Shallow marine waters are generally supersaturated in calcite, CaCO3, so as marine organisms (which often have shells made of calcite or its polymorph, aragonite) die, they will tend to fall downwards without dissolving. As depth (i.e. pressure) increases within the water column, the corresponding calcite saturation of sea-water decreases and the shells start to dissolve. At the lysocline, the rate of dissolution increases dramatically. Below this, there exists a depth known as the carbonate compensation depth (CCD) below which the rate of supply of calcite equals the rate of dissolution, such that no calcite is deposited. This depth is the equivalent of a marine snow-line.
The depth of the CCD varies as a function of the chemical composition of the sea-water and its temperature. Furthermore, it is not constant over time, having been globally much shallower in the Cretaceous through to Eocene.