Marine controls on modern sedimentation on the Antarctic continental shelf

Abstract

The extreme depth and rugged topography of the Antarctic continental shelf, its seasonal to perennial sea ice cover and its large, slowly calving ice shelves exert major influences on the adjacent ocean and sediments. Regionally variable exchanges of heat and momentum between the atmosphere and ocean result in net off-shelf transport of sea ice and net downward salt flux. This buoyancy partitioning drives a vertical convection that is followed by westward and northward flows of “ventilated” dense water across the continental shelves. Shelf waters meet, mix with and modify circumpolar deep water at a dynamic slope front near the continental shelf break, locally or periodically forming Antarctic Bottom Water. Nutrient-rich deep water upwells along this front, regionally intruding onto the continental shelf in long, thermally attenuated filaments. These intrusions are frequently linked to mass balance processes at the base of ice tongues and ice shelves. Melting, freezing, and tidal currents beneath glacial ice are related to the larger scale circulation and to the fate of basal debris in the ice sheet. The ocean circulation near ice fronts, and decadal variability in ice-front positions may account for several features of the sediment record. Time series (> 1 yr) current measurements in the Ross and Weddell seas have revealed moderate to strong currents that are seasonally variable, vertically coherent and dominated by the diurnal tide and longer term events. Few long-term current measurements exist in the shelf and slope bottom boundary layers that are most important to sediment deposition and erosion. Icebergs may frequent certain routes, e.g., in association with currents and fronts near the edge of the continental shelf, and are capable of scouring and resuspending sediments down to shelf-break depths. Sea ice plays a role in shelf sedimentation by rafting eolian and bottom debris away from the nearshore regions. In addition, generally offshore winds produce numerous leads and polynyas along the coastline and within the sea-ice fields. Higher biological productivity in these polynyas may be linked to higher local sedimentation rates. Biogenic marine sediments apparently comprise a major percentage of the regionally variable modern shelf deposits, despite the impressive glacial setting and polar environment.