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| '''Calcite compensation depth''' ('''CCD''') is the depth in the oceans below which the rate of supply of [[calcite]] ([[calcium carbonate]]) lags behind the rate of [[solvation]], such that no calcite is preserved. '''Aragonite compensation depth''' (hence '''ACD''') describes the same behaviour in reference to [[aragonite|aragonitic]] carbonates.
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| 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]] where the [[solubility]] increases dramatically. By the time the CCD is reached all [[calcium carbonate]] has dissolved according to this equation:
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| :<math>\mathrm{CaCO_3 + CO_2 + H_2O \ \rightleftharpoons \ Ca^{2+} (aq) + 2 \ HCO_3^- (aq)} </math>
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| 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, unless the seabed is above the CCD, for example along a [[mid-ocean ridge]]. As the sea floor spreads [[thermal contraction]] of the plate 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>
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| == Variations in value of the CCD ==
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| 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 [[Carbon dioxide|CO<sub>2</sub>]] 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 CO<sub>2</sub> 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 [[Carbon dioxide|CO<sub>2</sub>]] and [[calcium carbonate]] according to [[Le Chatelier's principle]].
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| 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 m. In the [[temperate]] and [[tropical]] [[Atlantic Ocean]] the CCD is at approximately 5000 m. In the [[Indian Ocean]] it is intermediate between the Atlantic and the Pacific. 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]]. Consequently the CCD is, on average, deeper in the Atlantic Ocean, shallower in the Pacific Ocean and of intermediate depth in the Indian Ocean. Relative ages of these basins can be inferred from the pattern of [[ocean currents]] which form a [[global conveyor belt]]. 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 carbon dioxide 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.
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| 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 carbon dioxide concentrations were much higher. Higher concentrations of carbon dioxide resulted in a higher [[partial pressure]] of carbon dioxide over the ocean. This greater pressure of atmospheric carbon dioxide leads to increased dissolved carbon dioxide in the ocean mixed surface layer. This effect was somewhat moderated by the deep oceans' elevated temperatures during this period.<ref>[http://www.physorg.com/news10978.html / ''Warmer than a Hot Tub: Atlantic Ocean Temperatures Much Higher in the Past,'' Physorg.com, 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.
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| Today, increasing atmospheric concentration of CO<sub>2</sub> from combustion of [[fossil fuels]] may elevate the CCD to some degree, with zones of [[downwelling]] first being affected.
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| [[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.
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| ==See also==
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| * [[Ocean acidification]]
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| * [[Lysocline]]
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| * [[Carbonate pump]]
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| == References ==
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| <references/>
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| {{DEFAULTSORT:Carbonate Compensation Depth}}
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| [[Category:Oceanography]]
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