Compensation depth is the level at which seawater dissolves calcite, or calcium carbonate. Near the surface of the ocean, where the water is warm and under low pressure, planktonic species are free to incorporate calcium and carbon into their mineralized shells. However, at compensation depth, the cold, acidic water rapidly dissolves calcium carbonate.
Compensation depth is the point at which the water completely dissolves calcium carbonate and leaches the mineral from seafloor sediments. The lysocline, as it is sometimes called, averages 4 to 5 kilometers deep in most areas, though it runs deeper where continental runoff causes turbulence and stirs up all but the deepest layers of the ocean. In places where little turbulence affects ocean layers, compensation depth is usually higher than average.
The acidity of deep water results from a build-up of CO2 released by deep-water species of animals. These animals extract oxygen from the surrounding water and excrete CO2 the same way other animals do. At depth, however, CO2 cannot escape the high pressure and extremely cold temperatures of the water it's suspended in. This renders the water considerably more acidic than surface waters and causes it to dissolve calcium that gets deposited by the steady rain of expired plankton from above. As deep water extracts calcite from rocks, limestone can't form at depth. This difference helps geologists determine the depth at which some rocks formed on ancient seafloors.