Sclerochronology, the study of periodic increments in skeletal organisms, can decipher the life history and environmental records preserved in fossil shells. Two powerful tools used in sclerochronologic studies are oxygen and carbon isotope profiles. Although many studies have applied isotopic analyses to marine shells, few have examined isotope profiles from estuaries. One common inhabitant of estuaries is the crassostreine oyster, which preserves a sclerochronologic record on the ligamental area surface in the form of external convex and concave bands. This study examines delta 18 O and delta 13 C profiles across ligamental increments found in Crassostrea virginica shells from the recent of Terrebonne Bay, Mississippi Delta, and the Pleistocene of Chesapeake Bay, Virginia. Results show that Mississippi Delta oysters calcified their shells close to isotopic equilibrium with respect to their environment. Three skeletal breaks found in one oyster formed from cessation of shell growth when summer temperature exceeded 27 degrees C. delta 18 O profiles show that ligamental increments formed annually from varying growth rates through the year, with convex bands forming from rapid growth during spring and summer, and concave bands forming from slower growth during winter. Chesapeake oysters have well-defined ligamental increments with deep concave bands that formed from slow growth and cessation during winter. Skeletal growth breaks suggest that temperature decreased below 10 degrees C during winter. Estimated Pleistocene delta 18 O water values are similar to published recent delta 18 O water values measured at the mouth of the Chesapeake Bay. Two important factors affecting shell delta 13 C values are metabolic changes and occurrence of phytoplankton blooms. Ranges of shell delta 13 C values are similar to delta 13 C dic values measured from water samples, which indicate that oyster delta 13 C profiles are tracking changes in environmental DIC, such as changes related to phytoplankton blooms and benthic respiration. Summer delta 13 C maxima may be related to recovery from gametogenesis. These results demonstrate that stable isotope sclerochronology of oyster shells facilitates the interpretation of past estuarine environments and oyster life histories.

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