Carbonate compensation depth

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In the deep ocean, carbonate particles in bottom sediments are supplied by the die-off of surface plankton that secrete calcium carbonate tests or shells. When these shells fall below a certain water depth, they begin to dissolve as ocean waters become undersaturated with respect to calcium carbonate because of increasing pressure, decreasing temperature and increasing amounts of dissolved CO2. With increasing depth, the rate of dissolution increases. The carbonate compensation depth, or CCD, is defined as the water depth at which the rate of supply of calcium carbonate from the surface is equal to the rate of dissolution. As long as the ocean floor lies above the CCD, carbonate particles will accumulate in bottom sediments, but below, there is no net accumulation. The sea floor near ocean ridges is typically above the CCD and carbonates are important sediment constituents, but with spreading and cooling, the sea floor descends below the CCD and deep sea clays become predominant.