CCD (Carbonate Compensation Depth)

CCD (Carbonate Compensation Depth)

In oceanography , the depth where carbonate ions under saturation in the water column or in the sediment pore and the water interface is large enough so that the rate of calcium carbonate (CaCO₃) sedimentation is totally compensated for by the rate of calcium carbonate dissolution, reaches the carbonate compensation depth (CCD). Alternately stated, the CCD is the depth at which calcareous skeletons of marine animals accumulate at the same rate at which they dissolve. Depending on the mineral structure of CaCO₃, the CCD is called calcite

compensation depth (trigonal structure) or aragonite compensation depth (rhombic structure), respectively.

Foraminifera, coccolithophorids, pteropods, and a few other benthic and planktic organisms build calcium carbonate shells or skeletons. Upon death or reproduction, the shells are discarded and sink to the sea-floor. Within the water column, calcium (Ca²⁺) content varies little, hence the calcium carbonate saturation state (CSS) is controlled by concentration of carbonate (CO₃²⁻) ions, pH , water pressure, temperature , and salinity:

CSS = (Ca²⁺) × (CO₃²⁻) ÷ K'_sp

whereas K'_sp is the equilibrium solubility product for the mineral phase of calcite or aragonite, respectively. It is CSS: supersaturated > 1 = saturated = 1 > undersaturated with carbonate ions. Position and thickness of the saturation horizon in the water column can be defined as the difference ΔCO₃²⁻ between the concentration of carbonate ions in situ and the concentration of saturated carbonate ion for the respective mineral phase. Since the concentration of carbonate ions cannot be measured directly, it is calculated using the dissociation constants of carbonic acid (H₂CO₃), and measurable parameters such as total inorganic carbon dioxide (Σ CO₂) dissolved in sea water, alkalinity, pH, and partial pressure of carbon dioxide exerted by sea water (pCO₂).

The water depth at which the sea water carbonate ion content and the concentration of carbonate ions in equilibrium with sea water for calcite or aragonite mineral phase intercept is called hydrographic calcite or aragonite lysocline, respectively. Below the lysocline, calcium carbonate dissolution begins and becomes progressively more intense in proportion to the fourth power of carbonate ion undersaturation. An undersaturation of about 10 μmol/kg is enough to dissolve almost all the calcite descending to the sea floor. At the CCD, the rate of calcium carbonate sedimentation is totally compensated for by the rate of calcium carbonate dissolution. These deepest parts of the global ocean, the abyssal plains , where depths exceed about 17,500 feet (4,500 meters), the bottom is no more covered with calcareous ooze but with a layer of redclay that contains no fossils at all.

From the surface water as the place of life-history down to the sedimentological archive, diversity of calcareous organism assemblages changes qualitatively and quantitatively due to carbonate dissolution. Each type of calcium carbonate shell architecture yields different crash behavior regarding foraminifera, coccolithophorids, pteropods, and other planktic and benthic calcium carbonate skeleton bearing organisms. For example, planktic foraminifer Globigerinoides ruber is rapidly dismantling into single chambers along sutures whereas Neogloboquadrina pachyderma is undergoing long lasting ultrastructural breakdown before the final smash. That is, calcium carbonate dissolution is a progress affecting individuals (of any level in Linné's system) in different extent.

Disintegration of calcium carbonate skeleton bearing organisms is a valuable tool to reconstruct modern (i.e., Holocene) and ancient (e.g., Pleistocene) deep and bottom water currents that are traceable through their different CO₂accumulation. CCD is found deepest in the North Atlantic Ocean (50°N) at about 5,000 m moving upwards continuously in the water column to 3,000 m in the Atlantic sector of the Southern Ocean (60°S) and, in turn, CCD is found deepest in the Pacific sector of the Southern Ocean (60°S) at about 4,500 m moving upwards to 3,000 m in the North Pacific Ocean (50°N). In more general terms, the CCD appears to coincide with the calcium carbonate saturation state of 0.75 in the Atlantic and 0.65 in the Pacific.

Reconstructing CCD of modern and ancient oceans means to elucidate the role that oceanic deep-water processes play in global climate change during various geologic time intervals and at different levels of precision. The location of CCD, lysocline and saturation horizon determine deep water CO₃²⁻ concentrations and thus the pCO₂ of surface waters. Hence, the ocean's ability to take up atmospheric pCO₂ is influenced by the balance of production and dissolution of calcium carbonate, and lifting or lowering the CCD has important consequences on the short and long term variations of CO₂ in the atmosphere.

See also Ocean circulation and currents