Reconstructing coastal eastern African climate from speleothems: Implications for human biogeography

There is strong evidence that cyclical changes to Earth’s orbital configuration during the Pleistocene led to the periodic greening of vast areas of the Sahara, East African Rift Valley, and Arabia. The opening of these humid corridors facilitated the dispersal of humans out of eastern Africa into Asia and beyond. However, less is known about what happened under opposite circumstances, when these corridors dried up due to waning orbital forcing. One hypothesis is that human populations sought refuge in eastern Africa’s coastal forests when conditions in the African interior were inhospitable. We test this hypothesis by evaluating the stability of climate in coastal Kenya from our reconstruction vis-à-vis climate in the African interior from previously published work. Speleothem samples collected from limestone quarries near Mombasa and Kilifi provide a novel record of long-term climate change in eastern Africa and offer new insight into human biogeography. Preliminary U-Th age results suggest that these samples grew throughout the last glacial period and possibly during older glacial-interglacial cycles. This sustained growth indicates that eastern African coastal climate was characterized by stable conditions and a positive moisture balance, supporting the refugia hypothesis. To further constrain the climate dynamics governing coastal eastern Africa, temperature and hydroclimate reconstructions are being developed using fluid inclusion and TEX86 analyses.