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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. 

Climate warming likely drives ocean deoxygenation, but models still cannot fully explain observed declines in oxygen. One unconstrained parameter is the oxygen demand per carbon respired for complete remineralization of organic matter (i.e., the total respiration quotient,r_Σ‐O2:C). Here, we tested ifr_Σ‐O2:Cdeclined with depth by quantifying suspended concentrations of particulate organic carbon (POC), particulate organic nitrogen (PON), particulate organic phosphorus (POP), particulate chemical oxygen demand (PCOD), and total oxygen demand (Σ‐O₂ = PCOD + 2PON) down to a depth of 1,000 m in the Sargasso Sea. The respiration quotient (r_‐O2:C = PCOD:POC) and total respiration quotient (r_Σ‐O2:C = Σ‐O₂:POC) declined with depth in the euphotic zone, but increased vertically in the disphotic zone. C:N andr_Σ‐O2:Nchanged with depth, but surface values were similar to values at 1,000 m. C:P, N:P, andr_Σ‐O2:Pmostly decreased with depth. We hypothesize thatr_Σ‐O2:Cis linked to multiple environmental factors that change with depth, such as phytoplankton community structure and the preferential production/removal of biomolecules. Using a global model, we show that the global distribution of dissolved oxygen is equally sensitive tor_‐O2:Cvarying between surface biomes versus vertically during remineralization. Additionally, adjusting the model'sr_‐O2:Cwith depth to match our observations resulted in less dissolved oxygen throughout the upper ocean. Most of this loss occurred in the tropical Pacific thermocline, where oxygen models underestimate deoxygenation the most. This study aims to improve our understanding of biological oxygen demand as warming‐induced deoxygenation continues.