Heavy Noble Gas Isotopes as New Constraints on the Ventilation of the Deep Ocean.

Past studies of noble gas concentrations in the deep ocean have revealed widespread, several percent undersaturation of Ar, Kr, and Xe. However, the physical explanation for these disequilibria remains unclear. To gain insight into undersaturation set by deep‐water formation, we measured heavy noble gas isotope and elemental ratios from the deep North Pacific using a new analytical technique. To our knowledge, these are the first high‐precision seawater profiles of 38Ar/36Ar and Kr and Xe isotope ratios. To interpret isotopic disequilibria, we carried out a suite of laboratory experiments to measure solubility fractionation factors in seawater. In the deep North Pacific, we find undersaturation of heavy‐to‐light Ar and Kr isotope ratios, suggesting an important role for rapid cooling‐driven, diffusive air‐to‐sea gas transport in setting the deep‐ocean undersaturation of heavy noble gases. These isotope ratios represent promising new constraints for quantifying physical air‐sea gas exchange processes, complementing noble gas concentration measurements. Plain Language Summary: The deep ocean inherits its dissolved gas content from exchange with the atmosphere at high latitudes and from biological and chemical processes. Noble gases, which are unaffected by biology and chemistry, are useful tools for understanding physical gas exchange. Past observations of dissolved noble gases throughout the deep ocean have revealed that Ar, Kr, and Xe concentrations fall below expected concentrations for water at solubility equilibrium with the atmosphere. However, a physical explanation for this well‐documented undersaturation of noble gases remains unclear. Here we have measured the isotope ratios of Ar, Kr, and Xe in the deep North Pacific as new tools to investigate physical mechanisms of disequilibrium. Our findings suggest that rapid cooling and sinking of surface water at high latitudes, driving air‐to‐sea gas transport with insufficient time for equilibration, is a key process in setting the observed deep‐ocean undersaturation of noble gases. Key Points: Ar, Kr, and Xe isotope ratios were measured in the deep North Pacific to investigate mechanisms of solubility disequilibriumSeawater solubility fractionation factors of heavy noble gas isotope ratios were determined, indicating a weak dependence on salinityObserved isotopic disequilibria point to incomplete cooling‐driven diffusive gas uptake prior to deep‐water formation [ABSTRACT FROM AUTHOR]