AFRICAN FRESHWATER BIVALVES AS CLIMATE AND ENVIRONMENTAL ARCHIVES

Bivalves are carbonate-secreting organisms that store critical information about the environment in which they grew. In this study, freshwater bivalves (Unionids) were collected from various locations in Africa (Central African Republic, Kenya, and Mali). By analyzing the isotopic ratios of δ¹⁸O and δ¹³C, it is possible to reconstruct the past environmental conditions they grew in. Two Aspatharia pfeifferiana shells and three Chambardia wissmanni from the Oubangui River in the Central African Republic (CAR) collected at three times over a two-year period were analyzed. Temperature and δ¹⁸O-water values collected fortnightly over ~three years (March 2010-November 2013) were used in a paleotemperature equation to calculate predicted shell δ¹⁸O values. All five shells accurately track predicted values, which are controlled mainly by variations in water δ¹⁸O values, illustrating that these shells are excellent environmental recorders. An Aspatharia rochebrunei shell collected from a smaller stream draining the Ngotto rainforest in CAR exhibited little variation in δ¹⁸O and δ¹³C values, probably due to the smaller variation in precipitation amounts and δ¹⁸O values in this region. Contrasting this is an Aspatharia dahomeyensis shell from a tributary of Lake Faguibine in Mali (in the Sahel region), which had δ¹⁸O values ranging from -5.0‰ to +11.3‰. These are the most positive bivalve shell δ¹⁸O values we are aware of; clearly this arid region undergoes intense evaporation. The Kenyan Chambardia wahlbergiwas specimen collected from the Tana River exhibits values similar to the other sub-Saharan shells. This survey of African freshwater shell stable isotope geochemistry provides a preliminary look at how these shells record their environment.