Carbonate Pump

The carbonate pump is the part of the biological pump that moves carbon to depth as calcium carbonate. Calcifying plankton such as coccolithophores, foraminifera, and pteropods build CaCO3 shells in the surface ocean; when they sink, part of that carbonate dissolves below the lysocline and part reaches the seabed. Building CaCO3 releases CO2 to the water, so the carbonate (or hard-tissue) pump works against the soft-tissue organic pump, a counterintuitive feedback in the ocean carbon budget. Its strength depends on the aragonite and calcite saturation state, which acidification is lowering, shoaling the saturation horizons and thinning carbonate sediments.