Dissolved gases in the deep North Atlantic track ocean ventilation processes

Gas exchange between the atmosphere and ocean interior profoundly impacts global climate and biogeochemistry. However, our understanding of the relevant physical processes remains limited by a scarcity of direct observations. Dissolved noble gases in the deep ocean are powerful tracers of physical air-sea interaction due to their chemical and biological inertness, yet their isotope ratios have remained underexplored. Here, we present high-precision noble gas isotope and elemental ratios from the deep North Atlantic (similar to 32 degrees N, 64 degrees W) to evaluate gas exchange parameterizations using an ocean circulation model. The unprecedented precision of these data reveal deep-ocean undersaturation of heavy noble gases and isotopes resulting from cooling-driven air-to-sea gas transport associated with deep convection in the northern high latitudes. Our data also imply an underappreciated and large role for bubble-mediated gas exchange in the global air-sea transfer of sparingly soluble gases, including O-2, N-2, and SF6. Using noble gases to validate the physical representation of air-sea gas exchange in a model also provides a unique opportunity to distinguish physical from biogeochemical signals. As a case study, we compare dissolved N-2/Ar measurements in the deep North Atlantic to physics-only model predictions, revealing excess N-2 from benthic denitrification in older deep waters (below 2.9 km). These data indicate that the rate of fixed N removal in the deep Northeastern Atlantic is at least three times higher than the global deep-ocean mean, suggesting tight coupling with organic carbon export and raising potential future implications for the marine N cycle.

Automated summary

Gas exchange between the atmosphere and ocean impacts global climate and biogeochemistry. Noble gases in the deep ocean provide valuable information about air-sea interaction, but their isotope ratios have not been well explored. High-precision noble gas isotope and elemental ratios from the deep North Atlantic reveal cooling-driven air-to-sea gas transport and bubble-mediated gas exchange. Using noble gases to validate air-sea gas exchange in a model allows us to differentiate physical from biogeochemical signals and study the marine nitrogen cycle.