Controls on the Barium and Strontium Isotopic Records of Water Chemistry Preserved in Freshwater Bivalve Shells

Biogenic carbonates, including bivalve shells, record past environmental conditions, but their interpretation requires understanding environmental and biological factors that affect trace metal uptake. We examined stable barium (δ¹³⁸Ba) and radiogenic strontium (⁸⁷Sr/⁸⁶Sr) isotope ratios in the aragonite shells of four native freshwater mussel species and two invasive species in five streams and assessed the effects of species identity, growth rate, and river water chemistry on shell isotopic composition. Shells were robust proxies for Sr, accurately reflecting ⁸⁷Sr/⁸⁶Sr ratios of river water, regardless of species or growth rate. In contrast, shell δ¹³⁸Ba values, apart from invasive Corbicula fluminea, departed widely from those of river water and varied according to species and growth rate. Apparent fractionation between river water and the shell (Δ¹³⁸Ba_shell-water) reached -0.86‰, the greatest offset observed for carbonate minerals. The shell deposited during slow growth periods was more enriched in lighter Ba isotopes than the rapidly deposited shell; thus, this phenomenon cannot be explained by aragonite precipitation kinetics. Instead, biological ion transport processes linked to growth rate may be largely responsible for Ba isotope variation. Our results provide information necessary to interpret water chemistry records preserved in shells and provide insights into biomineralization processes and bivalve biochemistry.