ORGANIC GEOCHEMICAL PROXIES OF CLIMATIC AND ENVIRONMENTAL CHANGE FROM THE SEDIMENTARY RECORD OF OLDUVAI GORGE (~1.85 - 1.88MA)

Over the past century, extensive studies in East Africa have yielded an unprecedented wealth of information that has shaped our understanding of hominin evolution, including evidence for the critical role played by climatic and environmental variability. Testing such hypotheses benefits from access to more complete and detailed stratigraphic records of the timing and nature of changes in climate and the environment experienced by hominins. Olduvai Gorge, a location in northern Tanzania rich in hominin fossil discoveries, exposes a sedimentary sequence of ~2.0 Ma, which includes multiple deep lake intervals. In 2014, the Olduvai Gorge Coring Project (OGCP) recovered a sequence of sediment cores that provide a record of the paleolacustrine systems in this area and are ideally suited for high-resolution analyses of organic geochemical proxies, providing an opportunity to build on previously published data for Olduvai that employed outcrop samples.

The section between 76.6 to 86.9 m depth, which spans a time interval of ~20-30 kyr, was sampled every 16 cm for organic geochemical analyses. The carbon isotopic composition of organic matter (δ¹³C_TOC) from this section varies between values representative of more forested (~-27‰) and open grassland (~-15‰) ecosystems over ~21 kyr, consistent with the Milankovitch precession cycle previously observed in lower resolution studies of outcrop samples. Biomarker profiles for this section, including n-alkane chain-length distributions and the ratio of the plant wax n-C₃₁ to the microbial hop-17(21)-ene, document shifts in the productivity of the lake consistent with the precession cycle observed in the δ¹³C_TOC. The δ¹³C_TOC also provides evidence for sub-Milankovitch changes superimposed on the precession cycle, suggesting that environments inhabited by hominins varied even more than previously documented. To characterize in greater detail both climate and environmental variability associated with these sub-Milankovitch changes, a full suite of biomarker distributions (e.g., alkenones, sterenes, GDGTs) are being analyzed to match the high resolution of the δ¹³C_TOC record, and thereby to provide a sub-millennial-scale temporal record of the climatic and environmental variability experienced by hominins during a critical time in their evolutionary history.