A 240-Year Stable Oxygen and Carbon Isotopic Record in a Coral from South Florida: Implications for the Prediction of Precipitation in Southern Florida

This study reports on the δ ¹⁸ O and δ ¹³ C composition of the skeleton from a 240-year-old specimen of Montastraea faveolata growing in Biscayne National Park, South Florida. Annual variations in the δ ¹⁸ O of the skeleton deposited during the summer months show a bimodal correlation with summer rainfall. During wetter years, the δ ¹⁸ O 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 δ ¹⁸ O. During years in which summer rainfall is less than normal, increases in precipitation are positively correlated with skeletal δ ¹⁸ O (r = +0.6) reflecting the input of freshwater from the Everglades higher in δ ¹⁸ O. Based on this correlation the δ ¹⁸ O 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 δ ¹⁸ O with the minimum in δ ¹³ C occurring several months after the minimum in δ ¹⁸ O. Since the mid 1930s there has been a decrease in δ ¹³ C 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 δ ¹³ C associated with increased addition of fossil fuel-derived CO ₂ .