Temperature dependence of oxygen-18 concentration in reef coral carbonates

Abstract

Oxygen isotope ratios in the aragonite precipitated by the scleractinian reef-building corals indicate that the skeletal carbonate is not in isotopic equilibrium with the sea water of the reef environment in which the corals grew. Despite this apparent isotopic disequilibrium, values are temperature dependent. The $xsO versus temperature curves for various coral genera are parallel or nearly parallel to the isotopic 'paleotemperature scale' established by Epstein et aJ. (1953) but in each case the curves are displaced toward lower xsO/X60 ratios at a given temperature. Regression coefficients for 44 genera have been determined. These findings suggest that if skeletal carbonate is derived from both sea-water bicarbonate and metabolic carbon dioxide, the relative proportions contributed by each of these two sources of carbon and oxygen are essentially constant for a given genus, but are different from one genus to another. Thus it is possible that isotopic composition can indicate relative excretory efficiency and that stable isotope ratios can be of considerable value in elucidating some of the processes involved in reef coral development. Epstein et al. [1951, 1953] demonstrated that the skeletal carbonates of some marine invertebrates, molluscs for example, have the same oxygen isotopic composition as CaC08 precipitated inorganically from sea water at the same temperature and under conditions of isotopic equilibrium. Although the octocoral Heliopora and some hydrozoan corals appear to secrete calcium carbonate in isotopic equilibrium with sea water [Weber and Woodhead, 1971], the scleractinian corals have been found to have variable isotope ratios. Some unknown physiological process was proposed by Epstein et al. [1951] to account for the apparent isotopic disequilibrium. A model for carbon and oxygen isotope fractionation in the calcification of corals was developed by Weber and Woodhead [1970] to serve as a preliminary working hypothesis for explaining the variability of 8C/C and 0/60 ratios in a large collection of specimens from a single reef at Heron Island, Australia. Among the implications of the model is that the 80 of reef-building corals will be temperature dependent, despite apparent isoCopyright ¸ 1972 by the American Geophysical Union. topic disequilibrium between the skeletal carbonate and ambient sea water. The purpose of the study reported here is to verify this prediction with analyses of corals collected over a wide range of environmental temperatures. The significance of the results is then considered in terms of relative metabolic efficiency.