A large source of atmospheric nitrous oxide from subtropical North Pacific surface waters

Nitrous oxide (N 2 O), a trace gas whose concentration is increasing in the atmosphere, plays an important role in both radiative forcing and stratospheric ozone depletion 1 , 2 . Its biogeochemical cycle has thus come under intense scrutiny in recent years. Despite these efforts, the global budget of N 2 O remains unresolved, and the nature and magnitude of the sources and sinks continue to be debated 3 , 4 , 5 despite the constraints that can be provided by characterizations of the gas 6 , 7 . We report here the results of dual-isotope measurements of N 2 O from the water column of the subtropical North Pacific Ocean. Nitrous oxide within the lower-euphotic and upper-aphotic zones is depleted in both 15 N and 18 O relative to its tropospheric and deep-ocean composition. These findings are consistent with a prediction, based on global mass-balance considerations, of a near-surface isotopically depleted oceanic N 2 O source 4 . Our results indicate that this source, probably produced by bacterial nitrification, contributes significantly to the ocean–atmosphere flux of N 2 O in the oligotrophic subtropical North Pacific Ocean. This source may act to buffer the isotopic composition of tropospheric N 2 O, and is quantitatively significant in the global tropospheric N 2 O budget. Because dissolved gases in near-surface waters are more readily exchanged with the atmospheric reservoir than those in deep waters, the existence of a quantitatively significant N 2 O source at a relatively shallow depth has potentially important implications for the susceptibility of the source, and the ocean–atmosphere flux, to climatic influences.