Carbon dioxide is one of the most important gases that dissolve in the ocean. Some of it stays as dissolved gas, but most reacts with the water to form carbonic acid or reacts with carbonates already in the water to form bicarbonates. This removes dissolved carbon dioxide from the water.

Many plants and animals use the bicarbonate to form calcium carbonate shells. When these organisms die, some of the bicarbonate is returned to the water, but a lot of it settles down to the sea bed. This process locks up, for long periods of time, carbon that originated in carbon dioxide in the atmosphere.

Carbon dioxide