THE INFLUENCE OF SPONGE BORINGS ON ARAGONITE-TO-CALCITE INVERSION IN LATE PLEISTOCENE STROMBUS-GIGAS FROM GRAND-CAYMAN, BRITISH-WEST-INDIES

Large, robust Strombus gigas lived in a variety of habitats in the Iro nshore Lagoon that existed on Grand Cayman during the Late Sangamon hi ghstand, approximately 125,000 years ago. Some well-preserved shells a re still entirely aragonitic, whereas others have been transformed com pletely to calcite. Permeability of the host rock and sponge-boring in tensity were the most important controls on the extent of aragonite-to -calcite inversion. Sponge borings played a critical role in the inver sion process because they provided conduits through which the reactive diagenetic fluids could penetrate the dense gastropod skeleton. Each boring acted as a nucleus from which inversion fronts could spread int o the shell. As a result, the radial fabric of the neomorphic calcite crystals around the sponge borings contrasts sharply with the mosaic o f equant calcite that formed in areas devoid of sponge borings. It see ms that bioerosion is, at least on a local scale, as important as othe r factors in controlling the meteoric alteration of carbonate sediment . The deltaO-18 values of neomorphic calcite (-7.1 parts per thousand to -4.9 parts per thousand), approximately 3 parts per thousand more d epleted in O-18 than the original aragonite, confirm the role of meteo ric water in the aragonite-to-calcite transformation. DeltaC-13 values of the diagenetic calcite (+0.2 parts per thousand to +4.5 parts per thousand) are similar to those in the original aragonite, suggesting t hat the former was inherited from the latter.