THE LAST GLACIAL MAXIMUM IN TROPICAL EAST AFRICA

The mechanisms which influenced the last glacial to interglacial transition (i.e., the last termination) and the collapse of most Northern Hemisphere ice sheets are still not well known. As proposed by Milankovitch, changes in summer insolation at high northern latitudes are generally regarded as the pacemaker of glacial-interglacial cycles. However, Denton et al. (2010) point out that rising summer insolation does not always influence terminations and some terminations occur during relatively low-amplitude insolation changes. Other proposed mechanisms for the last termination include changes in atmospheric CO₂ and tropical processes, such as changes in the western Pacific warm pool. We suggest that tropical glaciers provide a means to evaluate various hypotheses for the causes of the last termination. Tropical glaciers are far from the local influences of the large ice sheets and many respond primarily to changes in air temperature. We present a record of glacier fluctuations in the Rwenzori Mountains (~1°N, 30°E) of tropical East Africa. Three nested moraine sets document Rwenzori glacier fluctuations during the Last Glacial Maximum. We anticipate new ¹⁰Be ages on the outermost of the three moraines. The moraine just proximal to the outermost is dated at ~23.4 ka. Subsequently, at ~20.1 ka, a moraine was deposited that crosscuts the older moraines and marks the largest glacier extent during the Last Glacial Maximum. A comparison of these glacier fluctuations with tropical east African climate proxy records shows that the Rwenzori glaciers were extensive contemporaneously with dry and cold conditions, indicating a dominant influence of temperature on the glacier mass balance. We compare these glacier fluctuations with changes in summer insolation at high northern latitudes, as well as with other hypothesized causes of the last termination, such as changes in atmospheric CO₂ and the western Pacific warm pool.