FRESHWATER TAPHONOMY: EXPLORING THE EFFECT OF TIME ON THE TRACE ELEMENT AND STABLE ISOTOPE RECORD IN UNIONID SHELLS

Representing a border zone between terrestrial and marine systems, geochemical records of ancient freshwater environments can offer valuable insights into environmental change upstream. The shells of freshwater bivalves provide a promising repository of trace element and stable isotope proxies for environmental conditions such as precipitation, primary productivity and temperature. To use these proxies it is important to understand how the passage of time may alter the fossil or sub-fossil chemical record. Using live-collected Elliptio icterina, a two year in-stream deployment of empty shells was conducted to determine the rate of shell loss and whether the shell composition: δ¹⁸O, δ¹³C, δD, [Mn], [Cu], [Sr] and [Ba], changed over the course of the study. An additional objective was to understand the role of the periostracum in preserving shell material and chemical composition. Of forty-two valves, the periostracum was removed from half, and then the valves were measured, weighed and photographed before being placed in two streams on the Department of Energy's Savannah River Site in South Carolina. The study streams are characterized by slightly acidic water, undersaturated with respect to aragonite. Valves were collected at four month intervals. Shells were then photographed, weighed and analyzed for bulk stable isotope and trace element composition. In addition, six matching valves, one deployed and one undeployed, were analyzed at high resolution by micromilling samples for stable isotope analysis and by laser ablation ICP-MS for the analysis of trace elements. Extrapolating from the results, it is indicated that shells persist in the study streams for less than 20 years with smaller shells more likely to undergo higher rates of shell loss, and shells without a periostracum losing shell mass twice as quickly. The bulk shell stable isotope composition did not change, while some increase in Mn concentration was evident. A comparison of the trace element profiles from opposite valves, though, indicates that the interior composition remains unaltered. These results suggest that where Unionid shells persist they retain a stable isotope and trace element record that may be useful in the reconstruction of continental paleoenvironments.