Depth related changes in the 13C /12C ratio of skeletal carbonate deposited by the Caribbean reef-frame building coral Montastrea annularis: further implications of a model for stable isotope fractionation by scleractinian corals
Systematic variations in the isotopic composition of skeletal carbonate deposited by the Caribbean reef-frame building coral Montastrea annularis are correlated with water depth, location of the corallites within the corallum, and polyp packing density, as is demonstrated by isotope ratio measurements for 426 samples collected at 4.6 m depth intervals between 0 and 27.4 m at St. Croix, U.S. Virgin Islands. These data support a model, based on a study of Indo-Pacific scleractinians, proposed earlier for stable isotope fractionation by corals. Of particular interest is the fact that, within this species, ecotypic differentiation into shallow-water and deep-water subpopulations, with a boundary close to 20m, is reflected by changes in skeletal ° 13C. Stable isotope geochemical studies of both modern and fossil coral-derived carbonate may contribute to the solution of several problems having geologic and paleontologic significance.