Biomarker <i>δ</i> ¹³ C values record consistent savanna vegetation and variable alkalinity of Lake Olduvai during Pleistocene wet/dry cycles

Abstract

Climate records for the early Pleistocene in eastern Africa aid reconstruction of the ecology and landscapes occupied by hominins. Environmental changes associated with alternating wet/dry cycles during this critical interval in hominin evolution are recorded in isotopic profiles from lake sediments from Olduvai Gorge. The Olduvai Gorge Coring Project targeted the depocenter of Paleolake Olduvai, recovering a stratigraphic succession between the Bed I Basalt (~1.90 Ma) and Tuff IF (~1.80 Ma) datums that span hominin fossil horizons in nearby outcrops. Lacustrine claystones from the lower part of this interval are characterized by high C org (avg. 2.5 %). They display variations in δ 13 C org reflecting temporal changes in organic matter (OM) sources. Stratigraphic profiles for n -alkane δ 13 C values derived from plant waxes are consistent throughout wet/dry cycles indicating stability in savanna vegetation, refuting evidence for increased C 4 grasses during drier intervals. δ 13 C values for n -alkanes derived from aquatic macrophytes and for algal biomarkers increase during drier episodes, reflecting use of bicarbonate as a carbon source triggered by enhanced lake alkalinity. δ 13 C org profiles reflect variations in inputs of terrestrial vs. aquatic OM during wet/dry cycles, which are mirrored by δ 13 C values for hop-17(21)-ene derived from heterotrophic bacteria utilizing sedimentary OM. The δ 13 C records from Paleolake Olduvai sediments show temporal changes in OM sources rather than shifts in the proportion of C 3 and C 4 vegetation, indicating that precession-driven climate cycles primarily affected lake environments and associated aquatic resources. Thus, intervals of high climate variability did not necessarily cause changes in savanna vegetation affecting hominin habitats.