FIRST DOCUMENTATION OF THICK MICROBIAL ENCRUSTERS ON PLEISTOCENE CORALS FROM TURKS AND CAICOS ISLANDS

To better understand the origin and distribution of thick microbial encrusters on Pleistocene corals across the broader tropical western Atlantic region, we visited Turks and Caicos Islands (TCI) in January 2024. We have previously documented up to ~15 cm thick microbialites on Eemian (MIS 5e or Last Pleistocene Interglacial) Acropora cervicornis coral branches in outcrop at Cockburn Town on the west coast of San Salvador Island, Bahamas, and in cores from the same locality and The Gulf on the island’s south coast. We also observed cm-scale thick microbialites among branches of Pocillopora palmata (the only extinct Pleistocene Caribbean coral) from storm-transported coastal boulders at The Gulf, as well as on Orbicella annularis coral in a limestone boulder dredged from Matthew Town Marina on the west coast of Great Inagua Island, Bahamas.

In preparation for field work on TCI, we examined the cores drilled on West Caicos Island by Harold R. Wanless (Univ. of Miami, Florida). The cores contain common MIS 5e corals without thick microbialites. We confirmed this observation during our visit to the superb exposures of an MIS 5e coral reef along the island’s west coast. We also noticed the absence of microbialites from other MIS 5e in situ reefs exposed on Providenciales, and North and Middle Caicos Islands. The only documentation of thick microbial encrusters on TCI came from storm-transported limestone boulders along the west coast of Providenciales. These boulders contain common A. cervicornis with up to ~5 cm thick microbialites of both laminated and clotted morphologies, similar to those from San Salvador.

These observations suggest that microbial encrusters may be restricted to an older/lower stratigraphic horizon of MIS 5e reefs on TCI. We documented a similar uneven distribution of microbialites in outcrop at Cockburn Town on San Salvador. There, thick microbialites are common on branching A. cervicornis near the top of Reef 1, below the intra-MIS 5e Devil’s Point Discontinuity, but are absent on other types of corals from overlying Reef 2 that represents the highest interglacial sea level. This indicates that formation of thick and abundant microbialites may require a unique combination of coral substrates and environmental conditions, potentially associated with restriction during sea-level fall.