BIOGEOCHEMICAL EVIDENCE OF CHANGE IN DEPOSITIONAL SETTING IN PALEOLAKE OLDUVAI AT ~1.85 MA ASSOCIATED WITH COMPLEX INTERACTIONS OF CLIMATE, TECTONICS, AND VOLCANISM

A series of sediment cores recovered by the Olduvai Gorge Coring Project (OGCP) provide a long-term record of lacustrine systems through most of the Pleistocene. A section of OGCP core 2A includes two tuffs – Tuff 1B at ~72 m and Tuff 1F at ~66 m – that can be correlated stratigraphically and geochemically with equivalent units from outcrop dated as ~1.848 ± 0.003 Ma and ~1.803 ± 0.002 Ma, respectively. This study examines biogeochemical signatures in this section of Upper Bed I that spans the “Zinjhorizon,” and represents a critical time in hominin evolution. It focuses on changes in depositional setting manifest from characteristics of the lacustrine sediments temporally associated with Tuff 1B. Visually the section changes from (i) a darker claystone rich in organic matter (OM) to (ii) a lighter colored OM-poor sandy claystone, suggestive of a more oxidized setting, which is confirmed by evidence of bioturbation (burrows and rootlets). The lower portion of the section includes an abundance of aquatic biomarkers derived from algae (e.g., hop-17(21)-ene, steradienes, and alkenones), sponges (e.g., A-norsteranes), and cyanobacteria (e.g., n-C₁₇). However, immediately prior or post deposition of Tuff 1B, these aquatic indicators decrease markedly in abundance to minimal concentrations or below detection limits. This decrease in aquatic biomarkers in the upper section above Tuff 1B is accompanied by an increase in terrestrial markers (e.g., long chain n-alkanes) suggesting a switch from a dominance of autochthonous to allochthonous OM, also corresponding with a decrease in OM from >1 % to <0.3 %. A section of OGCP core 2A corresponding to Bed I below 76 m is characterized by precession-scale wet-dry cycles but also exhibits sub-Milankovitch scale variations likely reflecting local climatic influences. It appears that the wetter interval immediately below 76 m, characterized by abundant aquatic biomarkers, continues into the lower half of the section studied here, prior to Tuff 1B. Subsequently, increased volcanism and possibly tectonic activity appear to have changed basin morphology and/or water flow to the site of OGCP core 2A, creating a shallower lake environment where the preserved OM derives predominantly from terrestrial rather than aquatic sources.