Oyster harvesting in ancient Tampa Bay

Our new study, published in Environmental Archaeology, compared Mg/Ca elemental maps produced with laser-induced breakdown spectroscopy (LIBS) with oxygen isotope (δ¹⁸O) analysis of the same shells.

In temperate seas, oyster growth bands line up neatly with the seasons, so reading them reveals both the age of the animal and the season it was harvested. Subtropical estuaries like Tampa Bay are much more messy and growth bands do not necessarily track seasonal cycles as cleanly, so that swings in salinity and water temperature blur the oxygen isotope signal that archaeologists usually rely on. Mg/Ca ratios offer a way around this problem, because they are much less sensitive to salinity while still responding to water temperature. That said, Mg/Ca in mollusc shells and oysters in particular have not always been successfully used as seasonal proxies, so it was worth investigating further.

The oysters came from Bishop Harbor, a shallow sub-basin on the shore of Lower Tampa Bay where Indigenous Floridians built shell mounds and middens over roughly a millennium. We selected 20 whole oyster valves for analysis.

At the Leibniz-Zentrum für Archäologie (LEIZA) in Mainz, we used LIBS to produce detailed two-dimensional maps of Mg/Ca across a section of each shell. This builds up an image of the shell’s seasonal growth in a fraction of the time that isotope sampling takes. We then “virtually sampled” each map, turning the 2D image into a one-dimensional profile that could be compared directly with the oxygen isotope record from the same shell.

For 18 of the 19 shells with both records, the Mg/Ca and δ¹⁸O profiles were strongly correlated (R² > 0.70) and produced consistet season of capture signatures.

Being able to read the season of collection quickly and reliably from many shells is ideal for archaeological reconstructions of harvesting practices at the scale of whole sites and we hope to carry this step out in the future.

Overall, the study establishes Mg/Ca mapping by LIBS as a fast and reliable proxy for investigating oyster harvesting in estuarine environments, and a valuable complement to stable isotope analysis in regions where the isotopes alone are hard to interpret.

This research was funded by the Association for Environmental Archaeology AEA through its small grant scheme, for the project “Investigating Oyster Management Systems”.

Reference: Rogers, J. A., Arniz-Mateos, R., & Hausmann, N. (2026). Testing Oxygen Isotope Analysis and Mg/Ca Ratios for the Investigation of Oyster (Crassostrea virginica) Harvesting in Ancient Tampa Bay, Florida, USA. Environmental Archaeology. https://doi.org/10.1080/14614103.2026.2705152