Vegetation change in the Baringo Basin, East Africa across the onset of Northern Hemisphere glaciation 3.3–2.6 Ma

Vegetation in East Africa is generally thought to have shifted from forests to more open grasslands and savannas as global climate cooled and high-latitude ice sheets expanded during the Plio-Pleistocene. Such a shift would have greatly influenced landscape resources, and potentially hominin evolution as well. Existing records of African vegetation spanning these time-scales are generally derived from offshore marine records that record continental-scale changes, or paleosol carbonate records that record very local vegetation changes during the short time intervals of soil carbonate formation. Here we present a new record of basin-scale vegetation change from the late Pliocene (~3.3–2.6 Ma) derived from a drill core from the Chemeron Formation, located in the Baringo Basin/Tugen Hills region of the Kenya Rift Valley. Specifically, we present a new record of the relative abundance of C₄ grasses and C₃ vegetation based on the carbon isotopic composition of leaf wax biomarkers (δ¹³C_wax), which captures a signal of regional vegetation change. These data demonstrate that vegetation in the Baringo Basin varied greatly between C₃ forests and C₄ grasslands, and that vegetation exhibits both long-term (secular) trends and orbital-scale variations. The contribution of C₃ plants was lower than estimates based on low-resolution carbon isotope data from paleosol carbonates and organic matter in the basin. C₃ plants averaged ~53% of the vegetation during the late Pliocene, from ~3.3 to ~3.04 Ma, after which time δ¹³C_wax indicates more open vegetation and ~41% C₃ plants. This transition may have been driven by changes in basin geomorphology, but also possibly occurred as part of larger-scale drying and expansion of C₄ vegetation in East Africa. In addition to this secular change, we observe high amplitude variability in the δ¹³C_wax record including oscillations between ~80 and ~0% C₃ plants. These vegetation changes are correlated with changes in precipitation inferred from δ²H_wax and lake level oscillations inferred from sedimentary facies, implying that high-amplitude, orbital-scale variations in precipitation drove significant changes in vegetation resources during the late Pliocene in the Baringo Basin. These variations have important implications for changes in terrestrial resources in light of the evolutionary innovations in the hominin fossil record related to changes in foraging strategies.