NATURE 396, 63-66

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

John E. Dore¹, Brian N. Popp², David M. Karl³ & Francis J. Sansone³

¹Aquasearch, Inc., 73-4460 Queen Kaahumanu Highway, Suite 110, Kailua-Kona, Hawaii 96740, USA

²School of Ocean and Earth Science and Technology, Department of Oceanography, University of Hawaii, Honolulu, Hawaii 96822, USA

³School of Ocean and Earth Science and Technology, Department of Geology and Geophysics, University of Hawaii, Honolulu, Hawaii 96822, USA

Nitrous oxide (N₂O), a trace gas whose concentration is increasing in the atmosphere, plays an important role in both radiative forcing and stratospheric ozone depletion. Its biogeochemical cycle has thus come under intense scrutiny in recent years. Despite these efforts, the global budget of N₂O remains unresolved, and the nature and magnitude of the sources and sinks continue to be debated despite the constraints that can be provided by characterizations of the gas. We report here the results of dual-isotope measurements of N₂O from the water column of the subtropical North Pacific Ocean. Nitrous oxide within the lower-euphotic and upper-aphotic zones is depleted in both ¹⁵N and ¹⁸O 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 N₂O source. Our results indicate that this source, probably produced by bacterial nitrification, contributes significantly to the ocean-atmosphere flux of N₂O in the oligotrophic subtropical North Pacific Ocean. This source may act to buffer the isotopic composition of tropospheric N₂O, and is quantitatively significant in the global tropospheric N₂O 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 N₂O source at a relatively shallow depth has potentially important implications for the susceptibility of the source, and the ocean-atmosphere flux, to climatic influences.