Michael Frenz

Coccolith contribution to South Atlantic carbonate sedimentation

In order to assess the significance of calcifying organisms for the carbonate budget it is necessary to have reliable estimates of the relative proportion of carbonate production of the different organism groups. In this chapter we firstly review the carbonate distribution patterns of both bulk coccoliths and planktic foraminifera, estimated by means of different carbonate calculation techniques. These studies clearly reveal a variable pattern of carbonate sedimentation and accumulation for planktic foraminifera and coccolithophores, respectively. Whilst coccolith carbonate dominates the oligotrophic gyres of the South Atlantic, carbonate produced by planktic foraminifera is more important in more fertile, mesotrophic to eutrophic areas, such as the equatorial divergence zone. Mass estimates of coccolith carbonate burial in surface sediments of the South Atlantic document coccoliths to be major carbonate contributors in most mid-Atlantic Ridge sediments, exceeding up to 70 wt.-%. In contrast to these oligotrophic areas, they are of lesser importance in sediments accumulating on the continental margins, where they account for only a fifth of the carbonate fraction.

Surface Sediment Bulk Geochemistry and Grain-Size Composition Related to the Oceanic Circulation along the South American Continental Margin in the Southwest Atlantic

Surface sediments from the South American continental margin surrounding the Argentine Basin were studied with respect to bulk geochemistry (CaCO3 and Corg) and grain-size composition (sand/silt/clay relation and terrigenous silt grain-size distribution). The grain-size distributions of the terrigenous silt fraction were unmixed into three end members (EMs), using an end-member modelling algorithm. Three unimodal EMs appear to satisfactorily explain the variations in the data set of the grain-size distributions of terrigenous silt. The EMs are related to sediment supply by rivers, downslope transport, winnowing, dispersal and re-deposition by currents. The bulk geochemical composition was used to trace the distribution of prominent water masses within the vertical profile. The sediments of the eastern South American continental margin are generally divided into a coarse-grained and carbonate-depleted southwestern part, and a finer-grained and carbonate-rich north eastern part. The transition of both environments is located at the position of the Brazil Malvinas Confluence (BMC). The sediments below the confluence mixing zone of the Malvinas and Brazil Currents and its extensions are characterised by high concentrations of organic carbon, low carbonate contents and high proportions of the intermediate grain-size end member. Tracing these properties, the BMC emerges as a distinct north-south striking feature centered at 52–54°W crossing the continental margin diagonally. Adjacent to this prominent feature in the southwest, the direct detrital sediment discharge of the Rio de la Plata is clearly recognised by a downslope tongue of sand and high proportions of the coarsest EM. A similar coarse grain-size composition extends further south along the continental slope. However, it displays better sorting due to intense winnowing by the vigorous Malvinas Current. Fine-grained sedimentary deposition zones are located at the southwestern deeper part of the Rio Grande Rise and the southern abyssal Brazil Basin, both within the AABW domain. Less conspicuous winnowing/accumulation patterns are indicated north of the La Plata within the NADW level according to the continental margin topography. We demonstrate that combined bulk geochemical and grain-size properties of surface sediments, unmixed with an end-member algorithm, provide a powerful tool to reconstruct the complex interplay of sedimentology and oceanography along a time slice.