Distribution of encrusting foraminifera at Cat Island, Bahamas: Implications for foraminiferal assemblages in the geologic record

Abstract

Benthic foraminifera that are cemented by calcium carbonate or are otherwise firmly fixed to hard surfaces are commonly known as encrusting foraminifera. Very few studies focus on the use of encrusting foraminifera as paleoenvironmental indicators. However, their use as environmental indicators has many potential advantages over free foraminifera: they are sensitive to environmental factors such as light and water energy, and they are fixed in place, meaning they are more likely to stay in the original habitat. This means their tests remain in place after death, leaving a post-mortem history. Previous research on San Salvador, Bahamas, has established a potential model for the occurrence of encrusting foraminifera on small carbonate platforms. In this study, the encrusting foraminifera of neighboring Cat Island were studied and compared with those found at San Salvador. Cobbles were collected by SCUBA at eight sites representing different environments from near-shore reefs to patch reefs and the platform-margin. Encrusting organisms were examined in 10-cm2 quadrats by recording the number of individuals of each species and the area covered by each species. The difference between the various sites is readily apparent from both the overall density of encrusting organisms and the relative proportion of the different encrusting foraminiferan species. The near-shore localities are marked by high density assemblages dominated by well-preserved Homotrema rubrum and Nubecularia, with site-to-site variability as to which of the two species is dominant. The patch-reef localities have a lower density of encrusting foraminifera as well as a greater proportion of Planorbulina in relation to Homotrema rubrum. The encrusting foraminifera at the patch-reef localities are also less well preserved. The shelf-edge locations are distinctive because of their even lower density of encrusting foraminifera and the dominance by Gypsina plana. Average size of species decreases offshore with the exception of G. plana, and quality of preservation also decreases from onshore to offshore. Most of this data is consistent with prior research on San Salvador, indicating possible trends of encrusting foraminifer occurrence. These trends could be used to perform paleoenvironmental analysis of ancient carbonate deposits.