Carbonate compensation depth

The carbonate compensation depth (CCD) is the depth, in the oceans, at which the rate of supply of calcium carbonates 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 (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 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. 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 clay materials. In seawater, a dissolution boundary is formed as a result of temperature, pressure, and depth, and is known as the saturation horizon. Above this horizon, waters are supersaturated and CaCO3 tests are largely preserved. Below it, waters are undersaturated, because of both the increasing solubility with depth and the release of CO2 from organic matter decay, and CaCO3 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 CaCO3 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. 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.