The depositional environment of the Laptev Sea continental margin: preliminary results from the R/V Polarstern ARK IX-4 cruise

Abstract

Marine geological investigations were performed across the Laptev Sea continental shelf and slope. Thirty sampling sites were selected covering a depth range of ca 3500 m. Maximum core recovery was 9 m. PARASOUND sub-bottom profiling was used for site surveying and provided important information on the depositional environment of the continental margin together with sedimentological and stratigraphical investigations. Undisturbed horizontal layering of the sea-floor sediments is a common feature for the Laptev Sea shelf. There is no indication for glaciation 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 intensive sediment reworking on the shelf. Several broadly incised river channels recorded near the shelf edge are related to Pleistocene drainage systems of large Siberian rivers which cut into the dry shelves during the Last Glacial Maximum and were subsequently filled during the Holocene. During the Last Glacial we therefore suspect a significant freshwater contribution from the Eurasian continent to the Arctic Oceans. The 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 to the central arctic deep sea basin. Sediment cores from the upper and middle continental slope exhibit only minor lithological changes. Bioturbated, fine-grained sediments with high organic carbon contents dominate. The presence of free hydrogen sulphide gas within the sediment column indicates that an intense decay of organic matter under reducing conditions is taking place. Sedimentation rates are estimated to be ca. 50 cm/1000 years at the upper slope of the western Laptev Sea, being approximately 10 times higher than at the continental rise. The suboxic to anoxic environment diminishes at deep sea sites of the western Laptev Sea, where sedimentation rates and influx of organic matter are reduced.