A sediment record of cyclic bottom-current variability from the northwest Weddell Sea

Cores and 3.5 kHz, sub-bottom profiles collected from the continental slope and rise near 64°S, 52°W, off the tip of the Antarctic Peninsula, reveal transport and depositional processes in a high-energy environment. A large channel runs downslope from the shelf break; the remaining slope area is smooth and shows no acoustic penetration, except locally in the lee of topography. Sediments on the slope and rise were mainly deposited by suspension from a 150 to 300 m thick nepheloid layer. Periodic resuspension and winnowing by bottom currents has produced coarse lags of ice-rafted debris throughout these deposits and has also produced an elongated field of mudwaves running parallel to the base of the slope. These low-amplitude, asymmetric features were formed by currents flowing from west to east, parallel to the local slope. Short gravity cores show a compositional and textural cyclicity developed between two distinct lithologies. Intervals of poorly sorted sediment, barren of fauna and with mud-dominated size distributions, are interpreted to result from a cold climate and low bottom-current strength. Production of Weddell Sea Bottom Water probably lessened during these times and there was a greater duration of sea-ice cover. These sediments are interbedded with winnowed and better-sorted deposits comprising fine sands and silts that contain abundant planktonic foraminifera, interpreted as implying warmer climatic conditions and stronger current activity. A varied calcareous benthic foraminiferal assemblage also indicates that these intervals lay above the CCD during deposition. Renewed and invigorated production of bottom water is implied, contemporaneous with a shorter duration of sea-ice cover. Petrographic evidence indicates an Antarctic Peninsula source throughout. The cycles are considered to be the result of climatic fluctuations occurring over the timespan of glacial interstadials, although confirmation of this timescale awaits acquisition of an absolute chronology.

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