The Carbonate Compensation Depth (CCD) is a specific boundary in the deep ocean where the rate at which calcium carbonate (\(text{CaCO}_3\)) dissolves exactly matches the rate at which it is supplied from the surface. Tiny marine organisms, such as foraminifera and coccolithophores, build shells made of \(text{CaCO}_3\) in the surface waters. When these organisms die, their shells sink toward the seafloor, forming a continuous rain of carbonate material.
The CCD is the depth below which this sinking material cannot accumulate on the seabed. It dissolves completely in the corrosive deep water before it can be incorporated into the sediment. This boundary separates deep-sea areas where carbonate sediment can be preserved from those where it is entirely absent.
The Chemical Mechanism of Dissolution
The CCD is controlled by the changing chemistry and physics of seawater with increasing depth. As calcium carbonate shells sink, pressure and temperature increase their solubility. The immense pressure at great depths increases the tendency of \(text{CaCO}_3\) to dissolve, and the near-freezing temperatures of the deep ocean also enhance this solubility.
The most significant factor is the increasing concentration of dissolved carbon dioxide (\(text{CO}_2\)) in the deep water. When organisms die and sink, their organic matter decomposes, releasing \(text{CO}_2\) through respiration. This \(text{CO}_2\) then reacts with water to form carbonic acid (\(text{H}_2text{CO}_3\)), making the deep water more acidic.
The resulting acidity lowers the concentration of carbonate ions (\(text{CO}_3^{2-}\)), which are needed to keep calcium carbonate from dissolving. When the water becomes undersaturated with respect to \(text{CaCO}_3\), the carbonate shells begin to dissolve rapidly. This chemical process, combined with pressure and temperature, ensures the dissolution rate of \(text{CaCO}_3\) overwhelms the rate of its supply from the surface.
Locating the Lysocline and Compensation Depth
The transition from preserved carbonate sediment to dissolved material occurs across a zone defined by two distinct boundaries. The first is the lysocline, the depth at which the dissolution of calcium carbonate shells begins to increase rapidly. Above the lysocline, carbonate preservation is good, but below this boundary, the shells become increasingly corroded and fragmented.
The CCD is located deeper than the lysocline and marks the point where dissolution is complete, meaning no calcium carbonate material is preserved in the sediment. The difference in sediment composition above and below the CCD provides physical evidence of this boundary. Above the CCD, the seafloor is covered with a fine-grained deposit called calcareous ooze, which is rich in the microscopic shells of plankton.
In contrast, seafloor sediment collected from below the CCD is carbonate-free. It consists instead of fine-grained terrigenous material, volcanic ash, and the shells of siliceous organisms. This material is often referred to as red clay or abyssal clay.
Factors Influencing Depth Variability
The depth of the Carbonate Compensation Depth is a dynamic boundary that varies significantly between ocean basins and with latitude. For example, the CCD is deeper in the Atlantic Ocean (around 5,000 to 5,500 meters) compared to the Pacific Ocean (typically 4,200 to 4,500 meters). This difference is largely due to the global pattern of deep-water circulation.
Deep water in the Atlantic is relatively “young,” having recently been in contact with the atmosphere, meaning it has lower concentrations of dissolved \(text{CO}_2\) and is less corrosive. As this deep water travels across the globe, particularly through the Pacific, it accumulates more \(text{CO}_2\) from decomposing organic matter, making it older and more acidic. This causes the CCD to shoal, or become shallower.
The rate of biological productivity in the surface waters also influences the CCD. High productivity leads to a greater flux of sinking organic matter and carbonate shells, which increases the amount of \(text{CO}_2\) released at depth, driving the CCD to a shallower position. The increasing concentration of atmospheric \(text{CO}_2\) from human activity is also causing ocean acidification, which increases deep-water acidity and is causing the CCD to rise globally.
Reading Earth History in the Sediments
The Carbonate Compensation Depth is a powerful tool for reconstructing the history of Earth’s climate and ocean chemistry. The position of the CCD acts as a direct proxy for the ocean’s saturation state with respect to calcium carbonate. By analyzing deep-sea sediment cores, geologists can track the rise and fall of the CCD over millions of years.
A shallower CCD in the past indicates the deep ocean was more acidic, often due to higher atmospheric \(text{CO}_2\) levels. A deeper CCD suggests a less acidic, more saturated ocean. Analyzing these shifts allows researchers to identify past episodes of rapid ocean acidification and changes in the global carbon cycle. The record preserved in these sediments provides a long-term context for understanding how the ocean responds to major changes in the Earth system.