Carbonate Compensation Depth

The carbonate compensation depth (CCD) is the depth in the ocean below which calcium carbonate shells dissolve faster than they accumulate, so that deep-sea sediments contain no calcite. In today's ocean it lies at roughly 4 to 5 kilometers, and it varies between basins.

Cold, high-pressure water holds more dissolved carbon dioxide, which makes it more acidic, so it becomes undersaturated with respect to calcite at depth. The depth where dissolution begins is the lysocline, and the CCD lies below it. Aragonite, a more soluble form of calcium carbonate, dissolves at shallower depths. The CCD is shallower in the Pacific than in the Atlantic because deep Pacific water is older and has collected more CO₂ from decaying material, a feature of the thermohaline circulation.

Above the CCD the seafloor is covered with white ooze made of shells of foraminifera and coccolithophores, which can harden into chalk and limestone. Below it, only clay and silica remain, forming dark red clays. Geologists use the depth of the boundary in old sediment cores to reconstruct past ocean chemistry.

During the Paleocene-Eocene Thermal Maximum, about 56 million years ago, a massive release of carbon raised the CCD's shallower limit by about two kilometers, dissolving carbonate in deep-sea sediments. Today's ocean acidification is expected to raise the CCD again, though more slowly, and the dissolved carbonate will eventually neutralize some of the CO₂, as part of the carbon cycle and carbonate-silicate cycle.