LATE QUATERNARY ENVIRONMENTAL CHANGES FROM AEOLIAN AND FLUVIAL GEOARCHIVES IN THE SOUTHWESTERN KALAHARI,SOUTH AFRICA: IMPLICATIONS FOR PAST AFRICAN CLIMATE DYNAMICS

摘要

Precipitation in southern Africa and specifically in the southwest Kalahari – tropical summer rains in the north and the east, and extratropical winter rains in the southwest–is strongly influenced by a common climate force that also governs the seasonal shift of the Intertropical Convergence Zone (ITCZ). However, the displacement of the southern ITCZ since the Last Glacial Maximum (LGM, ca. 22 to 18 ka) is controversial. We present new field results, 14C- and optically stimulated luminescence (OSL)-data on fluvial and aeolian geoarchives in the catchment area of the lower Molopo River, southwest Kalahari, which indicate different periods of aeolian and fluvial activity in the last 40 ka. Based on our new data, we present a reconstruction of the late Quaternary climatic dynamics and a conceptual model indicating a southward shift of the ITCZ during the LGM, contemporaneously with a northward shift of the westerlies. This model implies that since 24 ka the southwest Kalahari has received summer and winter rains, which resulted in pluvial conditions that annually led to pan flooding throughout most of the year, cessation of lunette dune development due to stagnated deflation, and onset of perennial flow in a part of the lower Molopo River south of the Nossob-Molopo confluence. In addition, glacial climate circulation patterns over the southern African subcontinent can further resolve controversies about palaeoclimate reconstructions, which have important implications for modelling past and future climate scenarios.