SEDIMENTATION PROCESSES WITHIN CHANNEL-RIDGE SYSTEMS, 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 th e Crary Fan. Swath mapping of the bathymetry reveals three southwest-n ortheast trending ridges up to 300 m high with channels on their south eastern side. The structures occur on a terrace of the continental slo pe in water depths of 2000 - 3300 m. We carried out sedimentological s tudies on cores from three sites. Two of the studied cores are from ri dges, one is from the northwestern part of the terrace. The stratigrap hy of the recovered sediments is based on accelerator mass spectromete r C-14 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 predomina ntly of terrigenous components. We distinguish four different sediment ary facies and assign them to processes controlling sedimentation. Mic rolaminated muds and cross-stratified coarse-silty sediments originate d from contour currents. Bioturbated sediments reflect the increasing influence of hemipelagic sedimentation. Structureless sediments with h igh contents of ice-rafted debris characterize slumps. The inferred co ntour currents shaping the continental slope during the LGM were canal ized within the channels and supplied microlaminated mud to the wester n sedimentary ridges due to deflection to the left induced by the Cori olis force. The lamination of the sediments is attributed to seasonal variations of current velocities. The thermohaline bottom currents wer e directed to the northeast and hence opposite to the Weddell Gyre. Cr oss stratified coarse-silty contourites on the ridges are intercalated with the muds and indicate spillover of faster thermohaline flows. Av erage sedimentation rates on the terrace of the continental slope were unusually high (250 cm/ka) during the LGM, indicating active growth p hases of the Crary Fan during glacial intervals. A substantial environ mental change at 19.5 - 20 ka is documented in the sediments by a grad ual change from lamination to bioturbation. During the recent intergla cial, 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 sedime nts from northeast, gain a greater influence on sedimentation on the c ontinental slope. Higher percentages of microfossils indicate enhanced biogenic productivity. Increased iceberg activity is documented by gr eater amounts of ice-rafted debris. The interglacial sedimentation rat es decrease to a few cm/ka and indicate that the Crary Fan became rela tively sediment-starved during interglacial intervals.