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Abstract 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.