it is the largest pool of actively cycled carbon in the world, containing 50 times more than the atmosphere<ref name=GlobalCarbonCycle/>—but the timescale to reach equilibrium with the atmosphere is hundreds of years: the exchange of carbon between the two layers, driven by [[thermohaline circulation]], is slow.<ref name=GlobalCarbonCycle/>

Carbon enters the ocean mainly through the dissolution of atmospheric carbon dioxide, a small fraction of which is converted into [[carbonate]]. It can also enter the ocean through rivers as [[dissolved organic carbon]]. It is converted by organisms into organic carbon through [[photosynthesis]] and can either be exchanged throughout the food chain or precipitated into the oceans' deeper, more carbon-rich layers as dead soft tissue or in shells as [[calcium carbonate]]. It circulates in this layer for long periods of time before either being deposited as sediment or, eventually, returned to the surface waters through thermohaline circulation.<ref name=Prentice_etal_2001/> 

Oceans are basic (with a current [[Ocean#pH|pH value]] of 8.1 to 8.2). The increase in atmospheric CO<sub>2</sub> shifts the pH of the ocean towards neutral in a process called [[ocean acidification]]. Oceanic absorption of CO<sub>2</sub> is one of the most important forms of [[carbon sequestering]]. The projected rate of pH reduction could slow the biological precipitation of [[calcium carbonate]]s, thus decreasing the ocean's capacity to absorb CO<sub>2</sub>.<ref name=Klyepas1999>{{Cite journal |last1=Kleypas |first1=J. A. |last2=Buddemeier |first2=R. W. |last3=Archer |first3=D. |last4=Gattuso |first4=J. P. |last5=Langdon |first5=C. |last6=Opdyke |first6=B. N. |title=Geochemical Consequences of Increased Atmospheric Carbon Dioxide on Coral Reefs |doi=10.1126/science.284.5411.118 |journal=Science |volume=284 |issue=5411 |pages=118–120 |year=1999 |pmid=10102806 |bibcode=1999Sci...284..118K}}</ref><ref name=Langdon2000>{{Cite journal |last1=Langdon |first1=C. |last2=Takahashi |first2=T. |last3=Sweeney |first3=C. |last4=Chipman |first4=D. |last5=Goddard |first5=J. |last6=Marubini |first6=F. |last7=Aceves |first7=H. |last8=Barnett |first8=H. |last9=Atkinson |first9=M. J. |doi=10.1029/1999GB001195 |title=Effect of calcium carbonate saturation state on the calcification rate of an experimental coral reef |journal=Global Biogeochemical Cycles |volume=14 |issue=2 |pages=639 |year=2000 |bibcode=2000GBioC..14..639L|s2cid=128987509 |doi-access=free }}</ref>