Description: A distinct porcellanite layer from the Southwest Indian Ridge intercalated in Pleistocene diatom ooze was studied using nondestructive physical property measurements and sedimentological data. This bed was sampled by two piston cores at a water depth of 2615 m. The 3–5 cm thick porcellanite layer appears in the cores at a depth of 6.03 m (Core PS2089-2) and 7.73 m (Core PS2089-1) below the seafloor. Due to its characteristic physical properties the porcellanite bed can be detected with core measurements, and its distribution and lateral extent mapped with echosounding. The physical index properties, wet bulk density and electrical resistivity, increase significantly across this bed. Magnetic susceptibility is used to compare the lithological units of both cores and to distinguish whether resistivity anomalies are caused by a higher amount of terrigenous components or by the presence of porcellanite. The porcellanite has the special characteristic to affect a positive anomaly in resistivity but not in susceptibility. Most marine sediments, in contrast, show a positive correlation of magnetic susceptibility versus electrical resistivity; therefore a combination of electrical resistivity and magnetic susceptibility logs yields a definite detection of the porcellanite bed. Images from the X-ray CT survey indicate that the porcellanite is lithified and brittle and fragmented when the piston corer penetrated the bed.

Description: Production of biogenic silica and dissolution processes in the water column and surface sediment are important aspects for the investigation and reconstruction of present and past productivity of the ocean. Although the geological record of biogenic silica is often used as a proxy for paleoceanographic processes in the Southern Ocean, little is known about the present regional distribution of biogenic silica flux and accumulation and their relation to primary production in surface waters. Based on more than 130 sediment and pore water samples, the regional differences of the biogenic silica flux to the sea floor of the southern South Atlantic were investigated. In contrast to biogenic silica content, the dissolved Si-flux through the sediment/water interface, caused by intense dissolution of BSi in surface sediments, reflects biogenic production in surface waters. This was inferred by observed increases of Si-fluxes in regions of recurrent polynya formation or in the vicinity of Marginal Ice Zones as at the Weddell-Scotia Sea boundary. In the Scotia Sea, where no benthic fluxes were reported before, we found a considerable burial of biogenic silica and biogenic silica fluxes to the sea floor of ∼800–1300 mmol m-2 a-1. This is a significantly higher flux than derived for the known opal accumulation area in the SE Atlantic, further to the east in the Antarctic Circumpolar Current, where a flux of ∼600–767 mmol m-2 a-1 was observed. This shows that the Scotia Sea is not a gap within the Circumpolar Antarctic Opal Belt as previously assumed. The geochemical budget for different sub-regions of the South Atlantic was considered by a Geographic Information System. In contrast to most previous attempts, this ensures the accurate consideration of the spatial distribution of sampling sites, a crucial aspect for the accuracy of geochemical budgets. For the South Atlantic we calculated the flux of biogenic silica to the sea floor as ∼5.1×1012 mol a-1. Only ∼0.84×1012 mol a-1 is buried in these sediments, which is considerably less than previous estimates.

Description: Surface samples, mostly from abyssal sediments of the South Atlantic, from parts of the equatorial Atlantic, and of the Antarctic Ocean, were investigated for clay content and clay mineral composition. Maps of relative clay mineral content were compiled, which improve previous maps by showing more details, especially at high latitudes. Large-scale relations regarding the origin and transport paths of detrital clay are revealed. High smectite concentrations are observed in abyssal regions, primarily derived from southernmost South America and from minor sources in Southwest Africa. Near submarine volcanoes of the Antarctic Ocean (South Sandwich, Bouvet Island) smectite contents exhibit distinct maxima, which is ascribed to the weathering of altered basalts and volcanic glasses. The illite distribution can be subdivided into five major zones including two maxima revealing both South African and Antarctic sources. A particularly high amount of Mg- and Fe-rich illites are observed close to East Antarctica. They are derived from biotite-bearing crystalline rocks and transported to the west by the East Antarctic Coastal Current. Chiorite and well-crystallized dioctaedral illite are typical minerals enriched within the Subantarctic and Polarfrontal-Zone but of minor importance off East Antarctica. Kaolinite dominates the clay mineral assemblage at low latitudes, where the continental source rocks (West Africa, Brazil) are mainly affected by intensive chemical weathering. Surprisingly, a slight increase of kaolinite is observed in the Enderby Basin and near the Filchner-Ronne Ice shelf. The investigated area can be subdivided into ten, large-scale clay facies zones with characteristic possible source regions and transport paths. Clay mineral assemblages of the largest part of the South Atlantic, especially of the western basins are dominated by chlorite and illite derived from the Antarctic Peninsula and southernmost South America and supported by advection within the Circumantarctic Deep Water flow. In contrast, the East Antarctic provinces are relatively small. Assemblages of the eastern basins north of 30°S are strongly influenced by African sources, controlled by weathering regimes on land and by a complex interaction of wind, river and deep ocean transport. The strong gradient in clay mineral composition at the Brazilian slope indicate a relatively low contribution of tropically derived assemblages to the western basins.