ENCRUSTING FORAMINIFERA FROM PLEISTOCENE CORAL REEFS IN THE BAHAMAS

Pleistocene coral reefs (Grotto Beach Fm., Cockburn Town Mb.) exposed on San Salvador and Great Inagua islands in the Bahamas offer important insights into paleoenvironmental and sea level changes during the last interglacial highstand (Eemian; MIS 5e). Field and petrographic observations provide information on presence, distribution and abundance of encrusting organisms, their role in taphonomic modifications of corals, and the major environmental transition from healthy to microbially encrusted reefs. Stromatolites, clotted microbialites, red crustose coralline (RCC) algae, serpulids, and foraminifera form up to several cm thick encrustations on Acropora, Pocillopora, and Orbicella corals.

This study focused on encrusting foraminifera whose identification is commonly based on surficial morphology, but here many are embedded within other encrusters and had to be studied in thin sections. The result is a robust set of criteria for the recognition of the following four species present at all study sites: 1) Homotrema rubra is the most common type, characterized by red tests of variable morphology (globose, hemispherical, flat, branching, spiny), up to 8 mm in diameter (d), and made of multiple layers of chambers (d = 40-200 µm) with continuous or perforated walls 20-60 µm thick; 2) Gypsina plana makes ~5% of the total specimen count; its discoidal tests are up to 3 mm wide and consist of few layers of elliptical chambers (d up to 1.4 mm) with convex upper surface and perforated septae 60-200 µm thick; 3) Carpenteria utricularis has plano-convex, trochospiral tests (d ~2 mm) of conical or globose morphology and highly convex umbilical side; its chambers are up to 1 mm in diameter and have 50-400 µm thick septae; and 4) Planogypsina acervalis is the rarest type; it has thin discoidal tests and elliptical chambers (d up to 300 µm), with thin septae (20 µm) organized in a single layer. Corals are almost exclusively directly encrusted by RCC algae, which are in turn encrusted by foraminifera and serpulids, and commonly covered by more algae. Homotrema and Carpenteria are also embedded within thicker microbialites, with their abundance decreasing away from the coral surface. The prevalence of Homotrema and the paucity of Gypsina suggest that the encrustation took place in a near-shore, high energy, well-lit marine environment.