The record of temperature, wind velocity and air humidity in the delta D and delta(18)O of water inclusions in synthetic and Messinian halites
Résumé
Deuterium and oxygen isotope fractionations between liquid and vapor water were experimentally-determined during evaporation of a NaCl solution (35 g L(-1)) as a function of water temperature and wind velocity. In the case of a null wind velocity, slopes of delta D-delta(18)O trajectories of residual waters hyperbolically decrease with increasing water temperatures in the range 23-47 degrees C. For wind velocities ranging from 0.8 to 2.2 m s(-1), slopes of the delta D-delta(18)O trajectories linearly increase with increasing wind velocity at a given water temperature. These experimental results can be modeled by using Rayleigh distillation equations taking into account wind-related kinetics effects. Deuterium and oxygen isotope compositions of water inclusions trapped by the precipitated halite crystals were determined by micro-equilibration techniques. These isotopic compositions accurately reflect those of the surrounding residual waters during halite growth. Isotopic compositions of water inclusions in twenty natural halites from the Messinian Realmonte mine in Sicily suggest precipitation temperatures of 34(-4)(+10)degrees C that match the homogenization temperatures obtained by microthermometry (median = 34 +/- 5 degrees C). The similarity between the measured and experimental slopes of the delta D-delta(18)O evaporation trajectories suggests that the effect of wind was negligible during the genesis of these halite deposits. Hydrogen and oxygen isotope compositions of water inclusions from Realmonte halite also define a linear trend whose extrapolation until intersection with the Mediterranean Meteoric Water Line allows the characterization of the water source with delta D and delta(18)O values of -70 +/- 10 parts per thousand and -11.5 +/- 1.5 parts per thousand, respectively. These results reveal that the huge amounts of salts deposited in Sicily result from the evaporation of seawater mixed with a dominant fraction (>= 50%) of meteoric waters most likely deriving from alpine fluvial discharge.