text: Particulate inorganic carbon (PIC) usually takes the form of calcium carbonate (CaCO), and plays a key part in the ocean carbon cycle. This biologically fixed carbon is used as a protective coating for many planktonic species (coccolithophores, foraminifera) as well as larger marine organisms (mollusk shells). Calcium carbonate is also excreted at high rates during osmoregulation by fish, and can form in whiting events. While this form of carbon is not directly taken from the atmospheric budget, it is formed from dissolved forms of carbonate which are in equilibrium with CO and then responsible for removing this carbon via sequestration.
  CO + HO → HCO → H + HCO
  Ca + 2HCO → CaCO + CO + HO
  While this process does manage to fix a large amount of carbon, two units of alkalinity are sequestered for every unit of sequestered carbon.  The formation and sinking of CaCO therefore drives a surface to deep alkalinity gradient which serves to raise the pH of surface waters, shifting the speciation of dissolved carbon to raise the partial pressure of dissolved CO in surface waters, which actually raises atmospheric levels. In addition, the burial of CaCO in sediments serves to lower overall oceanic alkalinity, tending to raise pH and thereby atmospheric CO levels if not counterbalanced by the new input of alkalinity from weathering.  The portion of carbon that is permanently buried at the sea floor becomes part of the geologic record. Calcium carbonate often forms remarkable deposits that can then be raised onto land through tectonic motion as in the case with the White Cliffs of Dover in Southern England. These cliffs are made almost entirely of the plates of buried coccolithophores.