Interpretation of Seasonality from Geochemistry and Sclerochronology of Holocene and Pleistocene Freshwater Bivalves from the Omo-Turkana Basin, Ethiopia and Kenya

Abstract

Reconstructing past environmental variability at short (annual-decadal) time scales is critical for understanding ecological drivers of evolution, including that of our own near relatives, the hominins of Africa. One promising approach to reconstructing that variability is through the use of stable isotope geochemistry and sclerochronology, the study of growth banding in organisms that undergo accretionary growth. In this study we obtained sclerochronology and stable isotope ratios from bivalve shells collected from deposits of the Omo River and Lake Turkana/paleolake Lorenyang, Kenya, a region that experiences a highly seasonal (tropical semi-desert) climate. We analyzed δ18O and sclerochronology from six Etheria elliptica (river oyster) shells from the Omo River delta and East Turkana outcrops that range from modern to mid-Holocene age, and six early Pleistocene (~1.6Ma) Pseudobovaria sp. (extinct African mussel) shells from the WTK-13 Hominin Sites and Paleolakes Drilling Project (HSPDP) drill core, from West Turkana, Kenya. One modern river oyster has exclusively negative δ18O values through ~1.5yrs of growth consistent with a riverine provenance. Three late Holocene shells show a much broader range of isotopic variability with overall more positive values and lived for ~2.5yrs. This range in δ18O suggests a deltaic provenance for these shells: lower δ18O values reflect Omo River flood discharge, whereas more positive δ18O values reflect evaporated Lake Turkana water during reduced Omo River runoff. Two mid-Holocene shells exhibit less isotopic variability with δ18O values consistent with lacustrine growth coinciding with a time of fresher Lake Turkana water at the end of the African Humid Period. The six early Pleistocene shells have large seasonal δ18O cycles over ~0.5yr to ~3yrs of growth, with δ18O values comparable to modern and Holocene deltaic shells. The large range in δ18O may reflect deltaic growth under significant seasonality in paleo-Omo River runoff. Oxygen isotope cycles suggest high degrees of seasonality of paleo-Omo River (or other major influent) runoff into paleolake Lorenyang at ~1.6Ma which may be similar to modern hydroclimate conditions.