RESPONSE OF THE TERRESTRIAL AND AQUATIC ENVIRONMENTS AT PALEOLAKE OLDUVAI TO PLEISTOCENE PRECIPITATION CHANGES

It is widely hypothesized that hominin evolution was influenced by climatic variability as a driving force for selective pressure via the availability of food, shelter, water, and other resources. Evaluation of these hypotheses is aided by elucidation of high-resolution records of climate and environmental variability from anthropologically significant locations. Lacustrine sediments recovered by the Olduvai Gorge Coring Project (OGCP) facilitate analyses of biogeochemical proxies that reflect climatic and environmental changes during time intervals associated with hominin fossils at Olduvai Gorge. The stratigraphic sequence of OGCP core 2A includes a lacustrine interval deposited between two distinctive dated horizons, the Bed I Basalt and Tuff IB, which contains high total organic carbon contents (>1% TOC) and is therefore ideally suited for applying biogeochemical approaches to paleoclimate and paleoenvironment reconstruction. The interval in OGCP core 2A between 76.6-86.9 m is characterized by precession-driven wet-dry cycles recorded in the hydrogen isotopic composition of n-C₃₁. These changes in precipitation drove shifts in terrestrial vegetation observed in δ¹³C_TOC and n-C₃₃/n-C₃₁ profiles and influenced the distributions of biomarkers (e.g., C₂₈ steradienes, n-C₁₇, A-norsteranes, and chromans) that reflect aquatic productivity in the lake environment. There is also evidence of rapid changes in lake levels and corresponding aquatic communities superimposed on these Milankovitch-scale cycles. In addition, the rate of temporal changes in the aquatic biomarkers is markedly different from those previously characterized for measures of the terrestrial environment. As the influences on hominin evolution likely include not only climatic variability but also the comparative rate and nature of changes in both terrestrial and aquatic ecosystems, it becomes important that the separate and collective effects of such factors should be considered when evaluating the role of climate as a driving force for hominin evolution.