The oceanic carbon cycle is a long-term exchange of carbon between the atmosphere and the oceans. It occurs because mollusks and other shelled marine animals make their shells by secreting calcium carbide, which they create from the calcium and carbonate that are dissolved in the ocean water.
Carbonate is formed from carbonic acid, which, in turn, is formed when carbon dioxide from the atmosphere dissolves in the ocean and reacts with water. When the shelled marine animals die, their bodies decompose and leave behind their hard shells, which accumulate on the seafloor and are eventually broken down and compacted under enormous pressure to form limestone. When this limestone is eventually exposed to air after thousands or millions of years and gets weathered, the carbon in it is released back into the atmosphere in the form of carbon dioxide.