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 often controlled 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 tend to respond independently to both high and low frequency climate variability. This is particularly true in the case of sedimentary records from tectonically active basins of the East African rift system.

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 untangle the salinity signal and generate the first record of paleolake water salinity for the early Pleistocene ( 1.8 - 1.92 Myrs). We first estimated the δ¹⁸O of the paleolake water (δ¹⁸O_LW) by applying equilibrium fractionation constants based on X-ray diffraction and assuming 25˚C. We then calculated paleolake water salinity (Total Dissolved Solids, g/L) 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 suggest that during the early Pleistocene, Olduvai δ¹⁸O_LW varied between approximately -4.5‰ and -1.5‰ (VSMOW) corresponding to a TDS range of about 0.3 to 1.2 g/L.

Although the paleosalinity record is not strictly in line with published geochemical indicators of freshening (Al₂O₃/MgO) from the same site (Deocampo et al., 2017), intervals of very low salinity coincide with high Al₂O₃/MgO values and the new proxy faithfully captures all events of significant freshening. The most extreme episodes of salinity in the basin are likely not represented by this proxy, as lakewaters during those times did not produce the authigenic clays examined here. Isotopic indicators of freshening are in phase with precession maxima and peak Southern Hemisphere insolation, demonstrating the orbital sensitivity of the East African monsoon to high latitude climate change and warming in the southern hemisphere.

References

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

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