Triple isotope composition of oxygen in atmospheric water vapor

[1] Recently, an excess of ¹⁷O (¹⁷O-excess) has been demonstrated in meteoric water and ice cores. Based on theory and experiments, it has been suggested that this excess originates from evaporation of ocean water into under-saturated air. However, there has never been direct demonstration of this excess in marine vapor. Here, we present results of the first measurements of d¹⁷O and d¹⁸O in vapor samples collected over the South Indian and the Southern Oceans. Our data show the existence of ¹⁷O-excess in marine vapor and also clear negative correlation between ¹⁷O-excess and relative humidity. Thus, ¹⁷O-excess is useful for constraining oceanic humidity in hydrological and climatic models. Using the obtained values of ¹⁷O-excess, we estimated the fractionation factor between H₂¹⁸O and H₂¹⁶O for diffusion in air above the ocean (¹⁸α_diff). The new estimation of ¹⁸α_diff (1.008) is larger than the widely accepted value in hydrological studies.