Atlantic Ocean carbonate compensation depth

Figure 7. The depth distribution of the Ro and calcite saturation levels, the foraminiferal lysocline and the calcium carbonate compensation depth in the Western and Eastern Atlantic Ocean (after Ref. 40)...

Observations from studies of surface sediments have allowed definition of regionally varying levels in the ocean at which pronounced changes in the presence or preservation of calcium carbonate result from the depth-dependent increase of dissolution on the seafloor. The first such level to be identified was simply the depth boundary in the ocean separating carbonate-rich sediments above from carbonate-free sediments below. This level is termed the calcite (or carbonate) compensation depth (CCD) and represents the depth at which the rate of carbonate dissolution on the seafloor exactly balances the rate of carbonate supply from the overlying surface waters. Because the supply and dissolution rates of carbonate differ from place to place in the ocean, the depth of the CCD is variable. In the Pacific, the CCD is typically found at depths between about 3500 and 4500 m. In the North Atlantic and parts of the South Atlantic, it is found... 

Atlantic Ocean Nodule abundance in the Atlantic Ocean appears to be more limited than in the Pacific or Indian Oceans, probably as a result of its relatively high sedimentation rates. Another feature which inhibits nodule abundance in the Atlantic is that much of the seafloor is above the calcium carbonate compensation depth (CCD). The areas of the Atlantic where nodules do occur in appreciable amounts are those where sedimentation is inhibited. The deep water basins on either side of the Mid-Atlantic Ridge which are below the CCD and which accumulate only limited sediment contain nodules in reasonable abundance, particularly in the western Atlantic. Similarly, there is a widespread occurrence of nodules and encrustations in the Drake Passage-Scotia Sea area probably due to the strong bottom currents under the Circum-Antarctic current inhibiting sediment deposition in this region. Abundant nodule deposits on the Blake Plateau can also be related to high bottom currents. 

A third example is the increasing solubility of calcite with depth in the ocean. Surface seawater is generally supersaturated with respect to calcite, whereas at depths below about 500 m the oceans become undersaturated with calcite (cf. Drever 1988). For this reason there is a tendency for carbonate materials to dissolve and disappear with increasing depth as they settle into ocean bottom sediments. Reasons for the shift to undersaturation with increasing ocean depth include declining temperatures, increased pressure (see Chap. 1, Section 1.6.2), and increasing CO2 concentrations. The depth at which a pronounced reduction in the carbonate content of ocean sediments is observed is termed the calcite compensation depth (CCD). Because of the slow rate at which the calcite dissolves, the CCD is found well below the approximately 500 m depth at which calcite becomes undersaturated. The CCD is thus at about 3.5 km depth in the Pacific Ocean and 5 km depth in the Atlantic Ocean (Drever 1988). 

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