Nitrogen and oxygen isotopomeric constraints on the origins and sea‐to‐air flux of N2O in the oligotrophic subtropical North Pacific gyre

Abstract

Although the oceans are a significant source of the greenhouse gas nitrous oxide (N2O) to the atmosphere, the magnitude and characteristics of this source are poorly constrained. We present here stable isotope and isotopomer (intramolecular distribution of 15N within the linear NNO molecule) results for N2O and oxygen stable isotopic data for dissolved O2 from Station ALOHA in the subtropical North Pacific gyre near Hawaii. The results indicate shallow (∼100–300 m) in situ N2O production. Results of isotope mass balance models constrain the rate of N2O production and the sea‐to‐air flux of N2O. Results of an isotope mass balance model that takes into account the ratios of the vertical gradients in the isotopic abundances of N, O, Nα (central N) and Nβ (terminal N) of N2O and the measured gradients of N2O concentration through the thermocline indicate that shallow in situ production contributed 40% to 75% of the sea‐to‐air flux of N2O. This model also indicates that the net sea‐to‐air flux of N2O was at least 0.4 μmol m−2 d−1 and could be as high as 1.0 μmole m−2 d−1. These model results are not statistically different from the sea‐to‐air flux of N2O calculated using an empirical relationship between wind speed and gas transfer rate (1.1 ± 0.7 μmole m−2 d−1) derived from measured values for wind speed, temperature and the surface mixed layer concentration of N2O. These results can be used to better constrain the global N2O budget.