Particle fluxes, benthic processes and the palaeoenvironmental record in the Northern Weddell Sea

At the northern edge of the Weddell Gyre, Antarctica, fine-grained sediments accumulate beneath eastward-flowing Weddell Sea Bottom Water. We report on sediment trap material and core tops from four sites, with relevant water-column data. In this cold, low-productivity oceanographic setting it is the processes in the benthic nepheloid layer (resuspension, transport, winnowing of fines) and in the upper few cm of sediment (bioturbation, dissolution of biogenic silica and carbonate, degradation of organic matter) that dominate the palaeoenvironmental record. Biogenic productivity is low and seasonal because of sea-ice cover; terrigenous sediment is supplied by resuspension of Weddell Sea slope and rise sediments and by ice-rafting. The benthic nepheloid layer is up to 150 m thick, with particulate matter concentrations of up to 0.51 mg/l. Sediment traps moored for 2 years 827 m above the seabed collected mainly biogenic opal with well-preserved and diverse diatoms and radiolarians. Traps 21 m above the seabed recorded additional high fluxes of terrigenous silt and clay. Core tops are terrigenous with very little organic matter or opal except at the northernmost site and a very restricted diatom assemblage. Core tops consist of silty clay or sandy silty clay, depending on current regime; the proportion of fine sand is related to the annual frequency of high ( > 15 cm/sec) current speeds at each site. AMS 14C dates on organic carbon in sediment traps are 965±50 and 1895±55 radiocarbon years, reflecting the high reservoir age of Southern Ocean surface waters. Core top ages of particulate organic carbon are 10500 and 13000 calendar years B.P. These high ages are related to the glacial-age source of resuspended slope and rise sediments. Cores from this area contain a textural record of Weddell Sea Bottom Water flow, but only fragmentary data on palaeo-surface conditions.

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