Olduvai Gorge is renowned for discoveries of hominin fossils and tools in a well-resolved sedimentary context, representing one of the foremost sites in East Africa that has afforded critical evidence of hominin evolution. In 2014, the Olduvai Gorge Coring Project (OGCP) recovered the first deep sediment cores from this location. These cores provide the first opportunity to examine an extensive stratigraphic record from one location and independent of the weathering and related degradation typical of outcrop samples. Samples from these cores have been correlated to Bed I tuffs and basalt marker beds that are well characterized from outcrops within the gorge. This study focuses on the biogeochemical investigation of a ~10 m segment from OGCP sediment core 2A, where age constraints indicate deposition occurred over ~50,000 years (from 1.8055 ± 0.003 Ma to 1.8551 ± 0.013 Ma). The segment includes Tuff IB and IF, which were deposited at a time that prior research indicates represents marked changes in the paleoenvironment at Olduvai, with indications that Paleolake Olduvai may have been completely desiccated by the time Tuff IF was deposited. This environmental change is recorded by the composition of sedimentary organic matter (OM) in terms of bulk organic geochemical properties observed as a decrease in % TOC and δ13Corg. In addition, temporal variations in source-specific biomarkers (n-alkanes, phytane, steradienes, A-norsteranes and alkenones) reflect a shift from a deeper lake setting with a prevalence of aquatic-sourced OM combined with input from terrestrial plant waxes to a more shallow, perennial lake where evidence of burrows and erosional surfaces is associated with microbially degraded OM and intermittent aquatic biomarkers. Thus, variations in biogeochemical proxies complement the sedimentological evidence confirming increased periods of prolonged desiccation of Paleolake Olduvai beginning around the time of Tuff IB deposition.
Bibliographical noteFunding Information:
We would like to acknowledge the Stone Age Institute for organizing and funding the Olduvai Gorge Coring Project (OGCP) with grants from the Kamen Foundation , the Gordon and Ann Getty Foundation , the John Templeton Foundation , the Fred Maytag Foundation , Kay and Frank Woods , and NSF ( BCS1623873 ). Additionally, we would like to thank the Tanzanian Commission for Science and Technology, Tanzanian Department of Antiquities, Ministry of Natural Resources & Tourism, and the Ngorongoro Conservation Area Authority (NCAA) for enabling the collection of samples and continuing support of OGCP research. Funding for the organic geochemical preparation was provided by the Dept. of Earth and Atmospheric Sciences ( Indiana University ). Funding for the organic geochemical analysis on the QTOF was provided by the Mass Spectrometry Facilities at Indiana University and NSF grant CHE1726633 . We would like to thank A. Schimmelmann (Indiana University) for providing both an n-alkane standard mix, and the squalane standard.
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