RECONSTRUCTING PAST AFRICAN RIVER HYDROLOGY BY MEANS OF FRESHWATER BIVALVE GEOCHEMISTRY

Freshwater bivalves occur in nearly all river basins and different types of river ecosystems. Microsampling of the growth increments in their shells allows high-resolution reconstruction of intra-yearly changes in ambient river water geochemistry. Specimens were collected from locations across Africa (Niger, Mali, Central African Republic, D.R. Congo, Kenya, Botswana, Namibia, and South Africa). Isotope ratios of δ¹⁸O were measured in order to reconstruct δ¹⁸O of ambient water. Based on 3+ year monitoring of temperature and water δ¹⁸O in Bangui, Kinshasa and Kisangani (all Congo Basin), we show that different species, including Chambardia, Aspatharia and Etheria accurately record water δ¹⁸O in their shell carbonate, and can be used to reconstruct past river discharge. Intra-annual variability of δ¹⁸O in central Africa is relatively low, between -4.0 ‰ and -2.0 ‰ around the equator and between -4.0 and 0.0 ‰ at 5° N. Towards southern Africa, the seasonal range increases to 6.9 ‰, reflecting both temperature and δ¹⁸O-water variability. By contrast, specimens from Niger show a 12 ‰ range, from -8.0 to +4.0 ‰, and an Aspatharia from a Lake Faguibine tributary in Mali shows previously unrecorded δ¹⁸O values ranging up +11.3‰, reflecting contrasting humidity and highly evaporative conditions. These data provide a preliminary investigation of continental-scale variability in riverine δ¹⁸O variability as recorded by freshwater bivalves.