ENCRUSTERS ON PLEISTOCENE CORALS FROM THE BAHAMAS: NEW DATA FROM A CORE ON SAN SALVADOR AND COMPARISON WITH OUTCROP OBSERVATIONS FROM GREAT INAGUA AND SAN SALVADOR ISLANDS

This research examines distribution, type and abundance of encrusters on corals from a core through a Pleistocene (MIS5) reef at The Gulf site on the south coast of San Salvador. Results are compared with data from outcrop samples of the Eemian (MIS 5e) Cockburn Town Mbr. (Grotto Beach Fm.) at the Cockburn Town Fossil Reef (CTFR) site on San Salvador’s west coast and from Devil’s Point (DP) and Matthew Town Marina on Great Inagua. Our goal was to evaluate environmental conditions during the last interglacial that resulted in coral encrustation and elucidate the role of encrusters in modifying reservoir properties of reefal rocks.

High-resolution scans of petrographic thin sections from core samples were analyzed in Adobe Illustrator to document the distribution of encrusters (red algae, foraminifera, serpulids, and microbial coatings), and Fiji/ImageJ software was used to quantify their abundances (% surface area). Corals (Acropora cervicornis, Orbicella annularis, and Colpophyllia natans) occupy up to ~75% of samples analyzed. Red algae encrust directly onto corals, make up to ~35% of samples and form crusts up to 1.5 cm thick. Scattered among and on top of algal crusts are encrusting foraminifera (Homotrema rubra, Carpenteria utricularis, Planogypsina acervalis, Gypsina plana), covering 1-10% of all samples. Serpulids have similar distribution but occupy a smaller area (0-8%). Microbialites are present in 40% of samples, covering up to ~45% of area, succeeding red algae, and incorporating encrusting foraminifera. In core samples, laminated microbialites form crusts up to 14 cm thick on A. cervicornis corals near top of the reef.

In comparison, CTFR and DP outcrops expose the Devil’s Point discontinuity separating Reefs I and II. CTFR Reef I has A. cervicornis with microbial encusters up to 9 cm thick, in addition to red algae and foraminifera, whereas DP Reef I lacks thick encrustation. CTFR and DP Reef II corals range from pristine to moderately encrusted by red algae and foraminifera only. Corals from dredged boulders at the Marina site bear all encruster types, suggesting a localized setting for microbialite development. Even though locally distributed, thick microbial encrusters can occupy a substantial portion of reefs and indicate increased coral stress related to temperature and sea level fluctuations.