Reconstruction of past climatic events using principal comp-onent analysis of the trace element profiles in a sediment core (Joides Basin, Antarctica)

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Antarctic Bottom Water of the world ocean is derived from dense Shelf Water that is carried downslope by gravity currents at specific sites along the Antarctic margins. Data gathered by the AnSlope and CLIMA programs reveal the presence of energetic gravity currents that are formed over the western continental slope of the Ross Sea when High Salinity Shelf Water exits the shelf through Drygalski Trough. Joides Trough, immediately to the east, offers an additional escape route for less saline Shelf Water, while the Glomar Challenger Trough still farther east is a major pathway for export of the once supercooled low-salinity Ice Shelf Water that forms under the Ross Ice Shelf. The Drygalski Trough gravity currents increase in thickness from ∼100 to ∼400 m on proceeding downslope from ∼600 m (the shelf break) to 1200 m (upper slope) sea floor depth, while turning sharply to the west in response to the Coriolis force during their descent. The mean current pathway trends ∼35° downslope from isobaths. Benthic-layer current and thickness are correlated with the bottom water salinity, which exerts the primary control over the benthic-layer density. A 1-year time series of bottom-water current and hydrographic properties obtained on the slope near the 1000 m isobath indicates episodic pulses of Shelf Water export through Drygalski Trough. These cold (<−1 °C), salty (>34.75) pulses correlate with strong downslope bottom flow. Extreme examples occurred during austral summer/fall 2003, comprising concentrated High Salinity Shelf Water (−1.9 °C; 34.79) and approaching 1.5 m s−1 at descent angles as large as ∼60° relative to the isobaths. Such events were most common during November–May, consistent with a northward shift in position of the dense Shelf Water during austral summer. The coldest, saltiest bottom water was measured from mid-April to mid-May 2003. The summer/fall export of High Salinity Shelf Water observed in 2004 was less than that seen in 2003. This difference, if real, may reflect the influence of the large iceberg C-19 over Drygalski Trough until its departure in mid-May 2003, when there was a marked decrease in the coldest, saltiest gravity current adjacent to Drygalski Trough. Northward transport of cold, saline, recently ventilated Antarctic Bottom Water observed in March 2004 off Cape Adare was ∼1.7 Sv, including ∼0.4 Sv of High Salinity Shelf Water.