Abstract

Sediments of the Colombia Basin, Caribbean Sea, contain evidence of current winnowing. Although bottom photographs reveal a relatively tranquil bottom over most of the basin, current smoothing of the seafloor is observed on and adjacent to the Hess and Beata Escarpments. Current scour is observed at one location on the Hess Escarpment. The proportion of foraminiferal, coccolith, and clay components in 52 surface sediment samples suggests that fine particles (including coccolith carbonate) are being sorted and removed. The foraminifera to total CaCO 3 ratio (F/C) is highest on and adjacent to the major escarpments and reflects the increased foraminiferal content relative to total CaCO 3 caused by partial removal of the fine fraction coccolith carbonate. The increased currents associated with the escarpments are caused by streamlining and acceleration of water masses as they pass around the obstructions. The lack of erosional bed forms as revealed by bottom photographs and absence of fractionation towards larger foraminifera shows that only fine particles are being sorted and removed. The erosion associated with current smoothing of the bottom is probably responsible for the observed winnowing. Sediment cores from winnowed areas (F/C > .60) have irregular carbonate curves, more small-scale CaCO 3 variation, hiatuses, and mixed faunas when compared to cores from unwinnowed areas (F/C < .50). Thus, areas near escarpments which are being winnowed today have been winnowed throughout the past 150,000 years. Therefore, the geographic extent and intensity of winnowing must be known to interpret properly sediment cores in these areas.

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