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

Accepted version of the paper "Non-Equilibrium Fractionation Factors for D/H and ¹⁸O/¹⁶O During Oceanic Evaporation in the North-West Atlantic Region".

An edited version of this paper was published by AGU. Copyright (2022) American Geophysical Union.

Zannoni, D., Steen‐Larsen, H. C., Peters, A. J., Wahl, S., Sodemann, H., & Sveinbjörnsdóttir, A. E. Non‐Equilibrium Fractionation Factors for D/H and 18O/16O During Oceanic Evaporation in the North‐West Atlantic Region. Journal of Geophysical Research: Atmospheres, e2022JD037076. https://doi.org/10.1029/2022JD037076

In accordance with the the Dual Publication Policy of AGU.

Abstract

Ocean isotopic evaporation models, such as the Craig-Gordon model, rely on the description of non-equilibrium 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 non-equilibrium isotopic fractionation during ocean evaporation. In this work we present six months of water vapor isotopic observations collected from a meteorological tower located in the northwest Atlantic Ocean (Bermuda) with the objective of estimating non-equilibrium fractionation factors (k, ‰) for ocean evaporation and their wind speed dependency. The Keeling plot method and Craig-Gordon model combination was sensitive enough to resolve non-equilibrium fractionation factors during evaporation resulting into mean values of k₁₈= 5.2±0.6 ‰ and k₂= 4.3±3.4 ‰. Furthermore, we evaluate the relationship between k 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 (i) in the observed wind speed range [0 – 10 m s^-1] the sensitivity of k to wind speed is small, in the order of -0.2 ‰ m^-1s for k₁₈, and (ii) 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 d-excess variability predicted under the closure assumption using the k values estimated in this study is in agreement with observations over the Atlantic Ocean.

Acknowledgments

The work was supported by the Danish Council for Independent Research – Natural Sciences grant number 10-092850 and the Carlsberg Foundation, and the AXA Research Fund. The Tudor Hill Marine Atmospheric Observatory in Bermuda was supported by NSF award OCE1829686. We acknowledge an infrastructure grant (nr. 10/0244) from the Icelandic Research Council (Rannis), that partly covered the cost of the Picarro facilities in Bermuda. HCSL and DZ acknowledges funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement no. 821868. H.S. acknowledges support by the Norwegian Research Council (Project SNOWPACE, grant no. 262710) and by the European Research Council (Consolidator Grant ISLAS, project no. 773245). SW acknowledges funding from the European Research Council (ERC) under the European Union’s Horizon 2020 research and innovation program: Starting Grant-SNOWISO (grant agreement 759526).