View source for Carbonate compensation depth

{{short description|Depth in the oceans below which no calcium carbonate sediment particles are preserved}}

The '''carbonate compensation depth''' ('''CCD''') is the depth, in the oceans, at which the rate of supply of [[calcium carbonate]]s matches the rate of [[solvation]].  That is, solvation 'compensates' supply.  Below the CCD solvation is faster, so that carbonate particles dissolve and the carbonate shells ([[ Test (biology) |tests]]) of animals are not preserved.  Carbonate particles cannot accumulate in the sediments where the sea floor is below this depth.

[[Calcite]] is the least soluble of these carbonates, so the CCD is normally the compensation depth for calcite.  The '''aragonite compensation depth''' ('''ACD''') is the compensation depth for [[aragonite|aragonitic]] carbonates. Aragonite is more soluble than calcite, and the aragonite compensation depth is generally shallower than both the calcite compensation depth and the CCD.

==Overview==
[[File:Carbonate compensation concept.png|thumb|upright=2|right| {{center|Carbonate compensation concept{{hsp}}<ref name=Middelburg2019>{{cite book |doi = 10.1007/978-3-030-10822-9_5|chapter = Biogeochemical Processes and Inorganic Carbon Dynamics|title = Marine Carbon Biogeochemistry|series = SpringerBriefs in Earth System Sciences|year = 2019|last1 = Middelburg|first1 = Jack J.|pages = 77–105|isbn = 978-3-030-10821-2| s2cid=104368944 }} [[File:CC-BY icon.svg|50px]] Modified 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>}}]]

[[File:Calcareous sediment in the ocean.png|thumb|upright=2| Calcareous sediment can only accumulate in depths shallower than the calcium carbonate compensation depth (CCD). Below the CCD, calcareous sediments dissolve and will not accumulate. The lysocline represents the depth range in which the rate of dissolution increases dramatically.<ref name=Webb2019>Webb, Paul (2019) ''Introduction to Oceanography'', [https://rwu.pressbooks.pub/webboceanography/chapter/chapter-12-ocean-sediments/ Chapter 12: Ocean Sediments], page 273–297, Rebus Community. Updated 2020. [[File:CC-BY icon.svg|50px]] Modified text 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>]]

As shown in the diagram, biogenic [[calcium carbonate]] (CaCO<sub>3</sub>) [[Test (biology)|tests]] are produced in the [[photic zone]] of the oceans (green circles). Upon death, those tests escaping dissolution near the surface settle, along with clay materials. In seawater, a dissolution boundary is formed as a result of temperature, pressure, and depth, and is known as the ''saturation horizon''.<ref name="raven05">{{cite web |title=Ocean acidification due to increasing atmospheric carbon dioxide |url=https://royalsociety.org/topics-policy/publications/2005/ocean-acidification/ |website=The Royal Society }}</ref> Above this horizon, waters are supersaturated and CaCO<sub>3</sub> tests are largely preserved. Below it, waters are undersaturated, because of both the increasing solubility with depth and the release of CO<sub>2</sub> from organic matter decay, and CaCO<sub>3</sub> will dissolve.  The sinking velocity of debris is rapid (broad pale arrows), so dissolution occurs primarily at the sediment surface.

At the carbonate compensation depth, the rate of dissolution exactly matches the rate of supply of CaCO<sub>3</sub> from above. At steady state this depth, the CCD, is similar to the snowline (the first depth where carbonate-poor sediments occur). The [[lysocline]] is the depth interval between the saturation and carbonate compensation depths.<ref>{{cite journal |doi = 10.1038/s41561-018-0259-5|title = The role of calcification in carbonate compensation|year = 2018|last1 = Boudreau|first1 = Bernard P.|last2 = Middelburg|first2 = Jack J.|last3 = Luo|first3 = Yiming|journal = Nature Geoscience|volume = 11|issue = 12|pages = 894–900|bibcode = 2018NatGe..11..894B|s2cid = 135284130}}</ref><ref name=Middelburg2019 />

==Solubility of carbonate==
Calcium carbonate is essentially insoluble in sea surface waters today. Shells of dead calcareous [[plankton]] sinking to deeper waters are practically unaltered until reaching the [[lysocline]], the point about 3.5 km deep past which the [[solubility]] increases dramatically with depth and pressure. By the time the CCD is reached{{Clarify|reason=Above it says they reach the floor and then dissolve after|date=December 2023}} all [[calcium carbonate]] has dissolved according to this equation:
:<chem>CaCO3 + CO2 + H2O <=> Ca^2+ (aq) + 2HCO_3^- (aq)</chem>

Calcareous plankton and [[sediment]] particles can be found in the [[water column]] above the CCD. If the [[sea bed]] is above the CCD, bottom [[sediment]]s can consist of calcareous sediments called [[pelagic sediments|calcareous ooze]], which is essentially a type of [[limestone]] or [[chalk]]. If the exposed sea bed is below the CCD tiny [[sea shell|shell]]s of CaCO<sub>3</sub> will dissolve before reaching this level, preventing deposition of carbonate sediment. As the sea floor spreads, [[thermal subsidence]] of the plate, which has the effect of increasing [[Bathymetry|depth]], may bring the carbonate layer below the CCD; the carbonate layer may be prevented from chemically interacting with the sea water by overlying sediments such as a layer of [[siliceous ooze]] or [[abyssal zone|abyssal]] clay deposited on top of the carbonate layer.<ref>Thurman, Harold., Alan Trujillo. ''Introductory Oceanography''.2004.p151-152</ref>

== Variations in value of the CCD ==
{{Carbon cycle|Biogeochemical}}

The exact value of the CCD depends on the solubility of calcium carbonate which is determined by [[temperature]], [[pressure]] and the chemical composition of the water – in particular the amount of dissolved {{CO2|link=yes}} in the water. Calcium carbonate is more soluble at lower temperatures and at higher pressures. It is also more soluble if the concentration of dissolved {{CO2}} is higher. Adding a reactant to the above chemical equation pushes the equilibrium towards the right producing more products: [[Calcium|Ca<sup>2+</sup>]] and [[bicarbonate|HCO<sub>3</sub><sup>−</sup>]], and consuming more reactants {{CO2}} and [[calcium carbonate]] according to [[Le Chatelier's principle]].

At the present time the CCD in the [[Pacific Ocean]] is about 4200–4500 metres except beneath the equatorial [[upwelling]] zone, where the CCD is about 5000&nbsp;m. In the [[temperate]] and [[tropical]] [[Atlantic Ocean]] the CCD is at approximately 5000&nbsp;m. In the [[Indian Ocean]] it is intermediate between the Atlantic and the Pacific at approximately 4300 meters. The variation in the depth of the CCD largely results from the length of time since the bottom water has been exposed to the surface; this is called the "age" of the [[water mass]]. [[Thermohaline circulation]] determines the relative ages of the water in these basins. Because organic material, such as fecal pellets from [[copepods]], sink from the surface waters into deeper water, deep water masses tend to accumulate dissolved carbon dioxide as they age. The oldest water masses have the highest concentrations of {{CO2}} and therefore the shallowest CCD. The CCD is relatively shallow in high [[latitude]]s with the exception of the [[North Atlantic]] and regions of [[Southern Ocean]] where [[downwelling]] occurs. This downwelling brings young, surface water with relatively low concentrations of carbon dioxide into the deep ocean, depressing the CCD.

In the [[Geologic time|geological past]] the depth of the CCD has shown significant variation. In the [[Cretaceous]] through to the [[Eocene]] the CCD was much shallower globally than it is today; due to intense volcanic activity during this period atmospheric {{CO2}} concentrations were much higher. Higher concentrations of {{CO2}} resulted in a higher [[partial pressure]] of {{CO2}} over the ocean. This greater pressure of atmospheric {{CO2}} leads to increased dissolved {{CO2}} in the ocean mixed surface layer. This effect was somewhat moderated by the deep oceans' elevated temperatures during this period.<ref>{{Cite web
 |url=http://www.physorg.com/news10978.html
 |title=Warmer than a Hot Tub: Atlantic Ocean Temperatures Much Higher in the Past
 |publisher=Physorg.com
 |date=February 17, 2006
}}</ref>  In the late Eocene the transition from a [[Greenhouse and Icehouse Earth|greenhouse to an icehouse Earth]] coincided with a deepened CCD.

[[John Murray (oceanographer)|John Murray]] investigated and experimented on the dissolution of calcium carbonate and was first to identify the carbonate compensation depth in oceans.<ref>{{cite encyclopedia	|last=Berger	|first=Wolfgang H. |display-authors=etal |encyclopedia=Encyclopedia of Marine Geosciences	|date=2016	|publisher=Springer Netherlands	|series=Encyclopedia of Earth Sciences Series	|isbn=978-94-007-6238-1	|doi=10.1007/978-94-007-6238-1_47	|pages=71–73|chapter=Calcite Compensation Depth (CCD)	}}</ref>

=== Climate change impacts ===
{{See also|Effects of climate change on oceans}}
Increasing [[Carbon dioxide in Earth's atmosphere|atmospheric concentration of {{CO2}}]] from combustion of [[fossil fuels]] are causing the CCD to rise, with zones of [[downwelling]] first being affected.<ref>{{cite journal |last1=Sulpis |first1=Olivier |display-authors=etal |date=October 29, 2018 |title=Current CaCO3 dissolution at the seafloor caused by anthropogenic CO2 |journal=Proceedings of the National Academy of Sciences of the United States of America |volume=115 |issue=46 |pages=11700–11705 |bibcode=2018PNAS..11511700S |doi=10.1073/pnas.1804250115 |pmc=6243283 |pmid=30373837 |doi-access=free}}</ref> [[Ocean acidification]], which is also caused by increasing carbon dioxide concentrations in the atmosphere, will increase such dissolution and shallow the carbonate compensation depth on timescales of tens to hundreds of years.<ref>{{Cite journal |last1=Boudreau |first1=Bernard P. |last2=Middelburg |first2=Jack J. |last3=Hofmann |first3=Andreas F. |last4=Meysman |first4=Filip J. R. |date=2010 |title=Ongoing transients in carbonate compensation: COMPENSATION TRANSIENTS |journal=Global Biogeochemical Cycles |language=en |volume=24 |issue=4 |pages=n/a |doi=10.1029/2009GB003654|s2cid=53062358 |doi-access=free }}</ref>

== Sedimentary ooze ==
{{See also|Pelagic sediment}}
On the [[Ocean floor|sea floors]] above the carbonate compensation depth, the most commonly found ooze is [[calcareous ooze]]; on the sea floors below the carbonate compensation depth, the most commonly found ooze is [[siliceous ooze]]. While calcareous ooze mostly consists of [[Rhizaria]], siliceous ooze mostly consists of [[Radiolaria]] and [[diatom]]s.<ref>{{Cite journal |last1=Johnson |first1=Thomas C. |last2=Hamilton |first2=Edwin L. |last3=Berger |first3=Wolfgang H. |date=1977-08-01 |title=Physical properties of calcareous ooze: Control by dissolution at depth |url=https://dx.doi.org/10.1016/0025-3227%2877%2990071-8 |journal=Marine Geology |language=en |volume=24 |issue=4 |pages=259–277 |doi=10.1016/0025-3227(77)90071-8 |bibcode=1977MGeol..24..259J |issn=0025-3227}}</ref><ref>{{Cite book|last=Webb |first=Paul |chapter=12.6 Sediment Distribution|title=Introduction to Oceanography |chapter-url=https://rwu.pressbooks.pub/webboceanography/chapter/12-6-sediment-distribution/ |language=en|date=August 2023|via=Roger Williams University|access-date=3 July 2024}}</ref>

==See also==
* [[Carbonate pump]]
* [[Great Calcite Belt]]
* [[Lysocline]]
* [[Ocean acidification]]

== References ==

<references/>

{{DEFAULTSORT:Carbonate Compensation Depth}}
[[Category:Oceanography]]