HOLOCENE ENVIRONMENTAL CHANGES ON CUMBERLAND ISLAND, GEORGIA, BASED ON d13C AND d18O SHIFTS IN DONAX VARIABILIS SAY

The response of coastal barrier environments to late Holocene sea-level change and environmental fluctuation has long been of interest to coastal scientists. The coast of southeast Georgia and northeast Florida provides a relatively undeveloped setting in which to examine barrier response to changes in sea level, salinity, and water temperature. Four locations along the southern tip of Cumberland Island, Georgia, have been selected for sampling and analysis. Each site was sampled using a vibracoring device to depths of approximately 5 meters. Sub-samples were extracted at selected depths in the cores and analyzed for sediment properties. Shell samples of Donax variabilis were collected at several levels. The shell material was then cleaned and prepared for mass spectrometry in order to determine d¹³C and d¹⁸O values. Donax variabilis exhibits a life span of one year, with growth periods from April to September. This allows the d¹³C and d¹⁸O values to represent the average carbon and oxygen isotopic values for spring-summer of a given year. The carbon and oxygen data reveal how mean spring-summer water temperatures and salinities have varied through the late Holocene. In this barrier setting, the results provide some insight as to how the coastal environments of southeastern Georgia and northeastern Florida have responded to late Holocene fluctuations in sea level, salinity, and water temperature. The results may aid in predicting the response of such environments to future changes in sea level and water temperature.