Identifying the origin of nitrous oxide dissolved in deep ocean by concentration and isotopocule analyses

Nitrous oxide (N₂O) contributes to global warming and stratospheric ozone depletion. Although its major sources are regarded as bacterial or archaeal nitrification and denitrification in soil and water, the origins of ubiquitous marine N₂O maximum at depths of 100-800 m and N₂O dissolved in deeper seawater have not been identified. We examined N₂O production processes in the middle and deep sea by analyzing vertical profiles of N₂O concentration and isotopocule ratios, abundance ratios of molecules substituted with rare stable isotopes ¹⁵N or ¹⁸O to common molecules ¹⁴N¹⁴N¹⁶O, in the Atlantic, Pacific, Indian, and Southern oceans. Isotopocule ratios suggest that the N₂O concentration maxima is generated by in situ microbial processes rather than lateral advection or diffusion from biologically active sea areas such as the eastern tropical North Pacific. Major production process is nitrification by ammonia-oxidizing archaea (AOA) in the North Pacific although other processes such as bacterial nitrification/denitrification and nitrifier-denitrification also significantly contribute in the equatorial Pacific, eastern South Pacific, Southern Ocean/southeastern Indian Ocean, and tropical South Atlantic. Concentrations of N₂O below 2000 m show significant correlation with the water mass age, which supports an earlier report suggesting production of N₂O during deep water circulation. Furthermore, the isotopocule ratios suggest that AOA produce N₂O in deep waters. These facts indicate that AOA have a more important role in marine N₂O production than bacteria and that change in global deep water circulation could affect concentration and isotopocule ratios of atmospheric N₂O in a millennium time scale.