Δ13C TEMPORAL PROFILES FOR ALGAL, PLANT WAX, & BACTERIAL BIOMARKERS REFLECT WET/DRY CYCLES RECORDED IN PLEISTOCENE SEDIMENTS FROM PALEOLAKE OLDUVAI

In 2014 the Olduvai Gorge Coring Project (OGCP) recovered a Pleistocene succession of sediments from Paleolake Olduvai that correlates with outcrop sequences directly associated with horizons containing hominin fossils and stone tools. Coring at Site 2A targeted the depocenter of the ancient lake and recovered an almost continuous stratigraphic sequence between the dated horizons of the Bed I basalt (~1.900 Ma) and Tuff IF (~1.83 Ma) datums. The lower part of this interval (72.3 to 86.9 mbs) consists of lacustrine claystones interbedded with sandy claystones that are characterized by high C_org contents (avg. 2.5%), including several laminated intervals. It displays temporal shifts in δ¹³C_TOC from -15‰ to -27‰ that reflect changes in the sources of organic matter (OM) associated with precession-scale wet/dry climate cycles previously observed as lower-resolution signatures in outcrop samples. Determination of the δ¹³C composition of aliphatic hydrocarbons reveals consistent differences between the values for plant wax n-alkanes (C₃₁, C₃₃ & C₃₅; range -22‰ to -27‰) and those for sterenes (range -11‰ to -20‰) derived from algal sterols. The δ¹³C values for the plant waxes align with prior published data for δ¹³C_TOC during wet intervals whereas those of sterenes are comparable with δ¹³C_TOC during dry intervals. Collectively these data suggest a shift from a dominance of terrestrial OM to aquatic OM associated with the transition from wet to dry conditions, consistent with a decrease in terrestrial vegetation during drier intervals, augmented by reduced contributions from run-off. The consistency in the δ¹³C values for plant waxes suggests no significant shift from C₃ to C₄ vegetation during drier intervals. However, δ¹³C values for hop-17(21)-ene, derived from bacteria, parallel the shift in δ¹³C_TOC, suggesting that the predominant source of this compound either changes from a terrestrial source, such as acidobacteria in soils, to an aquatic origin, such as cyanobacteria as wetter conditions transition to a drier climate, or alternatively reflects contributions from heterotrophic bacteria utilizing sedimentary OM.