STABLE ISOTOPE STUDY OF ENCRUSTERS ON CORALS FROM THE COCKBURN TOWN MEMBER (GROTTO BEACH FORMATION; EEMIAN, MIS 5E) FROM SAN SALVADOR AND GREAT INAGUA ISLANDS, BAHAMAS

A 34 m-deep, 54 mm-diameter core through the Cockburn Town Member of the Grotto Beach Formation from The Gulf (TG) site on the south coast of San Salvador Island, Bahamas, contains coral reef deposits from the last Pleistocene interglacial highstand (Eemian, MIS 5e). Results of petrographic, stable isotope and XRD analyses of these deposits are compared with outcrops of this interval from the Cockburn Town Fossil Reef on the west coast of San Salvador and Matthew Town Marina and Devil’s Point sites on Great Inagua Island to better understand the conditions that resulted in heavy encrustation of corals, especially Acropora cervicornis, with microbialites.

Stable isotope analyses (δ¹³C and δ¹⁸O in ‰ VPDB) included various carbonate components throughout the entire TG core and focused on intervals with encrusters up to 14 cm thick at -9.2 to -9.7, -23.8 and -31.1 m. There is positive covariation (R² = 0.7) between data for coral (lower values; δ¹³C = -1.8 to 1.9‰; δ¹⁸O = -3.6 to -1.1‰) and laminated microbialites (higher values; δ¹³C = 1.2 to 4.4‰; δ¹⁸O = -3.1 to -0.2‰). Outcrop data have similar trends, with a sharp increase in δ¹³C and δ¹⁸O values at the transition between corals and microbial encrusters. These differences can in part be attributed to isotopic fractionation between aragonitic corals and mixed aragonitic and calcitic microbialites, and to vital effect and greater degree of meteoric diagenetic modification of corals.

Microbialites incorporate fine carbonate sand and mud, and their composition overlaps with skeletal-peloidal-ooid sand and micritic sediment from TG core (δ¹³C = -1.9 to 3.3‰; δ¹⁸O = -2.6 to 0‰). Rhizoliths from -19.4 to -24.4 m in TG core have δ¹³C values as low as -5.4‰, and outcrop caliche has the most negative δ¹³C value of -8.2‰, representing formation from meteoric water. Conversely, the most positive δ¹³C values of microbialites may indicate elevated organic productivity due to enhanced nutrient input to shallow marine settings, possibly during post-storm run-off. The transitions from corals to thick encrusters may also reflect changes in water depth, with elevated δ¹³C and δ¹⁸O values of microbialites indicating shallower and more restricted conditions. These results complement other sedimentological and paleontological studies to provide insights into reef microbialite formation.