The Origin of Variations in the Isotopic Record of Scleractinian Corals: I. Oxygen

Previous investigations of the δ ¹⁸ O of the skeletons of Florida specimens of the reef coral Montastraea annularis have failed to produce the full temperature range suggested by calibration studies of other corals. Explanations for this phenomenon include different relationships between temperature and the δ ¹⁸ O of skeletons of Floridian corals, changing δ ¹⁸ O of the water, physiological variables (“vital effects”), and an insufficient number of samples taken per year with consequent superposition of calcium carbonate precipitated at different times within an individual sample. In this study, we investigate all of these hypotheses, by measuring the δ ¹⁸ O of corals grown in the field which were periodically stained with alizarin-red S and where the δ ¹⁸ O of the water was measured and the temperature continuously recorded. We compare the effect of sampling the coral skeletons at different resolutions and the effect of sampling within different skeletal elements. Our study shows that discrete, high-resolution sampling of coral exotheca (fifty samples a year) is necessary to reproduce temperatures for this species in Florida waters. Coral skeletons sampled using lower resolution methods showed an artificial attenuation of the annual range in skeletal δ ¹⁸ O, with similar δ ¹⁸ O minima during the skeleton represented by the summer months, but larger differences in the winter δ ¹⁸ O maxima. Replicate isotope transects from fast and slow growing areas and different regions of the corallite were also compared. The δ ¹⁸ O of rapidly growing (8 mm/y) portions of the colony was 0.1 to 0.2‰ heavier than the slowest growing (1.1 mm/y) portions of the colony. This difference as well as the difference between the skeleton sampled at high and low resolutions appears to result in part from the attenuation of the δ ¹⁸ O signal as a result of the reduced sampling rate in slower growing sections of the coral and is not solely a result of variable kinetic effects.