ISOTOPIC VARIATIONS IN MOLLUSCAN BIVALVES RECORD CATCHMENT AND EVAPORATIVE CONTROLS IN THE OMO-TURKANA BASIN (Invited Presentation)

Within the East African Rift System (EARS), the Omo-Turkana Basin is situated in one of the hottest regions on Earth. In the past, the presence of water bodies in the basin provided rich resources to sustain human inhabitants in this otherwise inhospitable area. Today, the basin is occupied by Lake Turkana, which receives 80-90% of its water from the Omo River that drains the Ethiopian Highlands with their abundant monsoonal rainfall. Currently, Lake Turkana is hydrologically closed as it has no output other than evaporation. As a consequence, the lake water is highly alkaline compared to the inflowing riverine waters, inhibiting the growth of molluscs. However, large molluscan bivalves are relatively abundant in Pleistocene to early Holocene deposits, when Lake Turkana experienced high levels and freshwater conditions. Here, we present new incremental stable carbon and oxygen isotope records of molluscs from the Galana Boi and Kibish Formation to reconstruct environmental conditions and short-term climatic variability. The strontium isotopic compositions have been also measured from these shells to infer changes in water source areas. The isotopic compositions clearly differentiate between shells that had grown in fluvial, deltaic and lacustrine conditions as well as between different high stand levels. Radiocarbon dated specimens allow for direct comparison with isotopic distributions of time-equivalent ostracods shell assemblages from ancient Lake Turkana. These isotopic compositions record the integrated catchment and evaporative controls in the Omo-Turkana Basin, when (early) humans roamed the marginal environments. The obtained dataset provides a solid baseline for environmental and climatic reconstructions of older lake phases. For the Pleistocene Lorenyang lake phase, isotopic signatures have been determined for ostracod shells from the HSPDP ICDP WTK-13 drill core, and nearby outcrops in the Kaitio Laga. These isotopic compositions cover the entire range of the Holocene lake-level drop indicating considerable lake level fluctuations of paleo-Lake Lorenyang, which therefore probably provided a less stable resource for early hominins as previously thought.