Testing Oxygen Isotope Analysis and Mg/Ca Ratios for the Investigation of Oyster ( Crassostrea virginica ) Harvesting in Ancient Tampa Bay, Florida, USA

Shellfish remains are among the most abundant materials recovered from coastal archaeological sites and provide valuable archives for reconstructing past environments and human subsistence strategies. Mollusc shells incorporate chemical variations during growth that reflect environmental conditions, including the chemical composition of seawater and sea surface temperatures. This record can be used to infer the season of collection and the age and growth patterns of archaeological molluscs. In subtropical estuaries, interpreting the age and growth of oysters can be challenging because growth does not always correspond to seasonal cycles, and simultaneous fluctuations in salinity and water temperature can influence commonly used geochemical proxies, such as oxygen isotope values (δ18Oshell). Our study evaluates the use of Mg/Ca ratios from trace-element analysis by Laser-Induced Breakdown Spectroscopy (LIBS), which do not strongly depend on changes in salinity. Mg/Ca data from 20 archaeological oysters (Crassostrea virginica) from the Tampa Bay Estuary, Florida, were compared with δ18Oshell data from the same shells. Mg/Ca profiles show strong agreement with isotope-derived seasonal patterns and often provide clearer identification of annual growth cycles. These results demonstrate that Mg/Ca ratios can serve as a reliable proxy for archaeologists investigating patterns of oyster harvesting in estuarine environments.