Relationship between river water chemistry and shell chemistry of two tropical African freshwater bivalve species

• Published in: Chemical geology 2019-11, Vol.526, p.130-141
• Main Authors: Kelemen, Zita, Gillikin, David P., Bouillon, Steven
• Format: Article
• Language: English
• Subjects: Ba/Ca; Freshwater bivalve; Mg/Ca; Mn/Ca; Sclerochemistry; Shell geochemistry; Sr/Ca
• ISSN: 0009-2541; 1872-6836
• DOI: 10.1016/j.chemgeo.2018.04.026

Both climate change and land-use changes represent serious environmental issues throughout Africa, and are expected to lead to significant changes in river hydrology and geochemistry. Freshwater bivalve shell geochemistry may provide useful information about past river characteristics (discharge, geochemistry) providing a baseline to help understand modern environmental changes. Shells of two species of freshwater mussels (Chambardia wissmanni and Aspatharia dahomeyensis) were collected from two large African rivers (Oubangui and Niger Rivers) and were analyzed for Sr/Ca, Ba/Ca, Mg/Ca and Mn/Ca ratios along their maximal growth axis. Shell data were compared with geochemical data collected in these rivers every two weeks over a two-year period. To verify whether element ratios in shells were related to the host water, the elemental concentrations measured in the shells were set in a temporal context based on δ18Oshell cycles. Shells from the Oubangui River to some extent recorded water element ratios, with variations in water Sr/Ca ratios being reflected in all three bivalve specimens analyzed, while only the youngest specimen exhibited complete coverage. The two older specimens recorded only segments of the water data, which suggests an influence of growth rate. This relationship was not observed in A. dahomeyensis shells from the Niger, suggesting a species-specific relationship between freshwater shells and water. The Ba/Ca and Mg/Ca record showed no similarity between the water elements and analyzed shells, but an ontogenetic decrease was observed in older specimens. Shell Mn/Ca ratios tracked pH in shells from the Oubangui, with a time-lag, which might be due to bivalves taking up the increased Mn via phytoplankton which are expected to show a similar time-lag. This relationship was not observed in shells from the Niger, where due to the high turbidity throughout the year we expect very low primary production rates. In conclusion, no consistent relationships between water chemistry and shell chemistry were observed. While Sr/Ca and Mn/Ca ratios in shells of C. wissmanni from the Oubangui were most promising as proxies of water chemistry, the utility of element ratios as tracers is compromised by species-specific effects or by site-specific phenomena. •The relationship between water and shell chemistry is not consistent.•Element incorporation mechanisms in freshwater bivalve shells appear to be species-specific.•Ba/Ca and Mg/Ca ratios were found to be