THE DEPOSITIONAL ENVIRONMENT OF THE LAPTEV SEA CONTINENTAL-MARGIN - PRELIMINARY-RESULTS FROM THE R V POLARSTERN ARK IX-4 CRUISE/

Marine geological investigations were performed across the Laptev Sea continental shelf and slope. Thirty sampling sites were selected cover ing a depth range of ca 3500 m. Maximum core recovery was 9 m. PARASOU ND sub-bottom profiling was used for site surveying and provided impor tant information on the depositional environment of the continental ma rgin together with sedimentological and stratigraphical investigations . Undisturbed horizontal layering of the sea-floor sediments is a comm on feature for the Laptev Sea shelf. There is no indication for glacia tion of the broad shelf region during the Last Glacial, since moraine deposits are missing. However, a high number of plough marks in places points to recent to sub-recent ice-erosion which has led to an intens ive sediment reworking on the shelf. Several broadly incised river cha nnels recorded near the shelf edge are related to Pleistocene drainage systems of large Siberian rivers which cut into the dry shelves durin g the Last Glacial Maximum and were subsequently filled during the Hol ocene. During the Last Glacial we therefore suspect a significant fres hwater contribution from the Eurasian continent to the Arctic Ocean. T he composition of the normally consolidated core sediments indicates a strong flux of terrigenous material, which is mainly provided by the Siberian rivers. Currents distributing the suspension load and sea ice are supposedly major agents transporting sediments across the shelf t o the central arctic deep sea basin. Sediment cores from the upper and middle continental slope exhibit only minor lithological changes. Bio turbated, fine-grained sediments with high organic carbon contents dom inate. The presence of free hydrogen sulphide gas within the sediment column indicates that an intense decay of organic matter under reducin g conditions is taking place. Sedimentation rates are estimated to be ca. 50 cm/1000 years at the upper slope of the western Laptev Sea, bei ng approximately 10 times higher than at the continental rise. The sub oxic to anoxic environment diminishes at deep sea sites of the western Laptev Sea, where sedimentation rates and influx of organic matter ar e reduced.