PALEOCLIMATIC VARIABILITY AND THE ROLE OF SEASONALITY FROM THE EARLY MIOCENE DEPOSITS ON RUSINGA AND MFANGANO ISLANDS, LAKE VICTORIA, KENYA

Nearly a century of research on Rusinga and Mfangano Islands, Lake Victoria, Kenya has yielded a wealth of early Miocene fossils, including extensive remains of the early ape Ekembo. As a result, these sites are pivotal in understanding the early evolution and diversification of hominoids. In recent years, work has focused on reconstructing the paleoecology and paleoclimate of deposits on these islands to better understand the environment in which Ekembo and other contemporaneous mammals lived. Analyses of paleosols, sedimentology, and fossil leaves from the Wayando, Kiahera, and Hiwegi Formations, which were deposited between ~17.7 and ~20 Ma, reveal a complex and dynamic climate with variability in ecosystems through time. Macro- and micromorphological analyses of Calcisols from the Wayondo Formation indicate a semi-arid to arid climate in which evaporation exceeded precipitation and suggest a relatively open environment. New work in the Pisolitic Clay Member of the Kiahera Formation shows evidence of Vertisols, Calcic Vertisols, and Calcisols indicate a period of relative landscape stability. In particular, the presence of multiple well-developed, overthickened Vertisols with slickensides and wedge-shaped peds are the first to be documents on Rusinga, but are similar to those found at other early Miocene sites in East Africa, including Karungu, Buluk, and Moroto. Paleosol analyses, sedimentology and fossil leaves from the Hiwegi Formation indicate paleoenvironment varied throughout the formation from a relatively warm and wet closed-canopy forest at the base, to an arid environment in which evaporation exceeded precipitation in the middle, back to a wet, closed-canopy forests at the top of the stratigraphy. Taken together, these results indicate that climate, and in turn ecosystems, varied significantly over the ~2.5 myr history of deposition of the Wayando, Kiahera, and Hiwegi Formation. In particular, these results demonstrate that precipitation varied significantly over time. This variability in precipitation is likely the result of larger regional-scale climate processes, such as the development of the East African Monsoon. Furthermore, these results suggest that climatic variability during the Early Miocene likely played an important role in the evolution of early apes and hominoids.