THICK MICROBIAL ENCRUSTERS ON CORALS FROM PLEISTOCENE REEFS IN THE BAHAMAS

Thick microbial encrusters are present on Acropora cervicornis corals below the Devil’s Point Discontinuity (DPD) that separates Stages I (lower) and II (upper) of the Cockburn Town Fossil Reef interval (CTFR) – an Eemian/MIS 5e reef exposed at a quarry in Cockburn Town, along west coast of San Salvador Island, Bahamas. Corals were first coated with skeletal encrusters (coralline red algae, serpulids, foraminifera), and then overlain by microbialites that become more extensive upward toward DPD and consist of up to ~5 cm thick stromatolites on the upper sides of corals and up to ~10 cm of clotted microbialites engulfing some coral branches. Stromatolites incorporate fine-grained skeletal-peloidal-ooid sand within the micritic laminae. Clotted microbialites have patches of micrite with irregular fenestrae, medium- to coarse-grained sand, and may be crudely laminated.

In contrast, CTFR Stage II corals at the quarry and at Devil’s Point (west coast of Great Inagua Island) lack microbialites, which reflects differences in MIS 5e reef development in the Bahamas. Even though DPD is not pronounced in this interval of a 34-m core from The Gulf on the south coast of San Salvador, the CTFR interval in this core contains A. cervicornis with preliminary U/Th dates indicating MIS 5d-e age (~111-119 ka), and up to 14 cm thick, synsedimentary to slightly younger microbial encrusters (~104-108 +/-1.5-2 ka).

These deposits are the subject of a multifaceted study to elucidate formation of microbialites and their impact on reef properties. Petrographic, porosity, permeability, density, and image analyses reveal the significance of microbialites in porosity occlusion of the reef framework. The association with encrusting foraminifera and macroborings indicates that microbialites enhanced the rapid and firm lithification of the reefal deposits. Other stratigraphic (e.g., timing of formation relative to MIS 5e highstand) and geochemical (elevated δ¹⁸O and δ¹³C values) indicators suggest that high organic productivity and thick microbial encrustation were favored by lower sea level and increased restriction and nutrification during post-storm run-off in a nearshore back-reef setting. We continue to test these hypotheses through microbial biomarker analyses and re-examination of a 15-m core drilled in 1997 at Cockburn Town.