Controls on the oxygen and hydrogen isotopic composition of the waters of Florida Bay, U.S.A.

Abstract

The oxygen and hydrogen isotopic composition have been measured of waters in Florida Bay and fluids squeezed from sediments which make up Holocene islands in the bay. Although, these waters ranged in salinity from 27 to 120 g kg −1, most were found to have very similar hydrogen and oxygen isotopic compositions ( δ 18 O = + 2 to + 4‰ , δD= + 5 to + 25‰). In order to explain these observations we have applied the Craig-Gordon and Gonflantini evaporation models which account for oxygen and hydrogen isotopic fractionation during the desiccation of saline water bodies. These models provide excellent agreement for the evaporation of water into an environment with a relative humidity of 77%, a temperature of 25°C and atmospheric water vapor possessing δ 18O- and δD-values of −11 and −75‰, respectively. The salinity of fluids from one core on Cluett Key (26.9 g kg −1) was well below that of the surrounding bay (> 40 g kg −1 and the water from this locality was depleted in both D and 18O . These δ 18O- and δD-values, plotted together with other data from this study, fall on a line possessing a slope of 4.90 ( ± 0.41), similar to what would be expected from the evaporation models. This line intersects the meteoric water line at δ 18O and δD-values which are within error, similar to average δ 18O- and δD-values measured for rainfall in the Miami area.