THE ORIGIN OF VARIATIONS IN THE ISOTOPIC RECORD OF SCLERACTINIAN CORALS .1. OXYGEN

Previous investigations of the delta(18)O of the skeletons of Florida specimens of the reef coral Montastraea annularis have failed to produ ce the full temperature range suggested by calibration studies of othe r corals. Explanations for this phenomenon include different relations hips between temperature and the delta(18)O of skeletons of Floridian corals, changing delta(18)O of the water, physiological variables (''v ital effects''), and an insufficient number of samples taken per year with consequent superposition of calcium carbonate precipitated at dif ferent times within an individual sample. In this study, we investigat e all of these hypotheses, by measuring the delta(18)O of corals grown in the field which were periodically stained with alizarin-red S and where the delta(18)O of the water was measured and the temperature con tinuously recorded. We compare the effect of sampling the coral skelet ons at different resolutions and the effect of sampling within differe nt skeletal elements. Our study shows that discrete, high-resolution s ampling of coral exotheca (fifty samples a year) is necessary to repro duce temperatures for this species in Florida waters. Coral skeletons sampled using lower resolution methods showed an artificial attenuatio n of the annual range in skeletal delta(18)O, with similar delta(18)O minima during the skeleton represented by the summer months, but large r differences in the winter delta(18)O maxima. Replicate isotope trans ects from fast and slow growing areas and different regions of the cor allite were also compared, The delta(18)O of rapidly growing (8 mm/y) portions of the colony was 0.1 to 0.2 parts per thousand 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 lo w resolutions appears to result in part from the attenuation of the de lta(18)O signal as a result of the reduced sampling rate in slower gro wing sections of the coral and is not solely a result of variable kine tic effects.