Elemental Uptake in Botswanan Freshwater Bivalve Shells (Mutela zambesiensis and Coelatura kunenensis) Along a Strong Evaporative Gradient: Paleoclimate Reconstruction Implications

The chemical composition of bivalve shells has been suggested to reflect of the chemical composition of the water in which they grew. In this study, the Mn/Ca, Ba/Ca, Sr/Ca, and Mg/Ca ratios in the aragonitic freshwater bivalve shells of Mutela zambesiensis and Coelatura kunenensis harvested from ten different sites from the Linyanti River, Lake Liambezi, the Savuti River, the Selinda River, and the Zambesi River in both Botswana and Namibia which vary from humid to arid were analyzed. These analyses show that there is a weak correlation between [Sr/Ca]water and [Sr/Ca]shell of M. zambesiensis and C. kunenensis bivalves (R2~0.1). Using shell length as a proxy for growth rate, a strong correlation between growth rate of M. zambesiensis and [Sr/Ca]shell ratios (R2 = 0.89, N = 7) was observed. However, no such relationship was observed between the growth rate of C. kunenensis and [Sr/Ca]shell ratios. Additionally, no relationships were observed between Mn/Ca, Ba/Ca, and Mg/Ca ratios within the shell and neither the corresponding elemental ratios within the water, nor the shell length. These results show that Sr uptake into the shells of these bivalves is partially influenced by the concentration of Sr in the water in which they live, however, the correlation in this relationship is weak, suggesting that elemental ratios in the shells of M. zambesiensis and C. kunenensis would not make good proxies of environmental conditions.