δ13C VALUES OF ALGAL BIOMARKERS IN PLEISTOCENE SEDIMENTS FROM PALEOLAKE OLDUVAI RECORD TEMPORAL CHANGES IN LAKE ALKALINITY DURING WET/DRY CYCLES

The Pleistocene sedimentary succession recovered from the depocenter of Paleolake Olduvai by the Olduvai Gorge Coring Project (OGCP) afforded an almost continuous stratigraphic sequence between the dated horizons of the Bed I basalt (~1.900 Ma) and Tuff IF (~1.83 Ma) datums that correlate with horizons from outcrops that contain hominin fossils, stone tools, and other evidence of hominin activities [1]. The series of lacustrine claystones interbedded with sandy claystones in the 72.3 to 86.9 mbs interval are characterized by high C_org contents (avg. 2.5 %), including several laminated intervals. They display 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 [2,3] previously observed as lower-resolution signatures in outcrop samples [4]. The sediment sequence contains series of n-alkanes diagnostic of contributions from plant waxes and aquatic macrophytes and suites of algal biomarkers, including C₃₇-C₄₀ alkenones produced by haptophytes, C₂₈ & C₂₉ sterenes derived from algal sterols, and C₂₉ & C₃₀ 4-methylstanones originating from dinoflagellates. The δ¹³C compositions of the algal biomarkers are enriched in ¹³C during drier versus wetter intervals, with mean δ¹³C values for alkenones of -14.7 ‰ vs. -20.3 ‰ respectively. This substantive shift in biomarker δ¹³C values associated with the wet/dry cycles suggests that bicarbonate uptake likely facilitated continuity in primary productivity within Paleolake Olduvai during episodes of increased alkalinity. This interpretation raises the prospect that biomarker δ¹³C values may serve as a proxy for lake alkalinity and suggests that temporal changes in alkalinity need to be acknowledged as a potential influence on paleolimnologic records of sedimentary δ¹³C_org and considered in paleoenvironmental reconstructions.

[1] Njau et al., 2021. doi:10.1016/j.palaeo.2020.110059

[2] Colcord et al., 2018. doi:10.1016/j.palaeo.2018.01.023

[3] Shilling et al., 2020. doi:10.1016/j.palaeo.2020.109824

[4] Magill et al., 2013. doi:10.1073/pnas.1206276110