Age determination of sediment cores of the southeastern Weddell Sea, Antarctica

On the continental margin of the southeastern Weddell Sea, Antarctica, several channel-ridge systems can be traced on the eastern side of the Crary Fan. Swath mapping of the bathymetry reveals three southwest-northeast trending ridges up to 300 m high with channels on their southeastern side. The structures occur on a terrace of the continental slope in water depths of 2000 - 3300 m. We carried out sedimentological studies on cores from three sites. Two of the studied cores are from ridges, one is from the northwestern part of the terrace. The stratigraphy of the recovered sediments is based on accelerator mass spectrometer 14C determinations, stable oxygen and carbon isotopes analyses and paleomagnetic measurements. The sediments represent a period from the last glacial maximum (LGM) to recent time. They are composed predominantly of terrigenous components. We distinguish four different sedimentary facies and assign them to processes controlling sedimentation. Microlaminated muds and cross-stratified coarse-silty sediments originated from contour currents. Bioturbated sediments reflect the increasing influence of hemipelagic sedimentation. Structureless sediments with high contents of ice-rafted debris characterize slumps. The inferred contour currents shaping the continental slope during the LGM were canalized within the channels and supplied microlaminated mud to the western sedimentary ridges due to deflection to the left induced by the Coriolis force. The lamination of the sediments is attributed to seasonal variations of current velocities. The thermohaline bottom currents were directed to the northeast and hence opposite to the Weddell Gyre. Cross-stratified coarse-silty contourites on the ridges are intercalated with the muds and indicate spillover of faster thermohaline flows. Average sedimentation rates on the terrace of the continental slope were unusually high (250 cm/ka) during the LGM, indicating active growth phases of the Crary Fan during glacial intervals. A substantial environmental change at 19.5 - 20 ka is documented in the sediments by a gradual change from lamination to bioturbation. During the recent interglacial, bioturbated sediments were deposited in all parts of the terrace. Because of a reduction of the contour current velocities (4-7 cm/s), the water masses of the Weddell Gyre, supplying fine-grained sediments from northeast, gain a greater influence on sedimentation on the continental slope. Higher percentages of microfossils indicate enhanced biogenic productivity. Increased iceberg activity is documented by greater amounts of ice-rafted debris. The interglacial sedimentation rates decrease to a few cm/ka and indicate that the Crary Fan became relatively sediment-starved during interglacial intervals.

Sedimentological, geochemical, micropaleontological and rock magnetic proxies of sediment core GeoB6211-2

Surface currents and sediment distribution of the SE South American upper continental margin are under influence of the South American Monsoon System (SAMS) and the Southern Westerly Wind Belt (SWWB). Both climatic systems determine the meridional position of the Subtropical Shelf Front (STSF) and probably also of the Brazil-Malvinas Confluence (BMC). We reconstruct the changing impact of the SAMS and the SWWB on sediment composition at the upper Rio Grande Cone off southern Brazil during the last 14 cal kyr combining sedimentological, geochemical, micropaleontological and rock magnetic proxies of marine sediment core GeoB 6211-2. Sharp reciprocal changes in ferri- and paramagnetic mineral content and prominent grain-size shifts give strong clues to systematic source changes and transport modes of these mostly terrigenous sediments. Our interpretations support the assumption that the SAMS over SE South America was weaker than today during most of the Late Glacial and entire Early Holocene, while the SWWB was contracted to more southern latitudes, resembling modern austral summer-like conditions. In consequence, the STSF and the BMC were driven to more southern positions than today's, favoring the deposition of Fe-rich but weakly magnetic La Plata River silts at the Rio Grande Cone. During the Mid Holocene, the northern boundary of the SWWB migrated northward, while the STSF reached its northernmost position of the last 14 cal kyr and the BMC most likely arrived at its modern position. This shift enabled the transport of Antarctic diatoms and more strongly magnetic Argentinean shelf sands to the Rio Grande Cone, while sediment contributions from the La Plata River became less important. During the Late Holocene, the modern El Niño Southern Oscillation set in and the SAMS and the austral tradewinds intensified, causing a southward shift of the STSF to its modern position. This reinforced a significant deposition of La Plata River silts at the Rio Grande Cone. These higher magnetic silts with intermediate Fe contents mirror the modern more humid terrestrial climatic conditions over SE South America.