horizontal rule


By Dr. Richard M. Mitterer

horizontal rule

Virtually all calcium carbonate deposits in the oceans are formed by organisms. In shallow water environments the calcareous organisms are primarily corals, mollusks, and algae. In the open ocean the primary calcareous organisms are foraminifera (microscopic animals) and coccoliths (algae). Both forams and coccoliths are floating or planktonic organisms. This means that they live at or near the surface of the ocean, and they cannot swim but move wherever the currents carry them. When the planktonic organisms die their calcareous shells fall to the ocean bottom. If they accumulate in a high concentration (greater then 30 % of the sediment), an ooze is formed. These calcium carbonate shells, however, do not accumulate everywhere on the ocean floor. In general, calcareous sediments or oozes are not found where the sea floor is deeper than 4500 meters. The obvious explanation is that the shells falling through the longer water column are dissolved before they reach bottom, while the shells falling less than 4500 meters to the bottom are not to be dissolved

What causes the shells to be absent in the deepest part of the ocean? The answer cannot be that the organisms do not live there. Although true, the organisms do not live in the deep ocean, the organisms do not live anywhere in deep water even where the water is less than 4500 meters. The calcareous forams and coccoliths live at the surface of the ocean, not at the bottom. Furthermore, these organisms live virtually everywhere in the surface waters from the equator to the poles, so the answer to their preservation in deep water is not their distribution in surface waters.

The answer is that the shells dissolve due to the higher carbon dioxide content in the deeper waters of the oceans. Carbon dioxide is produced by animals during respiration. This process occurs everywhere in the ocean but in surface waters the excess carbon dioxide escapes to the atmosphere. Carbon dioxide produced in deep waters cannot escape and, furthermore, it increases with depth. That is, deeper waters have more carbon dioxide than shallow waters.

The significance of this excess carbon dioxide is that it dissolves calcium carbonate. The deeper the water, the higher the carbon dioxide content and the more likely that calcium carbonate will be dissolved.