Controls\non the Barium and Strontium Isotopic Records\nof Water Chemistry Preserved in Freshwater Bivalve Shells

Biogenic carbonates,\nincluding bivalve shells, record past environmental\nconditions, but their interpretation requires understanding environmental\nand biological factors that affect trace metal uptake. We examined\nstable barium (δ<sup>138</sup>Ba) and radiogenic strontium (<sup>87</sup>Sr/<sup>86</sup>Sr) isotope ratios in the aragonite shells\nof four native freshwater mussel species and two invasive species\nin five streams and assessed the effects of species identity, growth\nrate, and river water chemistry on shell isotopic composition. Shells\nwere robust proxies for Sr, accurately reflecting <sup>87</sup>Sr/<sup>86</sup>Sr ratios of river water, regardless of species or growth\nrate. In contrast, shell δ<sup>138</sup>Ba values, apart from\ninvasive Corbicula fluminea, departed\nwidely from those of river water and varied according to species and\ngrowth rate. Apparent fractionation between river water and the shell\n(Δ<sup>138</sup>Ba<sub>shell‑water</sub>) reached −0.86‰,\nthe greatest offset observed for carbonate minerals. The shell deposited\nduring slow growth periods was more enriched in lighter Ba isotopes\nthan the rapidly deposited shell; thus, this phenomenon cannot be\nexplained by aragonite precipitation kinetics. Instead, biological\nion transport processes linked to growth rate may be largely responsible\nfor Ba isotope variation. Our results provide information necessary\nto interpret water chemistry records preserved in shells and provide\ninsights into biomineralization processes and bivalve biochemistry.