Oxygen Isotope Analysis of Midden Mollusks to Assess Site Formation Processes and Subsistence Strategies at the Sapelo Island Shell Rings

Stable oxygen isotope profiles measured in midden mollusks, when coupled with detailed excavation and other analyses, yield insight into site formation processes and past human economic behavior. The Sapelo Island shell rings (Atlantic coast of Georgia, USA) represent classic examples of ring-shaped midden structures found worldwide. The largest of the three rings is ~3m high with a ~90 m diameter. Construction began in the Archaic Period prior to 4000 years cal. B.P. The rings are primarily composed of American oyster (Crassostea virginica) with some hard clams (Mercenaria sp.), but few other mollusk taxa are present. A shallow geophysical survey of the site permitted identification of sub-surface features related to early occupation, which was followed by detailed excavation of materials related to all stages of ring construction. More than 50 clams and oysters from selected features and strata were sequentially analyzed for oxygen isotope variation in the increments precipitated immediately prior to capture. The marginal oxygen isotope value of each shell was compared to seasonal oscillations prior in ontogeny to assess the season of capture. These data, coupled with zooarchaeological season of capture estimates, radiocarbon dates, and spatial analysis of features, produced a time-sequence of construction, human occupation, and subsistence strategy reconstructions. In addition to season of capture data, the isotope profiles in the shells give insight into the different oyster and clam habitats exploited by the site occupants. This habitat discrimination is a function of the relationship between salinity, water oxygen isotope values, and the complex mixing of fresh and marine waters around barrier islands.