Carbonate compensation depth (CCD) is the depth in the oceans below which the rate of supply of

calcite Calcite is a Carbonate minerals, carbonate mineral and the most stable Polymorphism (materials science), polymorph of calcium carbonate (CaCO3). It is a very common mineral, particularly as a component of limestone. Calcite defines hardness 3 on ...

( calcium carbonate) lags behind the rate of solvation, such that no calcite is preserved. Shells of animals therefore dissolve and carbonate particles may not accumulate in the sediments on the sea floor below this depth. Aragonite compensation depth (hence ACD) describes the same behaviour in reference to aragonitic carbonates. Aragonite is more soluble than calcite, so the aragonite compensation depth is generally shallower than the calcite compensation depth.

Overview

As shown in the diagram, biogenic calcium carbonate (CaCO₃)

tests Test(s), testing, or TEST may refer to: * Test (assessment), an educational assessment intended to measure the respondents' knowledge or other abilities Arts and entertainment * ''Test'' (2013 film), an American film * ''Test'' (2014 film), ...

are produced in the

photic zone The photic zone, euphotic zone, epipelagic zone, or sunlight zone is the uppermost layer of a body of water that receives sunlight, allowing phytoplankton to perform photosynthesis. It undergoes a series of physical, chemical, and biological proc ...

of the oceans (green circles). Upon death, those tests escaping dissolution near the surface, settle along with clays materials. In seawater, a dissolution boundary is formed as a result of temperature, pressure, and depth, and is known as the ''saturation horizon''. Above the saturation horizon, waters are supersaturated and CaCO₃ tests are largely preserved. Below the saturation, waters are undersaturated because of increasing solubility with depth and the release of CO₂ from organic matter decay and CaCO₃ will dissolve. Dissolution occurs primarily at the sediment surface as the sinking velocity of debris is rapid (broad white arrows). At the carbonate compensation depth, the rate of dissolution exactly matches rate of supply of CaCO₃ from above. At steady state this carbonate compensation depth 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 depth.

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 all calcium carbonate has dissolved according to this equation: :CaCO3 + CO2 + H2O <=> Ca^2+ (aq) + 2HCO_3^- (aq) Calcareous plankton and sediment particles can be found in the water column above the CCD. If the sea bed is above the CCD, bottom sediments can consist of calcareous sediments called calcareous ooze, which is essentially a type of limestone or chalk. If the exposed sea bed is below the CCD tiny shells of CaCO₃ 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 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 clay deposited on top of the carbonate layer.

Variations in value of the CCD

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 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 is higher. Adding a reactant to the above chemical equation pushes the equilibrium towards the right producing more products: Ca²⁺ and HCO₃⁻, and consuming more reactants 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 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 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 Copepods (; meaning "oar-feet") are a group of small crustaceans found in nearly every freshwater and saltwater habitat. Some species are planktonic (inhabiting sea waters), some are benthic (living on the ocean floor), a number of species have p ...

, 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 and therefore the shallowest CCD. The CCD is relatively shallow in high latitudes with the exception of the

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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 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 concentrations were much higher. Higher concentrations of resulted in a higher

partial pressure In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas as if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal gas ...

of over the ocean. This greater pressure of atmospheric leads to increased dissolved in the ocean mixed surface layer. This effect was somewhat moderated by the deep oceans' elevated temperatures during this period. In the late Eocene the transition from a greenhouse to an icehouse Earth coincided with a deepened CCD. John Murray investigated and experimented on the dissolution of calcium carbonate and was first to identify the carbonate compensation depth in oceans.

Climate change impacts

Increasing atmospheric concentration of from combustion of

fossil fuels A fossil fuel is a hydrocarbon-containing material formed naturally in the Earth's crust from the remains of dead plants and animals that is extracted and burned as a fuel. The main fossil fuels are coal, oil, and natural gas. Fossil fuels ...

are causing the CCD to rise, with zones of downwelling first being affected. 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.

Sedimentary ooze

On the 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 A diatom (Neo-Latin ''diatoma''), "a cutting through, a severance", from el, διάτομος, diátomos, "cut in half, divided equally" from el, διατέμνω, diatémno, "to cut in twain". is any member of a large group comprising sev ...

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