Non‐Equilibrium Fractionation Factors for D/H and ¹⁸O/¹⁶O During Oceanic Evaporation in the North‐West Atlantic Region

Abstract

Ocean isotopic evaporation models, such as the Craig-Gordon model, rely on the description of nonequilibrium fractionation factors that are, in general, poorly constrained. To date, only a few gradient-diffusion type measurements have been performed in ocean settings to test the validity of the commonly used parametrization of nonequilibrium isotopic fractionation during ocean evaporation. In this work, we present 6 months of water vapor isotopic observations collected from a meteorological tower located in the northwest Atlantic Ocean (Bermuda) with the objective of estimating nonequilibrium fractionation factors (<i>k</i>, ‰) for ocean evaporation and their wind speed dependency. The Keeling Plot method and Craig-Gordon model combination were sensitive enough to resolve nonequilibrium fractionation factors during evaporation resulting into mean values of <i>k</i> ₁₈ = 5.2 ± 0.6‰ and <i>k</i> ₂ = 4.3 ± 3.4‰. Furthermore, we evaluate the relationship between <i>k</i> and 10-m wind speed over the ocean. Such a relationship is expected from current evaporation theory and from laboratory experiments made in the 1970s, but observational evidence is lacking. We show that (a) in the observed wind speed range [0-10 m s^-1], the sensitivity of <i>k</i> to wind speed is small, in the order of -0.2‰ m^-1 s for <i>k</i> ₁₈, and (b) there is no empirical evidence for the presence of a discontinuity between smooth and rough wind speed regime during isotopic fractionation, as proposed in earlier studies. The water vapor <i>d</i>-excess variability predicted under the closure assumption using the <i>k</i> values estimated in this study is in agreement with observations over the Atlantic Ocean.