Mollusc Shell Encrustation and Bioerosion Rates in a Modern Epeiric Sea: Taphonomy Experiments in the Java Sea, Indonesia

Mollusc shells of several species were deployed on racks and on the seafloor for up to two years on eutrophic and mesotrophic reefs in the Java Sea, a modern epeiric sea. Taphonomic indicators of shell preservation decreased during the study, but some ligament, periostracum, and shell color persisted throughout. Shell fragmentation was negligible except for species with easily chipped margins; weight loss was less than 5 for sturdy shells and up to 15 for shells with chipped margins. Shells deployed in mesh bags on the sediment surface had low encrustation and bioerosion, probably because of partial or complete burial. Areal encrustation on shells in bags was greater at the mesotrophic site than the eutrophic site, but animal encrustation and biovolume of encrusters was greater at the eutrophic site. Shells elevated on racks were encrusted rapidly at all sites; animal encrustation rates were correlated positively with productivity, and biovolume of encrusters was greater on nearshore eutrophic reefs than on offshore mesotrophic reefs. Bioerosion rates were variable but also tended to be higher at the more productive site. Natural shells also exhibited a positive, though less strong correlation with productivity suggesting that encrustation intensity and shell bioerosion may serve as relative indicators of productivity in the fossil record.