ALBERT

Description: The main objective of the study was to investigate seasonal sediment dynarnics on the Laptev Sea shelf. The Laptev Sea comprises one of the largest Siberian shelf areas and is characterized by seasonal ice coverage and thus, by a strong seasonality in sediment input. The pathways and the final fate of the sediments derived from the Siberian hinterland are central questions for understanding the complex land-shelf-ocean interactions and their seasonal variations. In order to characterize seasonal variations in suspended particulate matter (SPM) dynamics on the eastem Laptev Sea shelf, one-year Acoustic Doppler Current Profiler (ADCP) records and complementary optical backscatter profiles from the ice-free period were analyzed. In order to use indirect measuring devices for the quantification of SPM concentration, optical (turbidity meter) and acoustic (ADCP) backscatter sensors were compared to assess their potential for the investigation of SPM dynamics on the Laptev Sea shelf. To estimate SPM concentrations from optical backscatter signals, these were converted using the linear relation between the backscatter signals and SPM concentrations derived from filtered water samples. Applying the theoretical interaction of sound in the water to SPM, the acoustic backscatter signals were transformed adapting a previously established approach. SPM concentrations estimated from the backscattered signals of both sensors showed a close similarity to SPM concentrations obtained from filtered water samples. In general both the ADCPs and the turbidity meters provided good estimations, with ADCPs underestimating and turbidity meters slightly overestimating SPM concentrations. Hence, both sensors can be used for the deterrnination of SPM dynamics On the Laptev Sea shelf with its comparably low SPM concentrations. However, ADCPs are more convenient for investigation of sediment transport dynamics as they provide reasonable SPM concentration and current records for the entire water colurnn simultaneously. Combined turbidity meter, pigment, plankton, and current records were analyzed to describe the con~positiont,r ansport dynamics, and short-term variability of SPM in the nepheloid layers (i.e., layers of increased SPM concentration in the water column) during the ice-free period. The combined measurements indicate that most of the sediment transport takes place in the bottom nepheloid layer On the eastem and the central Laptev Sea shelf. The bottom nepheloid layer comprises riverine material, resuspended bottom material, and decaying organic matter from the upper water column. The SPM concentration within the bottom nepheloid layer decreases from south to north and from east to west, respectively, mainly due to dispersion. On the inner shelf in the vicinity of the Lena Delta the SPM concentration in the surface nepheloid layer is strongly dependent On riverine discharge. On the mid-shelf the formation and dynamics of the surface layer are mainly related to changes in phytoplankton biomass and zooplankton migration. On the eastem Laptev Sea shelf paleo-river valleys act as transport conduits during the ice-free period, where bottom material is resuspended On the mid-shelf during and after storm events and transported onto the inner shelf. On the central Laptev Sea shelf resuspension events seem to be less common and SPM is mainly transported over the continental margin into the deep Arctic Ocean. To investigate seasonal variations in SPM dynamics on the eastem Laptev Sea shelf, one-year records On currents and SPM concentrations were examined. The data indicated that during and shortly after the river-ice breakup (June to early July) sediment transport on the inner shelf is dominated by riverine input and transport onto the mid-shelf within the surface nepheloid layer. When ice-free conditions prevail (mid-July to September), SPM is mainly trapped on the eastern Laptev Sea shelf: SPM discharged by the Lena River is transported within the surface layer onto the mid-shelf, where it sinks through the water column into the bottom nepheloid layer. In the bottom layer it is transported back onto the inner shelf with additional bottom material, which was resuspended during and after storm events. On the inner shelf the material is partly conveyed back into the surface layer by turbid mixing and carried out onto the shelf again. During freeze-up (October) SPM in the surface layer on the inner shelf is rather incorporated into newly formed ice and partly transported with the ice over the continental margin into the deep Arctic Ocean. Beneath the ice Cover (November to JuneIJuly) on the inner shelf SPM slowly sinks and sediment transport is of minor importance. However, beneath the polynya bottom material is still resuspended after storrn events and transported onto the inner shelf where it temporarily settles. The data suggest a quasi-estuarine sediment circulation and a sediment export dominated by ice export rather than bottom transport on the eastem Laptev Sea shelf. Since for the first time currents and SPM concentrations were recorded simultaneously for a one-year period, the unique dataset gave new insights into sediment dynamics on the Laptev Sea shelf and its complex land-shelf-ocean interactions. The data provided the basis for a conceptual model of sediment transport on the Laptev Sea shelf, which emphasizes the significance of sea ice export for the sediment budget of the Laptev Sea shelf and as a sediment source for the deep Arctic Ocean. The conceptual model can presumably be extended to other Siberian shelf seas.