IDENTIFICATION AND SPECIES ABUNDANCE OF ENCRUSTING FORAMINIFERA WITHIN PLEISTOCENE (MIS 5E AND 7) CORAL REEFS IN THE BAHAMAS

Encrusting foraminifera from coral reefs of the Grotto Beach Fm. outcrops (Eemian, MIS 5e) on San Salvador and Great Inagua Islands in the Bahamas are compared with those in The Gulf (TG) core from San Salvador. Below the Pleistocene/Holocene boundary terra rossa paleosol developed on eolian grainstones, the core contains four coral reef intervals. The uppermost MIS 5e interval (~9-11 m core depth) has the most extensive skeletal (red algae, foraminifera, serpulids) and microbial (up to 14 cm thick) encrusters on branching Acropora cervicornis corals. Intervals two (~16-22 m) and three (~23-30 m; MIS 7 Owl’s Hole Fm.) comprise diverse assemblages of corals, while A. cervicornis dominates the core base (~31-34 m).

Identification of foraminifera encrusting on corals and analysis of their species abundance, distribution and substrate preferences can provide information on water depth and turbulence, light availability, and nutrient levels. Four species were identified in outcrop samples: Homotrema rubrum (50-75% abundance), Carpenteria utricularis (10-25%), Planogypsina acervalis (2-10%), and Gypsina plana with 5% abundance. TG core thin-section analysis supports the species abundance with 68%, 17%, 11%, and 3%, respectively. No statistically significant variations in species abundance and diversity were observed with core depth. This suggests robustness of their ecosystem niche in this area during the MIS 5e and 7.

In the core, the majority of C. utricularis was documented from the interval with the thickest microbial encrusters, suggesting a substrate preference. The thick encrusters also suggest a change in environmental conditions, such as lower sea level and increased restriction, favored by both microbial and C. utricularis encrusters. Conversely, H. rubrum is common throughout the core on a variety of substrates. Past research on modern H. rubrum documented the relationships between environmental conditions and this species' 3D morphology. Such relationships, unfortunately, could not be adequately evaluated from the Pleistocene examples firmly lithified within the coral reef framework. The results, however, provide useful information on the ecological conditions within encrusting foraminiferal communities during multiple periods of Pleistocene reef formation in the Bahamas.