Memoirs of river life
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Freshwater unionid bivalves are potentially valuable sources of paleoenvironmental proxies in terrestrial systems. The accretionary, predominantly nacreous shell structure contains several potentially useful stable isotope and trace element records. The interpretation of these records is predicated on understanding how these records respond to environmental, biological and preservational factors. In this study, the shells of Elliptio complanata from several streams on the Savannah River Site in South Carolina were analyzed with high spatial resolution techniques including 1813micro-milling and laser ablation ICP-MS for ´O, ´C and ´D values as well as concentrations of Mn, Cu, Sr, and Ba to explore: whether the ´D values of shell material reflect aqueous ´D values; the influence of shell layer structure on trace element concentrations; the relationship between aqueous and shell trace element concentrations; and the preservation potential of paleoenvironmental proxies in relict shells over annual to decadal time frames. The results of this study indicate that the ´D values of the organic matrix of unionid bivalves reflect aqueous ´D values. Because a strong relationship often exists between meteoric 1818´D and ´O values, shell ´D values potentially offer estimates of aqueous ´O values allowing 1818for temperature calculations from shell ´O values that are independent of aqueous ´O values. The presence of organic matrix in archaeological shells of 2000 years suggests this proxy may endure in the recent geologic past. The analysis of unionid shells for trace element proxies must take into account shell layer, such as inner versus outer nacreous layer, even when shell fabric, in this case nacre, is similar. A two-fold difference in the concentration of Mn, Sr and Ba between layers may be due to differences in shell growth rate but more likely, indicates a strong metabolic control in shell chemistry. Despite this influence, there is a strong relationship between the average concentrations of Mn, Sr and Ba in a layer and the water in which the bivalves lived. Other trace element concentrations, such as Cu, do not show a similar relationship and may be more closely related to the organic matrix than the inorganic aragonite of the shell. And, despite the rapid loss of shell material in death assemblages, stable isotope and trace element records are not compromised over a two-year period.