SEDIMENTARY BIOMARKERS RECORD ENVIRONMENTAL CHANGES ASSOCIATED WITH SHALLOWING OF PALEOLAKE OLDUVAI (CA. 1.8 MYR)

The Olduvai Gorge Coring Project (OGCP) recovered a sequence of sediment cores that provide a long-term record of lacustrine systems within the Olduvai region extending through most of the Pleistocene. These sediments provide a unique opportunity to examine organic geochemical proxies as measures of changes in the environmental and climatic characteristics of this region during a critical interval of hominin evolution. Here we examine biomarker occurrences and distributions to evaluate the nature and timing of significant changes in the lacustrine environment. Development of an understanding of temporal variations in Paleolake Olduvai will document how environments were changing and help assess whether they might prompt adaptations of hominin dependent on resources connected to the lake. This report focuses on analyses of the sedimentary transition from dark anoxic claystone containing pyrite to light-colored sandy claystone. This stratigraphic interval (ca. 1.80 to 1.85 Myr) records the progressive shallowing of Paleolake Olduvai associated with volcaniclastic input and increasing aridity. Preliminary analyses of the sedimentary biomarker compositions for this interval (66.3 – 71.7 m) show that they reflect this change in depositional setting. The anoxic claystones preserve a suite of alkenones derived from phytoplankton, with a distribution (C₃₇, C₃₉ alken-2-ones; C₃₈, C₄₀ alken-3-ones) typical of saline lacustrine settings except that alkadienones are dominant and alkatetrenones are absent; they also contain series of microbial C₂₇-C₃₂ hopanes and their 2-methylhopane homologues. In the sandy claystones the biomarkers are dominated by n-alkanes maximizing at C₃₁ or C₃₃ and exhibit high CPI (carbon preference index) values for C₂₅-C₃₅ (6.1-8.7) typical of inputs from leaf waxes. Further evidence for contributions from terrestrial organic matter (OM) is provided by the prevalence of dehydroabietic acid, and abundant series of both n-alkan-2-ones and n-alkan-3-ones likely derived from soils. Thus, biomarker distributions document two distinct organic geochemical facies corresponding to a shift in inferred sources of OM from aquatic to terrestrial that occurs in conjunction with a change in lithofacies and depositional setting associated with shallowing of Lake Olduvai over ~50 kyr.