SPONGE BOREHOLE SIZE AS A RELATIVE MEASURE OF BIOEROSION AND PALEOPRODUCTIVITY

Bioerosion intensity has been proposed as a measure of paleoproductivi ty in fossil reefs, but it is difficult to measure directly because fo ssil corals are often incomplete and because it is difficult to infer the length of time a given coral was exposed to bioeroding organisms. Both nutrient availability and taphonomic factors can affect bioerosio n intensity as measured in dead corals. Here, we examine these two eff ects separately using data from previous studies on bioerosion in mode rn and fossil corals. Size of individual sponge borings accurately ref lects total bioerosion in modern massive and branching corals on the G reat Barrier Reef. Total bioerosion in both massive and branching cora ls decreases outward across the continental shelf, paralleling trends in nutrient availability. Size of individual Cliothosa hancocki boring s decreases across the shelf in branching Acropora but not in massive Porites. Fossil sponge borings Entobia convoluta and Uniglobites glome rata in massive corals from Oligocene and Miocene reefs in Puerto Rico are smallest in Oligocene shelf-edge reefs, intermediate in Oligocene patch reefs, and largest in Miocene patch reefs. Both facies-related influence, represented by Oligocene shelf-edge reefs vs. Oligocene pat ch reefs, and nutrient-related influence, represented by Oligocene vs. Miocene patch reefs, were reflected in the size of sponge boreholes. Size of sponge borings also varies among species of host corals, appar ently in relation to skeletal architecture. Borehole size is inversely correlated with skeletal density as measured by the relative proporti on of skeleton and pore space in transverse thin section. There is a w eak positive correlation between borehole size and corallite diameter. These findings contradict reported positive correlations between tota l bioerosion and bulk density in modern corals. Borehole size appears accurately to reflect intensity of total internal bioerosion in fossil corals. Facies-controlled taphonomic overprints and influence of skel etal differences between coral species limit the use of sponge borehol e size to a rough indicator of paleoproductivity in fossil coral reef environments.