Minor Element and Stable Isotope Records In Freshwater Bivalve Shells from a 2,000 Year Old Midden, South Carolina

Freshwater bivalve shells, which are common to archeological middens provide a potential record of river and watershed paleoenvironment over a period of years to decades, depending on the life span of a mollusk. Considered as a geological deposit, a midden may be used to reconstruct environmental information over entire periods of human habitation. The type of records include hydrogen and oxygen isotopes of shell material that may provide climatic information on rainfall and temperature, while varying concentrations of manganese and barium may indicate seasonal productivity upstream of where the bivalve lived. For this study several shells from a South Carolina midden layer dated to 2070 BP were sampled at high resolution by Micromill and LA-ICP-MS for stable isotopes of hydrogen, oxygen and carbon as well as concentrations of manganese, strontium, barium and copper. These results were then compared to those from a previous study of modern bivalves from several nearby streams. Our results show a strong similarity between ancient and modern shells suggesting that these shells do retain valuable information. For example, profiles of adjacent midden shells are as distinct as profiles from different streams in modern shells. Theoretically this could be used to estimate radius of collection by indigenous people that inhabited a particular site. Another observation is that manganese concentrations are consistently low in the midden shells analyzed compared to modern shells from several streams. This may indicate that midden shells could provide a preindustrial baseline measurement for some indicators of stream health. Our results indicate that midden shell material, even of freshwater origin, can be a valuable resource for archaeological environmental reconstruction.