SEASONAL AND REGIONAL VARIATION IN THE STABLE ISOTOPIC COMPOSITION (OXYGEN AND HYDROGEN) OF SEMI-TROPICAL RAINFALL: EXAMPLE FROM SOUTH FLORIDA

The stable isotopic composition of rainfall in coastal and tropical regions tends not to vary seasonally as compared to more temperate regions due to more stable temperatures. However, 10 years of rainfall data collected from 5 sites in south Florida indicate a strong seasonal and regional variation, despite the relatively limited geographic coverage and low-lying elevation of each of the collection sites. Based upon the weighted-mean stable isotope values, the sites were classified as coastal Atlantic, inland, and lower Florida Keys. The coastal Atlantic sites had weighted-mean values of d¹⁸O and dD of -2.86 ‰ and -12.85 ‰, respectively, and exhibited a seasonal variation with lower isotopic values in the summer wet-season precipitation (d¹⁸O = -3.38‰, dD = -16.57‰) as compared to the winter-time precipitation (d¹⁸O = -1.66 ‰, dD = -3.21 ‰). The inland site was characterized as having the highest d-excess value (+13.34 ‰), signifying a contribution of evaporated Everglades surface water to the local atmospheric moisture. In spite of its lower latitude location, the lower Keys site located at Long Key had the lowest weighted mean stable isotope values (d¹⁸O = -3.64 ‰, dD = -20.29 ‰) as well as the lowest d-excess value of (+8.83 ‰). Extremely low stable isotope values (d¹⁸O < -6 ‰, dD < -40 ‰) were observed prior to the passage of hurricanes from the Gulf of Mexico as well as from cold fronts from the north-west. These results suggest that a source region to the west, possibly the western Pacific, may be responsible for the extremely low values observed during some tropical storms and cold fronts.