Terrestrial paleoenvironments and paleoclimate during the Miocene climate optimum at Buluk, East Turkana, Kenya

Understanding the environmental and climatic context of Neogene fossil localities in eastern Africa is critical for determining potential drivers of evolution in our hominoid ancestors and their floral and faunal communities. In this study, we reconstructed paleoenvironments and paleoclimate conditions for strata associated with the Buluk fossil site in the Turkana Basin of northern Kenya. Recent geochronologic analysis situates the fossiliferous strata at Buluk between ca. 16.6 Ma and 16.1 Ma, making this one of a very few equatorial, terrestrial records coinciding with the Miocene climate optimum (16.9–14.7 Ma). Floodplain paleosols were described using field observations, micromorphology, bulk elemental geochemistry, and stable isotope geochemistry. The paleosols range from weakly developed Entisols to moderately mature Vertisols that contain co-occurring pedogenic slickensides, carbonates, illuviated clay, iron mottling, and Fe-Mn concretions. These features are consistent with a seasonally varying soil water budget, ranging from saturation to water deficit on annual scales. Two paleosol proxies for mean annual precipitation indicate that climate was subhumid: 588–1188 mm (± 512 mm, PPM1.0 model) and 615–998 mm (± 209 mm, RF-MAP2.0 model). Mean annual temperature inferred from clumped isotope analysis on pedogenic carbonates suggests that a thermic to hyperthermic soil temperature regime (24–31 ± 5 °C) existed during the peak of the Miocene climate optimum, notably lower than the Pliocene through modern conditions in the Turkana Basin. Stable carbon isotope ratios (δ13C) measured on bulk organic matter and plant waxes are consistent with vegetation dominated by C3 biomass, which supports existing tooth enamel δ13C records that indicate C3 diets in fossil taxa at Buluk. However, numerous carefully vetted pedogenic carbonate samples have δ13C values that fall above the C3 end member and suggest spatially and temporally variable abundances of C4 vegetation. These findings support interpretations of heterogeneous, dynamic landscapes at Buluk that likely hosted seasonal woodlands—not forests or grasslands—and seasonally varying rainfall possibly driven by monsoon circulation. This study provides clear evidence that the Turkana Basin was significantly wetter, but not hotter, during the Miocene climate optimum compared to today, but further work is needed to understand broader temporal patterns of hydroclimate change across the middle to late Miocene in the region.