# Biomarker δ13C values record consistent savanna vegetation and variable alkalinity of Lake Olduvai during Pleistocene wet/dry cycles

## TL;DR

This study uses isotopic data from lake sediments to show that savanna vegetation remained stable during Pleistocene climate changes, while lake alkalinity varied.

## Contribution

The study provides new isotopic evidence that climate cycles affected lake environments but not terrestrial vegetation in hominin habitats.

## Key findings

- δ13C values from plant waxes show consistent savanna vegetation through wet/dry cycles.
- Aquatic δ13C values indicate increased lake alkalinity during drier periods.
- Climate variability affected aquatic resources but not terrestrial ecosystems.

## Abstract

Fossil and archaeological records confirm that the lacustrine setting of Olduvai Gorge was a sustained habitat for hominins in the early Pleistocene. Sediments deposited in Paleolake Olduvai spanning forty thousand years record evidence of climatic and environmental changes and episodes of ecological stability in the landscapes occupied by hominins. Isotopic signatures diagnostic of plant waxes document consistency in savanna vegetation during a succession of wet/dry climate cycles, whereas those associated with aquatic biota attest to increased lake alkalinity during drier intervals. Hominins would have experienced a stable terrestrial ecosystem with a consistent mix of woodland and grassland, while water resources were affected by pronounced temporal variations in the hydrological cycle regulating the size and chemistry of Paleolake Olduvai.

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 Corg (avg. 2.5 %). They display variations in δ13Corg reflecting temporal changes in organic matter (OM) sources. Stratigraphic profiles for n-alkane δ13C values derived from plant waxes are consistent throughout wet/dry cycles indicating stability in savanna vegetation, refuting evidence for increased C4 grasses during drier intervals. δ13C 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. δ13Corg profiles reflect variations in inputs of terrestrial vs. aquatic OM during wet/dry cycles, which are mirrored by δ13C values for hop-17(21)-ene derived from heterotrophic bacteria utilizing sedimentary OM. The δ13C records from Paleolake Olduvai sediments show temporal changes in OM sources rather than shifts in the proportion of C3 and C4 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.