Contribution of Calcareous Plankton Groups to the Carbonate Budget of South Atlantic Surface Sediments

A total of more than 400 surface sediment samples from the equatorial, central and subpolar South Atlantic Ocean were investigated for their carbonate content as well as for the carbonate contribution from various calcareous plankton groups. The modem pattern of marine carbonate production is exemplified by comparing two sediment traps located in different domains of the South Atlantic. In addition, this paper presents new carbonate calculations for the content of coccoliths, calcareous dinocysts, planktic foraminifera, and pteropods in surface sediments. In general, carbonate input of the different organism groups is highly variable although dominated by both planktic foraminifera and coccolithophorids. Whereas coccolith carbonate dominates the oligotrophic gyres of the South Atlantic, carbonate derived from planktic foraminifera is much more important in more fertile, mesotrophic areas, such as the equatorial divergence zone. In contrast, calcareous dinocysts only supply a minor proportion of calcium carbonate to the sediments. The aragonite content, mainly derived from pteropod shells, is of regional importance at the continental margin of the western South Atlantic. Here, aragonite contents of up to 50 wt-% of the total sediments were measured. Carbonate dissolution has a major effect below the lysocline depth, but also in highly productive areas (supralysoclinal dissolution). Foraminiferal carbonate is much more affected by dissolution than either coccolith or calcareous dinocyst carbonate. Preservation of pteropod shells is restricted to relatively shallow parts of the ocean distant from continental margins, as aragonite is much more susceptible to dissolution than calcite. As a result, the maximum aragonite content is observed at an intermediate depth, i.e. between 2000 to 3000m.