Calibration of hydroclimate proxies in freshwater bivalve shells from Central and West Africa

Freshwater bivalve shell oxygen and carbon stable isotope ratios (d18O, d13C) may act as recorders of hydroclimate (e.g., precipitation-evaporation balance, discharge) and aquatic biogeochemistry. We investigate the potential of these hydroclimate proxies measured along the growth axis of shells collected from the Oubangui River (Bangui, Central African Republic) and the Niger River (Niamey, Niger). Biweekly water samples and in situ measurements collected over several years, along with daily discharge data from both sites allowed a direct comparison with proxies recorded in the shells. Data from a total of 14 unionid shells, including three species (Chambardia wissmanni, Aspatharia dahomeyensis, and Aspatharia chaiziana), confirmed that shells precipitate carbonate in oxygen isotope equilibrium with ambient water. Because water temperature variations were small, shell d18O values (d18Oshell) also accurately record the seasonality and the range observed in water d18O (d18Ow) values when calculated using an average temperature. Calculated d18Ow values were in good agreement over the entire record of measured d18Ow values, thus d18Oshell records can be reliably used to reconstruct past d18Ow values. Discharge and d18Ow values from both rivers fit a logarithmic relationship, which was used to attempt reconstruction of past hydrological conditions, after calculating d18Ow values from d18Oshell values. A comparison with measured discharge data suggests that for the two rivers considered, d18Oshell data are good proxies for recording discharge conditions during low(er) discharge levels, but that high discharge values cannot be accurately reconstructed due to the large scatter in the discharge-d18Ow relationship. Moreover, periods of bivalve shell growth cessation due to high turbidity or air exposure should be taken into account. While d13C values of dissolved inorganic carbon in both rivers showed clear seasonality and correlated well with discharge, most of the shells analyzed did not record these variations adequately, likely due to the complication of vital effects including the variable contribution of metabolic CO2. Thus, tropical African unionid d18Oshell values can be used to reconstruct d18Ow values with high confidence to provide insight on past hydroclimate such as precipitation-evaporation balance and periods of low discharge.status: publishe