LATE EOCENE PALEOTEMPERATURES: INSIGHTS FROM CRASSOSTREA GIGANTISSIMA

Articulated specimens of the fossil oyster Crassostrea gigantissima were collected in life position from oyster banks exposed in outcrop at Griffins Landing, GA, to better constrain Late Eocene (35±0.5Ma) climate for the southeast coast of the U.S. The specimens were removed from encasing sediment, they were cleaned and the chondrophore of each left valve was sectioned for further study. Primary shell material was identified by the presence of a non-luminescent foliated microstructure when the sections were viewed under cathodoluminescence. Sequential subsamples (~150 microgram) of primary material were collected from six specimens by micromilling material at ~0.3 mm increments along the maximum growth axis of a section from the umbo toward the outer shell margin just below the surface of the chondrophore. In addition, subsamples of dogtooth calcite, blocky calcite spar, and luminescent shell material were collected to determine an isotope signature for diagenetic alteration. Oxygen isotope profiles for three shells displayed periodic fluctuations from -2.5 to 0.6‰ (VPDB), from which a mean annual temperature of 19ºC (winter: 15º; summer: 23ºC) was estimated using a d18O value of -0.3‰ for late Eocene seawater and the paleotemperature equation of Epstein et al. (1953). Carbon isotope values for these shells averaged -1.1±0.2‰ and they did not correlate with the oxygen isotope data. While oxygen isotope profiles for the three other shells displayed similar periodicity and mean annual temperatures (20-22ºC), average d13C values (-3.0 to -1.8‰) were distinct and the data formed trends that are consistent with a mixing line between the isotope composition of primary Eocene marine calcite and soil derived carbonate, suggesting that some shells may be altered in subtle ways that are not easily recognized. Considering the paleotemperature records of the unaltered shells only, winter sea surface temperatures for the Late Eocene appear to be similar to the modern coast of Georgia while summer temperatures were 2-8ºC cooler. These results are consistent with paleotemperatures estimated from fossil bivalves of the Gulf Coastal Plain of the U.S. and they suggest that a regional cooling event impacted marine fauna in both the Gulf of Mexico and southeast U.S. during the late Eocene.