Ventilation versus biology: What is the controlling mechanism of nitrous oxide distribution in the North Atlantic?

The extent to which water mass mixing and ocean ventilation contribute to nitrous oxide (N 2 O) distribution at the scale of oceanic basins is poorly constrained. We used novel N 2 O and chlorofluorocarbon measurements along with multiparameter water mass analysis to evaluate the impact of water mass mixing and Atlantic Meridional Overturning Circulation (AMOC) on N 2 O distribution along the Observatoire de la variabilité interannuelle et décennale en Atlantique Nord (OVIDE) section, extending from Portugal to Greenland. The biological N 2 O production has a stronger impact on the observed N 2 O concentrations in the water masses traveling northward in the upper limb of the AMOC than those in recently ventilated cold water masses in the lower limb, where N 2 O concentrations reflect the colder temperatures. The high N 2 O tongue, with concentrations as high as 16 nmol kg −1 , propagates above the isopycnal surface delimiting the upper and lower AMOC limbs, which extends from the eastern North Atlantic Basin to the Iceland Basin and coincides with the maximum N 2 O production rates. Water mixing and basin‐scale remineralization account for 72% of variation in the observed distribution of N 2 O. The mixing‐corrected stoichiometric ratio N 2 O:O 2 for the North Atlantic Basin of 0.06 nmol/μmol is in agreement with ratios of N 2 O:O 2 for local N 2 O anomalies, suggesting than up to 28% of N 2 O production occurs in the temperate and subpolar Atlantic, an overlooked region for N 2 O cycling. Overall, our results highlight the importance of taking into account mixing, O 2 undersaturation when water masses are formed and the increasing atmospheric N 2 O concentrations when parameterizing N 2 O:O 2 and biological N 2 O production in the global oceans.