Late quaternary changes in Antarctic bottom water velocity inferred from sediment grain size in the northern Weddell Sea

Newly formed bottom water (θ ≤ −0.7°C) in the northern Weddell Sea flows E or NE at speeds up to 10–15 cm/s, with velocity decreasing towards the centre of the Weddell Gyre (preliminary results from long-term current meter moorings). Upper Quaternary sediments from this area contain a fine-grained terrigenous component (from the nepheloid layer) plus biogenic silica (mostly diatoms) with a small amount of ice-rafted debris. In cores from between 61° and 66°S and from 3300 m to 4700 m water-depth, the proportion of biogenic silica increases northwards (corresponding to increasing seasonal extent of open water vs sea-ice cover), and the proportion of silt and well-sorted fine sand in surface sediments increases with average current velocity. Downcore, diatom-rich and diatom-poor sediments alternate on a scale of 1–3 m, and intervals with more diatoms contain a higher proportion of silt to clay. Preliminary stratigraphy suggests the cyclicity in composition and texture is related to glacial-interglacial cyclicity. During warm periods (indirectly correlated with isotope stages 1, 3, 5 etc.) biogenic silica production takes place during several months of each year, and silt and fine sand are transported by bottom currents. During glacial periods with greater sea-ice cover than at present, biogenic productivity was suppressed and no silica was preserved in the sediments: in addition, a lower proportion of terrigenous silt implies that bottom currents were weaker. At sites with a present-day average velocity of 10 cm/s, a Last Glacial Maximum average velocity of 1 cm/s or less is inferred from grain-size measurements.