TRACE METAL CONCENTRATIONS AND ISOTOPE COMPOSITIONS OF ALLEGHENY RIVER DISSOLVED LOAD, SUSPENDED LOAD, AND SEDIMENT POREWATER: IMPLICATIONS FOR GEOCHEMICAL PROXIES IN FRESHWATER BIVALVE SHELLS

Recent analytical advances have allowed us to accurately measure the isotopes of barium (Ba) in environmental samples, which has led to a growing body of research on Ba isotope fractionation and global Ba isotope distribution. However, research into Ba isotope systematics and their use as geochemical proxies in freshwater environments is still in its infancy. Our lab has been refining the use of freshwater bivalve shells as geochemical proxies, with a particular focus on trace metal and isotope records. Bivalves are sedentary animals that interact with the river water and sediment porewater, while filter feeding suspended particulate from the water column throughout their lives. Ba, Sr, and Mn readily substitute for Ca in the aragonite lattice of bivalve shells, and thus the riverine isotopic compositions of Ba and Sr and concentrations of other trace metals are potentially preserved within the shell. With this study we seek to constrain the stable Ba and radiogenic Sr isotope composition of suspended particulate matter (seston) and sediment porewater, which are two potentially critical riverine components relevant to bivalve shell chemistry proxy development. We selected two sampling sites in the lower Allegheny River (western Pennsylvania, USA): 1) a relatively turbulent environment in the tailwaters of a navigational dam, and 2) the calmer pooled section above the dam. The results of ongoing sampling and elemental/isotopic analyses of sediment porewater, seston, dissolved river water, and bivalve shells (Actinonais ligamentina, Dreissena polymorpha, Corbicula fluminea) from each location will be reported. Initial results show that sediment porewater within the pooled section of the river is elevated in Ba, Sr, Ca, and Mn relative to the overlying river water by factors of 4, 1.3, 1.2, and 90, respectively. These elements are also concentrated in sediment porewaters at the turbulent site, but to a lesser extent. Elemental and Ba and Sr isotope measurements for all samples will provide insight into trace metal uptake and Ba isotope fractionation into freshwater bivalve shells and provide context for interpretations of these findings for bivalve lifecycles and geochemical proxies.