QUANTITATIVE LACUSTRINE PALEOSALINITY AND PLEISTOCENE ORBITAL CONTROLS FROM CLAY MINERAL OXYGEN ISOTOPES: OLDUVAI GORGE, TANZANIA

Few proxies quantitatively record important paleolimnological parameters such as temperature and salinity. Because lake hydrology is at times driven by non-climatic factors, such as basin morphology, deciphering climate history from lake sediments is complex. As a consequence, paleoclimate proxies within closely allied lake systems often respond independently to high and low frequency climate variability. This is particularly true in the case of sedimentary records from active tectonic settings.

In East African lakes, elevated salinity and alkalinity commonly leads to authigenic clay minerals enriched in Mg. Here we use isotopic measurements of structural oxygen in authigenic clay minerals (δ¹⁸O_C) in the Olduvai Basin to extract the salinity signal and generate the first record of paleolake water salinity for the early Pleistocene (~1.8 – 1.92 Myrs). We first caclculate the δ¹⁸O of the paleolake water (δ¹⁸O_LW) by applying equilibrium fractionation constants based on X-ray diffraction and assuming 25˚C. Paleolake water salinity (Total Dissolved Solids, g/L) is then calculated according to TDS=(0.3× δ¹⁸O_LW)+1.64, a relationship based on paired measurements of modern δ¹⁸O_LW and TDS in meteoric surface waters at Ngorongoro Crater (Deocampo, 2004).

Our results show that during the early Pleistocene, the clay minerals were in equilibrium with paleolake waters ranging between –4.5 and –1.5‰ (VSMOW) and this corresponds to a TDS range of 0.3 to 1.2 g/L. Although the paleosalinity record is not strictly consistent with geochemical indicators of freshening (Al₂O₃/MgO) from the same site (Deocampo et al., 2017), intervals of low salinity coincide with high Al₂O₃/MgO values. The new proxy faithfully captures all events of significant freshening and reveals that enhanced East African monsoon precipitation was sensitive to warming over high latitudes in the Southern Hemisphere. These changes were the main drivers of changing terrestrial habitats, providing resources and hazards to hominins and other vertebrates, and setting the stage of environmental variability and the evolution of early hominins (e.g. Behrensmeyer, 2006).

References

Deocampo, D.M., 2004. Applied Geochemistry: 19, 755–767.

Deocampo et al., 2017. Geology: 45 (8), 683-686.

Behrensmeyer, A. K., 2006. Science: 311, 476-478.