Environmental and climatic control on seasonal stable isotope variation of freshwater molluscan bivalves in the Turkana Basin (Kenya)

We present growth incremental stable isotope records (delta O-18 and delta C-13) of modern and similar to 2 Ma fossil bivalve shells from the semi-arid Turkana Basin (N. Kenya, S. Ethiopia). These data suggest that seasonal cyclicity in delta O-18 and delta C-13 recorded by modern and fossil shells is driven by wet-dry seasonal changes in host water chemistry, forced by monsoonal rainfall over the Ethiopian Highlands. Fully lacustrine shells show lower amplitude, or even absent seasonal cyclicity in comparison with deltaic or riverine shells because of the buffering effect of the large water volume in the lake setting. Riverine shells arguably have the clearest relation to seasonally variable climate parameters. Riverine molluscs thus provide a potentially valuable proxy for varying rainfall delta O-18 values in the Turkana Basin catchment. Cross plots of molluscan delta O-18 and delta C-13 data reveal dominant environmental control on molluscan isotope values with remarkably large isotopic differences between lacustrine, deltaic and riverine environments. We interpret this isotope pattern to directly result from the different mixing proportions of Omo River source water with evaporated lake water in these environments. The interpretation of fossil molluscan delta O-18 and delta C-13 data in a palaeoclimatological context is not straightforward, since the potential influence of temporal changes in lake water temperature, surface evaporation or river discharge on the delta O-18 budget of the lake is smaller than the isotopic shifts caused by shifting facies patterns in the sedimentary record. Even though it is clear from the rich molluscan faunas that the similar to 2 Ma palaeo-Lake Lorenyang must have been significantly less alkaline than modern Lake Turkana and likely provided good drinking water and abundant availability of food for the different species of hominins inhabiting the region at that time, stable isotope values of molluscan bivalves are not suitable to record the difference in alkalinity between these two settings. (C) 2013 Elsevier B.V. All rights reserved.