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This study reports on the δ18O and δ13C composition of the skeleton from a 240-year-old specimen of Montastraea faveolata growing in Biscayne National Park, South Florida. Annual variations in the δ18O of the skeleton deposited during the summer months show a bimodal correlation with summer rainfall. During wetter years, the δ18O of the coral skeleton and the amount of precipitation during the summer months are inversely correlated (r = -0. 7) reflecting dilution of the seawater by meteoric water lower in δ18O. During years in which summer rainfall is less than normal, increases in precipitation are positively correlated with skeletal δ18O (r = +0.6) reflecting the input of freshwater from the Everglades higher in δ18O. Based on this correlation the δ18O record of the coral skeleton suggests that the 19th and 18th centuries have been relatively dry compared to the 20th century. Carbon isotopic compositions of the skeleton are positively correlated with δ18O with the minimum in δ13C occurring several months after the minimum in δ18O. Since the mid 1930s there has been a decrease in δ13C of the skeleton. Explanations for this trend may be (1) it reflects the increased input of carbon derived from the destruction of terrestrial ecosystems, (2) its part of a long-term decrease in δ13C associated with increased addition of fossil fuel-derived CO2.
Swart et al. (Thu,) studied this question.