Carbonate Compensation Depth

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Carbonate compensation depth (CCD) is a level in the oceans below which the rate of supply of calcium carbonate (calcite and aragonite) equals the rate of dissolution, such that no calcium carbonate is preserved.

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.

Calcareous plankton and sediment particles can be found in the water column above CCD and, 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 sea bed is below the CCD tiny shells of CaCO₃ will dissolve before reaching this level so there will be no carbonate sediment.

The exact depth 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 in the water. Calcium carbonate is more soluble at lower temperatures and at higher pressures. It is also more soluble if the dissolved CO₂ is higher because some of this combines with water molecules to produce a weak acid.

At the present time the CCD in the Pacific Ocean is about 4,200-4,500 m except beneath the equatorial upwelling zone, where the CCD is about 5,000 m. In the temperate and tropical Atlantic Ocean the CCD is at approximately 5,000 m. In the Indian Ocean it is intermediate between the Atlantic and the Pacific. The CCD is relatively shallow in high latitudes.

The difference between CCD depths in different oceans today can largely be explained by variation in dissolved CO₂ content. This is related to the patterns of circulation of deep ocean waters like the North Atlantic Deep Water because decay of organisms in the water column causes a build up of dissolved CO₂ in water which has been at depth longer.

In the geological past the depth of the CCD has shown significant variation. For example, in the Cretaceous through to the Eocene the CCD was much shallower globally. In the late Eocene the development of Antarctic glaciers resulted in colder deep sea temperatures leading to a deeper CCD.

Global changes in CCD since the Mesozoic are broadly the same as changes in global sea level.

Sir John Murray investigated and experimented on the dissolution of calcium carbonate and was first to identify the carbonate compensation depth in oceans.