Microendoliths in Lower Pliocene Oysters from the Alt Empordà Basin,NW Mediterranean: Paleoenvironmental Inferences

The study of traces made by microendoliths on Neopycnodonte cochlear shells from a coquina in the Vilacolum area (Alt Empordà basin) has permitted us to identify for the first time 18 ichnotaxa in the Lower Pliocene sediments of the NW Mediterranean. A shallow, euphotic environment with local shaded zones (corresponding to deep shallow conditions) is proposed for this shell bed, owing to the presence of abundant heterotroph microborings and rare cyanobacterial traces as well as to the high number of Rhopalia catenata and Ichnoreticulina elegans. This refines a former interpretation, mainly based on body fossils and macroborings, which suggested simply a shallow depositional environment. The ichnospecies Ichnoreticulina elegans, Rhopalia catenata, ?R. clavigera, Saccomorpha clava, S. terminalis, Polyactina araneola, Entobia mikra, Orthogonum lineare and Aurimorpha varia are identified for the first time in marine Pliocene materials, thus furnishing a more complete picture of the temporal span of some microborers.     

Cross-shelf differences in the pattern and pace of bioerosion of experimental carbonate substrates exposed for 3 years on the northern Great Barrier Reef, Australia

Patterns of bioerosion of dead corals and rubbles on the northern Great Barrier Reef were studied by using blocks of the massive coral Porites experimentally exposed at six sites, located on an inshore–offshore profile, for 1 year and 3 years. Rates of microbioerosion by microborers, grazing by fish, and macrobioerosion by filter-feeding organisms were simultaneously evaluated using image analysis. Microbioerosion, grazing, and total bioerosion were lower at reefs near the Queensland coast than at the edge of the continental shelf (1.81 kg m⁻² and 6.07 kg m⁻² after 3 years of exposure respectively, for total bioerosion). The opposite pattern was observed for macrobioerosion. Bioaccretion was negligible. These patterns were evident after 1 year of exposure, and became enhanced after 3 years. Microborers were established and were the main agent of bioerosion after 1 year of exposure, and as the principal support for grazing, continued to be the main cause of carbonate loss after 3 years. Full grazing activity and establishment of a mature community of macroborers required more than 1 year of exposure. After 1 year, macroborers and grazers were the second most important agents of bioerosion on both inshore and offshore reefs. However, after 3 years, grazers became the main agents at all sites except at the inshore sites, where macroborers were the principal agents. Because the contribution of microborers, grazers, and macroborers to bioerosion varies in space and time, we suggest that the estimation of reef carbonate budgets need to take in account the activities of all bioerosion agents.