A 1.2 MA HISTORY OF TEMPERATURE AND ARIDITY IN THE LAKE MALAWI BASIN, EAST AFRICA

Variability in climate in the East African Rift Valley is driven largely by change in rainfall, and this undoubtedly impacted the behavior of our hominin ancestors, just as it affects humans in this region today. While change in temperature is not likely to have played as great a role as rainfall in the evolution of our species, we can now quantify the temperature history of the East African great lakes through the analysis of TEX₈₆ in lake sediments. The temperature records that are being generated through this approach are providing new insights into the climate dynamics of the Rift Valley, and the extent to which variations in temperature and rainfall may have combined to ease or intensify the environmental stress placed upon our ancestors. We present a history of temperature in the Lake Malawi basin (~ 10º S, 35º E) that spans the past 1.2 million years, based on analyses for TEX₈₆ on cores recovered by the Lake Malawi Drilling Project, and we compare this with the history of relative aridity in the region, based on calcareous sediment accumulation when the lake was a closed basin, with lake levels at times dropping by several hundred meters. The mean temperature in the basin has been about 24º C, somewhat cooler than today, but exhibiting no long-term trend over the 1.2 Ma history. The amplitude of glacial-interglacial temperature change has been on the order of 3º C over this time period, but the orbital-scale periodicity has shifted from about 70 ky(?) prior to about 500 ka to 100 ky since that time. Relative aridity in the region has shifted at much higher frequency (precessional?), indicating no consistent relationship between temperature and aridity on millennial time scales. This may be a result of interactions between a regional trend of warm-with-wet or cold-with-dry conditions and the warmer conditions expected from the atmospheric temperature lapse rate for lower lake surface elevations.