PALEO-SEASONALITY RECONSTRUCTIONS OF THE OMO-TURKANA BASIN USING SINGLE OSTRACOD STABLE ISOTOPE SIGNATURES

Seasonal monsoon variability exerts significant control on ecosystems past and present, including those that hominins and early humans inhabited. Despite significant progress in reconstructing continental paleoclimate change in eastern Africa, there remain many challenges to quantitatively estimating seasonal hydroclimate variability and changes in seasonal amplitude through time. To address this critical gap in our understanding of paleoenvironment and paleoclimate, we seek to quantify seasonal to interannual climate variability using single ostracod stable isotope data. The carbonate precipitated by these organisms is a function of the hydroclimate and temperature of the environment in which they lived. Given the relative constant water temperatures in the Omo-Turkana Basin, the stable isotopic composition of ostracod shells mainly reflect changes in the oxygen isotopic composition of the ambient water, which is a function of the amount of riverine runoff and evaporation from Lake Turkana. Modern Omo River water isotope data shows at least a 3‰ change of the course of a year, meaning that changes in fluvial input impact the isotope mass balance of Lake Turkana. As ostracods are short-lived crustaceans, the isotopic analyses of numerous individual shells (n > 20) provides a proxy for the degree of seasonality of hydrological conditions in the Omo-Turkana basin. Here, we present stable isotope distributions of different modern ostracods species together with those of different time-intervals from the Holocene-to-modern outcrop, spanning the last 14 kyrs, in the Omo-Turkana Basin. We obtain statistically significant differences in the inter-quartile range (1.5 IQR) for the inferred δ¹⁸O_water. We interpret that increased isotopic variability within these distributions correlates to periods of higher northern hemisphere summer insolation. We hypothesize that solar insolation maxima increased the strength of the East African monsoon, resulting in enhanced seasonality in eastern Africa during humid periods. Ultimately, this proxy can also be applied to older Pleistocene deposits to further explore this relationship and better factor seasonality into our understanding of the ecosystems in which hominins evolved.