DISTRIBUTION OF MICROBIAL ENCRUSTERS ON PLEISTOCENE CORALS FROM THE BAHAMAS AND TURKS AND CAICOS ISLANDS

Microbial carbonates are considered a common component of Quaternary coral reefs, but their temporal and spatial distribution varies greatly. Here we document uneven distribution of cm-scale microbial encrusters on Late Pleistocene corals from the Bahamas and Turks and Caicos Islands and consider implications for interpreting their formation.

The most extensive, up to ~15 cm thick microbialites of both laminated and clotted morphologies were observed on Acropora cervicornis coral branches in outcrop and core from Cockburn Town on the west coast of San Salvador Island, immediately below the Devil’s Point Discontinuity (DPD) that separates two stages of the Last Interglacial (Eemian/MIS 5e) reef development. The DPD is not pronounced in a core from The Gulf on the island’s south coast, but the uppermost reef interval contains A. cervicornis (U/Th age ~111-119 kya), with slightly younger (~104-108 kya +/-1.5-2 kyr) microbial crusts of similar thickness. Up to ~5 cm thick microbialites of uncertain age were observed among branches of Pocillopora palmata from storm-transported coastal boulders at The Gulf, on A. cervicornis from boulders along the west coast of Providenciales, and on Orbicella annularis in a boulder dredged from Matthew Town Marina on the southwest coast of Great Inagua.

Thick microbialites are absent from: 1) deposits dominated by domal corals above DPD at Cockburn Town; 2) excellent exposures of MIS 5e reefs at Devil’s Point on the west coast of Great Inagua, and outcrops and cores from West Caicos Island; 3) weathered MIS 5e reef exposures on Providenciales, and North and Middle Caicos Islands; and 4) older MIS 7 corals, dominated by O. annularis from The Gulf core, even though A. cervicornis near the bottom of the 34 m core (MIS 9?) has thin (<2 cm) microbial crusts.

The documented distribution relative to MIS 5e highstand together with stable isotope data suggest that formation of thick microbialites was favored by lower sea level and increased restriction and nutrification in a nearshore back-reef setting. Their uneven distribution was thus not only controlled by glacioeustatic variations in sea level and any accompanying changes in water temperature and chemistry but was also highly dependent on local availability of suitable coral substrates and conditions that eliminated competition with eukaryotes.