Coral skeletal chemistry: physiological and environmental regulation of stable isotopes and trace metals in Montastrea annularis

Abstract

A detailed study has been made of the $^{13}$C, $^{18}$O, calcium, magnesium, strontium, and iron contents of a coral skeleton deposited during a two-year period for which environmental data are available. Strong seasonal variations in $^{13}$C and magnesium contents have been found. These appear to be linked to changes in growth rate, and should be considered in future work. $^{18}$O content does not show equilibrium physicochemical temperature effects, and its lack of correlation with $^{13}$C indicates complex metabolic isotope fractionation. Strontium content shows little variation. This difference from magnesium is predicted on grounds of biochemical ion transport. Iron is detrital in origin. Seasonal records of trace constituents in coral skeleton are shown to differ from those predicted by previous investigators, indicating that metabolic effects cannot be ignored in paleoenvironmental interpretation of carbonate skeletal chemistry. Stable isotopes are demonstrated to be useful tools in understanding overall carbon metabolism of photosynthetic calcifying organisms. A model of carbon isotope fractionation is developed, and used to place bounds on the sources of carbon used in photosynthesis and calcification. It is estimated that approximately 40% of the carbon supply is from seawater bicarbonate and 60% from recycled respiratory CO$_{2}$.