CLIMATE SEASONALITY IN THE TURKANA BASIN, KENYA FROM 6-1 MA: ISOTOPIC EVIDENCE FROM FOSSIL EQUID TEETH.

Climate seasonality affects the distribution of vegetation and animal communities in modern ecosystems, and has been proposed as an environmental driver of hominin evolution over the past 6 million years. Records from paleosol carbonates in terrestrial sediments and leaf wax biomarkers from deep sea and lacustrine cores have suggested that climate seasonality has become more variable since the Miocene. However, these records lack the temporal and spatial resolution necessary to assess seasonal patterns accurately. Extant equids are water-dependent grazers, and oxygen isotope variation within teeth of modern zebra in eastern Africa correlates with intraannual ranges of fluctuations in the δ¹⁸O of local precipitation. We present new intratooth δ¹⁸O measurements (n =115) of fossil equid teeth (n=10) from the Nachukui, Kanapoi, and Nawata Formations, spanning 6.5 Ma to 1.39 Ma. Additionally, we compare intratooth variation from Turkana Basin equids to fossil equids from northern Tanzania and southern Kenya. Our findings reveal that intratooth δ¹⁸O ranges are lower in the Miocene and Pliocene equids compared to Pleistocene equids, which have intratooth δ¹⁸O ranges similar to modern equids from eastern Africa. These results indicate increasing isotopic seasonality of precipitation in the Turkana Basin from the Miocene to the Pleistocene, reflecting increasing variation on seasonal scales in precipitation amount or source. In the Pleistocene, isotopic seasonality in the Turkana Basin is higher than in northern Tanzania and southern Kenya, indicating regional variation in seasonality of precipitation amount or source. A shift in environmental seasonality may have played a role in ecological changes such as the expansion of C₄ grasses and faunal population dynamics, which may have influenced selective pressures experienced by hominins.