OSL INVESTIGATIONS OF A COASTAL SWIFT CREEK OCCUPATION AT HARRISON RING, BAY COUNTY, FLORIDA

The Gulf Coast of Florida preserves a rich history of archaeological culture that has been studied since the nineteenth and early twentieth centuries. Woodland burial mounds are well known in this region and have been studied extensively along with their closely associated shell middens. One of these shell middens, known as Harrison Ring, lies on the Tyndall Air Force peninsula and is considered to be of Swift Creek origin (~A.D. 150-800) based on the ceramic findings at the site. Historically, the temporal succession of Woodland cultural forms in this region have been based on radiocarbon dating, with some outstanding questions regarding the validity of local reservoir corrections applied to shell ages. In this study, we use optically stimulated luminescence (OSL) dating of coarse-grained quartz to determine the timing of occupation at Harrison Ring using high-resolution vertical sampling.

A total of 17 samples were collected for OSL dating at Harrison Ring with vertical sampling conducted at 10 cm intervals from the surface. We find OSL ages determined using both 0.5 mm aliquots and single grains at the archaeological levels (approximately 1751 ± 339 years ago) to be consistent with the timing of early Swift Creek cultures on the Florida Gulf Coast. In general, OSL equivalent doses show high overdispersion and skewness that we attribute to beta-microdosimetry and possible bioturbation in the profiles. Results are also presented from a test with a dosimetric technique employing Al₂O₃:C dosimeters. The beta dose rates determined from Al₂O₃:C dosimeters are broadly inconsistent with those determined from volumetrically averaged geochemical analyses (NAA & DNC). Moreover, large variability in the beta dose rates exist in the sedimentary profile at Harrison Ring, which is particularly evident in the results from Al₂O₃:C dosimeters. By testing a combination of dosimetric techniques at the site, we find that the best agreement with independent age control exists when calculating ages using a beta dose rate from NAA & DNC and a gamma dose rate from Al₂O₃:C dosimetry. This may be a useful technique to employ in areas where beta-microdosimetry is thought to be an issue (e.g. in environments with low potassium content), but should be tested further in other areas with independent age control.