COG3

Rachel Lupien
Brown University


Controls on Plio-Pleistocene East African Climate

Orbitally-induced insolation changes and ice sheet extent are hypothesized to have affected East
African hydroclimate on seasonal- to million-year-timescales. Evaluating the impacts of these forcings and the relationship between human evolution and the environment has been challenging due to the lack of long, high-resolution terrestrial hydroclimate records. Leaf wax biomarker hydrogen isotopes (δDwax) preserved in lacustrine archives allow us to quantitatively
analyze rainfall amount in East African basins over the last 4 million years. Three case studies examining centennial-scale abrupt change, orbital-scale variability, and long-term shifts in the Plio-Pleistocene demonstrate the evolving influence of low- and high-latitude forcings on East African precipitation. The establishment of large permanent ice sheets at ~2.7 Ma drove an aridification shift, perhaps through southward displacement of the tropical rain belt. However, we observe no trend in δDwax since the onset of Northern Hemisphere glaciation, suggesting a nonlinear response. The lack of mid-Pleistocene obliquity variations, together with 100-kyr variability in the late-Pleistocene, further indicates threshold responses to global climate boundary conditions associated with glacial-interglacial variations. Further, extremely rapid, dramatic changes in centennial-scale hydroclimate, similar to the onset of the African Humid Period, may have radically effected hominin habitats. This compilation and analysis of high- and low-frequency signals provide a climatic context to interpret how precipitation mean, variability,
and abrupt transitions influenced human evolution in East Africa.