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ABSTRACT RationaleThe isotopic composition of dissolved dinitrogen gas (δ¹⁵N‐N₂) in water can offer a powerful constraint on the sources and pathways of nitrogen cycling in aquatic systems. However, because of the large presence of atmosphere‐derived dissolved N₂in these systems, high‐precision (on the order of 0.001‰) measurements of N₂isotopes paired with inert gas measurements are required to disentangle atmospheric and biogeochemical signals. Additionally, the solubility equilibrium isotope fractionation of N₂and its temperature and salinity dependence are underconstrained at this level of precision. MethodsWe introduce a new technique for sample collection, processing, and dynamic dual‐inlet mass spectrometry allowing for high‐precision measurement of δ¹⁵N‐N₂and δ(N₂/Ar) with simultaneous measurement of δ(⁴⁰Ar/³⁶Ar) and δ(Kr/N₂) in water. We evaluate the reproducibility of this technique and employ it to redetermine the solubility equilibrium isotope effects for dissolved N₂across a range of temperatures and salinities. ResultsOur technique achieves measurement reproducibility (1σ) for δ¹⁵N‐N₂(0.006‰) and δ(N₂/Ar) (0.41‰) suitable for tracing biogeochemical nitrogen cycling in aquatic environments. Through a series of air–water equilibration experiments, we find a N₂solubility equilibrium isotope effect (ε = α/1000 − 1, where α = (²⁹N₂/²⁸N₂)_dissolved/(²⁹N₂/²⁸N₂)_gas) in water of ε(‰) = 0.753 − 0.004•TwhereTis the temperature (°C), with uncertainties on the order of 0.001‰ over the temperature range of ~2°C–23°C and salinity range of ~0–30 psu. We find no apparent dependence of ε on salinity. ConclusionsOur new method allows for high‐precision measurements of the isotopic composition of dissolved N₂and Ar, and dissolved N₂/Ar and Kr/N₂ratios, within the same sample. Pairing measurements of N₂with inert gases facilitates the quantification of excess N₂from biogeochemical sources and its isotopic composition. This method allows for a wide range of applications in marine, coastal, and freshwater environments to characterize and quantitatively constrain potential nitrogen‐cycling sources and pathways and to differentiate between physical and biological isotope signals in these systems. 

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 (~32°N, 64°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 SF 6 . 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.