Optically Stimulated Luminescence (OSL) dating and palaeoenvironmental studies of pan (playa) sediment from Witpan, South Africa

摘要

Robust. dateable sources of palaeoenvironmental proxy data are scarce in the southwestern Kalahari Desert, and this study investigates the potential of pan (playa) floor sediments as an archive of late Quaternary environmental change. Augering has revealed the presence of up to 3 m of clay- and silt-rich deposits in the base of Witpan, a small pan basin set amongst linear and lunette dunes. The weakly-stratified deposits include aeolian quartz sands and evaporites amongst a clay-rich groundmass, and are believed to have accumulated on a wetter-than-present pan surface. The aeolian quartz fraction has been dated with Optically Stimulated Luminescence (OSL), and reveals accumulation of at least 50 cm of sediment at around 20 ka, and previous accumulation at around 32 ka. The northern and southern sections of the pan are shown to maintain some hydrological independence, attributed to bedrock outcropping, and also show distinct differences in their physical sedimentology. Biogenic proxies are poorly preserved, with neither diatoms nor pollen found in adequate quantities, although phytoliths, found in both the pan-floor sediments and the fringing lunette dune, may offer a more resilient palaeoenvironmental indicator. The possible value of pan floor sediments as dryland archives of physical and chemical, and possibly selected biogenic, palaeoenvironmental proxies, combined with OSL-constrained chronologies, should not be discounted.