Present-day biosiliceous sedimentation in the northwestern Ross Sea, Antarctica

Sedimentological and oceanographic inferences have been obtained for the NW Ross Sea using sedimentary Pb-210 as a tracer together with determinations of biogenic silica and organic carbon. Pb-210 chronologies give apparent a ccumulation rates ranging between 14 and 80 mg cm(-2) yr(-1) (0.02-0.12 cm yr(-1)) in the shelf basins. Even if a profile of Pb-210 is present in sedi ments from the top of the banks, here sediment accumulation rate is practic ally null, and physical mixing is responsible for the downward transport of fine particles and associated Pb-210. The accuracy of Pb-210-derived accum ulation rates is discussed with respect to C-14 dates. The annual rate of b iogenic accumulation from Pb-210 appears to be ca. 8 times higher than the value derived using radiocarbon. Bioturbation is probably responsible for t he discrepancy but also temporal and spatial variations in opal accumulatio n play a key-role. Contrasting measured and expected inventories of Pb-210, a residence time of about 50 years has been tentatively estimated for the water in the NW Ross Sea. Furthermore, the data suggest that the pattern of present-day biosiliceous sediment accumulation in the Ross Sea is mainly d riven by biogenic silica production in the water column, the SW area being the most productive part of the Ross Sea, by high sediment accumulation rat e which enhances the seabed preservation, and by hydrodynamics, which is so effective to resuspend fine biogenic particles from the topographic highs. Resuspended particles are then deposited onto the flanks. The material whi ch accumulates in the central part of the basins derives basically from pri mary production and settling through the water column.