Fossil specimens of Acropora palmata and Acropora cervicornis (~125 Ka) were collected from the Cockburn Town fossil reef on San Salvador Island, Bahamas. These corals were collected to determine whether stable isotope data support a drastic temperature fluctuation during the late Pleistocene as implied by 230Th dating of similar corals and also the sedimentologic record of the Cockburn Town reef (Wilson et al., 1998). Thin sections of A. palmata and A. cervicornis were made to ensure accurate sampling and to assess the degree of diagenesis. Stable isotope data from skeletal carbonate material were collected to determine if the corals show residual signs of thermal stress — d18O values as a measure of temperature and d13C values to indicate whether the corals contained symbiotic algae at the time of their demise. Assuming ancient corals were physiologically similar to modern corals, if the corals from Pleistocene time did not contain zooxanthellae, they responded to thermal stress similarly to modern corals. Evidence of bleaching from the Pleistocene age Cockburn Town fossil reef would provide a precedent for modern coral bleaching.
The d18O and d13C values of A. cervicornis are similar to those of modern Acroporid corals (Carpenter et al., in preparation), but the values of A. palmata show contamination from marine cementation. More detailed petrographic work is needed to direct sampling around the cemented areas. The data from cements and inorganic materials indicate a typical L shaped pattern of diagenesis (Martin et al., 1985) based on changing water/rock ratios from input of organic material into fluids. Diagenetic products identified were similar to those found in previous studies, with extreme meteoric diagenesis at the hiatus surface. Light carbon values from the hiatus indicate subareal exposure, but it is difficult to determine exactly when this light carbon formed.
Valerie Gamble, ’01 St. Paul, MN
Major: Biology, Geology
Sponsor: Ben Greenstein