LATE EOCENE OCEAN TEMPERATURES: INSIGHTS FROM GIANT OYSTERS

Coupling sclerochronological techniques with carbon and oxygen isotope analysis provide powerful quantitative proxies for paleoclimatological and paleoecological information that have been applied sparingly to Eocene fossils in the Atlantic Coastal Plain. This approach was used to analyze fossil shells of the giant oyster Crassostrea gigantissima(Finch, 1824) collected from outcrops at Griffins Landing along the Savannah River in Burke County Georgia to better understand paleoclimate for a climatically critical period in the geological record.

Articulated specimens were collected in life position from outcrops and the left valve of each was sectioned for further analysis. Each section was evaluated under cathodoluminescence and by thin section petrography to ensure that pristine material was sampled for stable isotope analysis. Approximately 100 μg of carbonate was removed from unaltered shell by drilling small grooves (1-1.5mm) parallel to observed growth bands in the section slightly below the ligament surface using a Merchantek Micromill. Drill depth was <0.5mm to minimize any error that might result from drilling through multiple layers. Consecutive samples were drilled sequentially along the growth axis, providing a contiguous isotopic record of shell growth. The paleotemperature equation for calcite (Epstein et al., 1953) was used to calculate isotopic temperature. The oxygen isotopic composition of seawater was estimated at -1.2‰ for assumed ice-free conditions of the Tertiary.

The range in oxygen isotope composition of individual shells varied from +1.2 to +1.8‰ (n = 4), indicating that the mean annual range in temperature varied from 6 to 9ºC. Average oxygen isotope values varied from -0.9 to -1.7‰ indicating average temperatures of approximately 15 to 19ºC (assuming δ_w= -1.2‰). Carbon isotope compositions showed a distinct pattern of more negative values during warmer conditions (i.e., when δ¹⁸O values are low) and more positive values during cooler conditions (i.e., when δ¹⁸O values are high) which is consistent with increased phytoplankton productivity during the summer months.