Carbonate 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. 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 are produced in the photic zone 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