Biogenic sedimentation in McMurdo Sound, Antarctica

Abstract

Surface sediments from eastern McMurdo Sound and fjord basins of the Victoria Land Coast are enriched in organic carbon and opal. At depths below 600 m, opal contents commonly exceed 30%; organic carbon contents average 1.5% and are as high as 3.5%. Opal and organic carbon are supplied by local production within and below sea ice and by advection from open water areas of the southwestern Ross Sea. The distribution of organic carbon and opal at the sea floor is consistent with cyclonic (clockwise) water circulation in McMurdo Sound. Advective transport from the Ross Sea supplies biogenic sediment to eastern and northern McMurdo Sound; the southwestern shelf is bathed by waters derived in part from beneath the Ross Ice Shelf which transport very little allochthonous carbon. The supply of biogenic debris in southwestern McMurdo Sound is further curtailed by sea-ice conditions, e.g., more prevalent multi-year sea ice which reduces photosynthesis, and the absence of summer basal melting and ice breakout which restricts the flux of sea ice and open-water production to the sea floor. Consistent with this hypothesis, organic carbon fluxes measured via sediment trapping beneath fast ice are one to two orders of magnitude higher in eastern versus western McMurdo Sound. Pronounced cross-sound gradients in shallow water benthic biomass and species diversity have previously been attributed to east-west variations in productivity. Our surface sediment data suggest that similar or even more dramatic contrasts in benthic community structure may exist in the deeper water areas of McMurdo Sound. Pervasive resuspension is documented by pronounced near-bottom flux increases measured by sediment trapping. Resuspension events, which may be more common during the austral winter, act to winnow and transport biogenic debris and fine terrigenous sediment from shallow regions (< 400 m) to protected shelf basins. Despite oxidative loss within the water column, the accumulation rate of organic carbon in the deep basins of McMurdo Sound averages 45 mg C m −2 day −1, more than an order of magnitude higher than the world average organic carbon flux to continental margins and equivalent to accumulation rates observed in many anoxic settings. Preservation efficiency of opal during transport and deposition appears anomalously high and the most opal-rich sediments of McMurdo Sound are depleted in organic carbon relative to mid- and low-latitude siliceous deposits. Despite this observation, if similar patterns of biogenic sedimentation exist on many other parts of the shelf, the Antarctic continental margin is an important sink for sedimentary organic carbon, as has been previously suggested for the silica system.