CONSTRAINTS ON THE USE OF BARIUM AND STRONTIUM ISOTOPES IN BIVALVE SHELLS AS ENVIRONMENTAL TRACERS IN RIVERINE SYSTEMS

Barium (Ba) and strontium (Sr) readily substitute for calcium (Ca) in the aragonite lattice of bivalve shells. The Ba and Sr isotopic composition of the stream environment can therefore be preserved in shells, making them potentially sensitive recorders of metal geochemistry and other environmental parameters, including records of past conditions preserved in historical or fossil shells. To interpret these records, we need to understand factors that influence how trace metals from the environment are incorporated into shells. We examined how species identity and growth rate affect incorporation of trace metals. We measured stable Ba and radiogenic Sr isotopes in shells of 31 bivalves representing six taxa (four native species and invasive Corbicula fluminea and Dreissena polymorpha) and six different streams in the upper Ohio River basin. For three native species, we sampled rapidly growing portions of the shell produced during the juvenile stage and slower-growing portions of the adult shell. Ba isotope fractionation between shells and stream water differed widely among species and according to growth rate. Native species and D. polymorpha were depleted in heavy Ba isotopes compared to stream waters by 0.3 to 0.9‰ (typical analytical uncertainty <±0.05‰). In contrast, Ba isotope composition of C. fluminea was indistinguishable from that of its associated stream water. The radiogenic Sr isotope compositions of shells of all species were indistinguishable from those of the stream waters, suggesting that the dissolved load is the primary source of alkaline earth elements in shells. Species-dependent Ba fractionation could result from differences in (1) shell mineral ultrastructure, (2) biological processes associated with shell formation, and (3) Ca/Ba/Sr sources (e.g., dissolved in stream vs. bound to organic matter). The large differences in Ba isotope ratios among species and life history stages provide important information about shell growth mechanisms in freshwater bivalves, and controlling for these factors is critical to a robust interpretation of shell metal concentrations and the use of isotopes as environmental proxies.