EARLY MIOCENE PALEOCLIMATE AND PALEOENVIRONMENTS ACROSS EAST AFRICA

Early Miocene fossil sites in East Africa provide a unique opportunity to assess patterns of terrestrial climate and environmental change and their implications for the evolution of the hominoid clade. Although the systematics of the vertebrate faunas from these fossil sites have been well studied, relatively little research has focused on their paleoclimates or paleoenvironments. Here we present multi-proxy reconstructions of paleoclimate and paleoenvironment from several early Miocene sites in East Africa that span a ~5 myr interval from ~21.5 – 16.5 Ma: Moroto, Napak, and Bukwa in Uganda; Tinderet, Kisingiri, West Turkana, and Buluk in Kenya. Mean annual temperature estimates based on proxy reconstructions using paleosol bulk geochemistry and fossil leaves indicate a megathermal climate and that temperature was relatively stable through the Early Miocene. Mean annual precipitation estimates using fossil leaves and paleosol bulk geochemistry are considerably different among sites, indicating variable precipitation through time and probably also spatially across East Africa. Additionally, qualitative and quantitative estimates indicate that precipitation was typically seasonal and that most of the sites experienced intervals with little to no rain throughout the year. Reconstructions of vegetation based on fossil leaves, phytoliths, paleosols, and stable isotopes indicate considerable variation in paleoenvironment through time ranging from relatively open sites with common grass and evidence for at least some C4 vegetation to closed sites with evidence for multi-story closed canopy forest. Importantly, these differences in paleoenvironment appear to primarily track changes in precipitation, rather than seasonality, with more open sites being drier and more closed sites being wetter. Taken together these results indicate that paleoclimate across East Africa in the early Miocene was variable, and primarily driven by spatial and temporal changes in mean annual precipitation. This variability is also linked to fluctuations between more open and more closed environments, which suggesting that habitat and climatic variability likely played an important role in evolution of hominoids.