DISCRETE EPIBIOFACIES ON EXPERIMENTALLY-DEPLOYED SKELETAL HARDPARTS, ANALOGS FOR PHOTIC-ZONE DEPTH DETERMINATIONS IN CARBONATE ENVIRONMENTS

The Shelf-and-Slope Experimental Taphonomy Initiative (SSETI) is a long-term project that focuses on the fossilization potential of organisms in shallow-to-deep offshore environments. These environments provide a proxy for ancient epicontinental seas, as well as ancient coral reefs and associated carbonate facies. The skeletal hardparts found in these environments are themselves habitat islands of diversity and relative abundance, providing substrates for numerous eukaryotic taxa, such as invertebrates, plants, and protists. Epibionts (encrusters) and bioeroders evolved almost as soon as the first carbonate fossil appeared, and since that time, may have affected the fossilization potential of calcareous hardparts.

To determine the temporal dynamics of hardpart preservation in relation to epibionts and bioeroders, the SSETI project, with the use of submersibles, deployed molluscan shells from 15 m to 300 m in water depth, crossing the Bahamas Platform and into the lower slope of the Exuma Sound. At these deeper depths, sediments accumulate as mixed-environmental associations of skeletal sands. We wanted to determine if shell-inhabiting bionts formed distributional patterns indicative of photic-bathymetric depths, such that, when skeletal hardparts are difficult to identify, the encrusting organisms could be used as environmental indicators. Results revealed that general environmental trends with depth are generated using higher-level taxonomic descriptors (guild level), but when species-level analysis was done using calcareous encrusters (e.g., bryozoa, protists, serpulid polychaetes), their distributions fell into four discrete clusters: inner shelf (15 m), outer shelf (30 m), shelf/slope break (70 m), and slope (200-300 m). Bryozoa and encrusting foraminifera contributed the most to these discrete "epibiofacies" (after Brett). Importantly, these discrete epibiofacies remained relatively consistent over a long temporal interval (8 yrs), despite successional overprinting. Therefore, not only are shell-inhabiting bionts important for community evolution studies, they also provide a predictive tool for photo-bathymetric determinations in ancient shallow subtidal-to-offshore environments.