SAND RAMP EVIDENCE FROM SOUTHERN NAMIBIA OF LATE QUATERNARY EPISODES OF GREATER ARIDITY AND STRONGER WINDS

Our understanding of late Quaternary paleoclimate conditions in arid southwest Africa is largely based on the chronology of geomorphic features such as aeolian, fluvial and lake deposits. In the past two decades, numerous optically stimulated luminescence (OSL) ages of aeolian deposits have been obtained from the Kalahari and Namib deserts but interpretation of these ages in terms of paleoclimate inferences is challenging because the ages correspond to periods of both arid and humid conditions inferred from other records. Sand ramps are common in several valleys of southern Namibia but have received little attention. Compared to linear dunes, they contain complex internal structure with alternating aeolian and non-aeolian units associated with different depositional environments, thus they are a better source of paleoenvironmental information. Here we report the stratigraphy and OSL chronology of a sand ramp in the Konkiep River valley of southern Namibia, and discuss its significance in understanding late Quaternary paleoclimate of southwest Africa.

Variations in grain size and magnetic susceptibility of sediments exposed by a 2.3 m-deep pit dug into the sand ramp (Site 1) suggest two periods of active sand accumulation by stronger winds, each followed by a period of colluvial deposition and weak pedogenesis under wetter conditions. Two aeolian depositional sequences are also visible in a natural exposure along one side of the ramp (Site 2). Nine OSL samples from Site 1, taken at 25 cm depth intervals, and two samples from the aeolian units exposed at Site 2 show that aeolian deposition occurred at 46-31 and 22-17 ka, with the most rapid accumulation from 22.2 ka to 21.6 ka, during the peak of the Last Glacial Maximum. Aeolian deposition was also more rapid at 46-44, 32-31, and 18-16.8 ka, during Northern Hemisphere Heinrich Events H5, H3, and H1, respectively. These periods of increased aeolian activity correlate with evidence of enhanced aridity and strong trade winds in Southeast Atlantic marine cores. We infer that in southwest Africa, the periods of 46-31 ka and 22-17 ka were much drier and windier than today because of a strengthened South Atlantic Anticyclone; and conditions at 31-22 ka and after 17ka were wetter, due to enhanced summer rainfall.