A WOODLAND-SAVANNA MOSAIC IN THE EARLY MIDDLE MIOCENE AT BULUK, TURKANA BASIN, KENYA

Growing evidence for environmental heterogeneity across Miocene fossil localities in eastern Africa suggests that early great apes occupied a variety of habitats under a wider range of climate conditions than previously known. In this study, we reconstruct paleoenvironments for the Buluk fossil site in the eastern Turkana Basin of northern Kenya. Buluk is one of the few localities in eastern Africa that captures the transition from early to middle Miocene faunal communities. Using sedimentology and paleopedology, we reconstruct the depositional environment at Buluk as a meandering river complex with floodplain paleosols and sinuous, suspended-load channels flowing south-southwest. Locally, basalt flows blocked fluvial drainages and armored floodplain mudstones, impeding stream discharge and lateral migration. Floodplain paleosol features, such as pedogenic slickensides, carbonates, and iron redoximorphy, indicate seasonal waterlogging alternating with water deficit. Paleosol elemental weathering trends and micromorphological observations are consistent with in situ weathering of volcanic and volcaniclastic parent materials. Robust predictive models based on paleosol elemental composition estimate subhumid environments (600-1000 mm yr^-1). Clumped isotope analysis on pedogenic carbonates indicate soil temperatures of 25-31°C, slightly cooler than Pliocene paleosols and much cooler than modern soils in the Turkana Basin. Stable carbon isotopes measured on pedogenic carbonates and bulk organic matter indicate herbaceous C₄ vegetation within an open-canopy, C₃-dominated woodland to savannah mosaic. Our results add to the growing variety of environmental and climate space known to have been occupied by early hominoids and suggest that localized C₄ ecosystems were present in eastern Africa at least 6 million years prior to the expansion of C₄ grasslands at 10 Ma.