GEOCHRONOLOGY AND TERRESTRIAL PALEOENVIRONMENTS OF THE MIDDLE MIOCENE HOMINOID FOSSIL-BEARING STRATA AT BULUK, TURKANA BASIN, KENYA

The transition from the Early to Middle Miocene marked a substantial change in eastern African faunas, and particularly hominoids and primates. Understanding the age and environmental context of fossil localities across this transition is critical for determining potential drivers of hominoid and primate evolution. We report an updated geochronologic framework and reconstruction of the paleoenvironment and paleoclimate for strata associated with the Buluk fossil locality in the Turkana Basin of northern Kenya. Using Ar-Ar geochronology of multiple tuffs coupled with magnetostratigraphy, we provide new age determinations that constrain the age of most of the fossil beds to between ~16 and 16.35 Ma, which is more than 500 kyr younger than previous age interpretations. These revised age determinations shift the chronologic placement of Buluk into the latest early Miocene, coincident with the peak of the Miocene Climatic Optimum (MCO). Facies analyses of the deposits at Buluk indicate the presence of sinuous, perennial streams and rivers, variably developed paleosols, and rare small ponds. In situ vertebrate fossils are largely associated with the sand-dominated fluvial channels and point bars adjacent to the channels. The floodplain paleosols range from weakly developed Entisols to moderately mature Vertisols. Paleosol proxies indicate climate was subhumid (mean annual precipitation = 500 – 1200 mm yr^-1). Mean annual temperature estimates inferred from clumped isotopes (Δ₄₇) indicate a thermic to hyperthermic soil temperature regime (25-31°C). Stable isotopes of soil organic matter, pedogenic carbonate, and plant waxes are consistent with woodland vegetation with a locally variable fraction of C₄ vegetation. Taken together, these results indicate that during the MCO, the heterogenous, dynamic landscapes at Buluk hosted seasonal woodlands. Interestingly, these climate reconstructions indicate that Buluk was much wetter during the MCO than the current arid conditions in eastern Turkana. However, these new data also underscore the lack of Middle Miocene paleoclimate estimates in the region. This lack of data inhibits a clear understanding of the hydroclimatic response in eastern Africa to the MCO and demonstrates that importance of targeting paleoclimate and paleoenvironmental reconstructions in the Middle to early Late Miocene across eastern Africa.