Terrigenous sediment export from the western margin of South Africa on glacial to interglacial cycles

Abstract

Similar to many shelves globally, Pleistocene terrigenous sediment deposition on the western shelf of South Africa is strongly influenced by high-amplitude sea-level fluctuations over glacial to interglacial climate cycles. Here, variations in the glauconitic quartzose sand and terrigenous mud content of gravity cores from the western slope of South Africa are used to evaluate the dynamics of shelf sediment export over climate cycles of the last 450 kyr. Calcareous ooze on the slope contains a mean of 0.2 to 8 wt.% glauconitic quartzose sand derived from erosion of Neogene outer shelf sediments and 13 to 40 wt.% terrigenous mud sourced from the Orange River. Quartz sand content on the slope increases during glacial periods and is sharply reduced across glacial terminations and throughout interglacial highstands. Relative sea-level changes on the margin estimated from the quartz sand content on the slope are consistent with the timing, but not amplitude, of proposed eustatic sea-level records. A possible mechanism of sand erosion on the outer shelf is the vertical mixing forced by internal tides which may intensify over the shelf break as sea level is lowered during glacials. Turbidites are rare and bottom currents are generally capable of suspending mud but not sand except locally along the margin. Estimated variations in the bulk sedimentation rate and terrigenous mud mass accumulation rate on the slope suggest that export of terrigenous mud off the shelf during glacial lowstands is transported beyond the slope at intermediate and deeper water depths to the Southern Ocean.