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 (N 2 O) to the atmosphere, the magnitude and characteristics of this source are poorly constrained. We present here stable isotope and isotopomer (intramolecular distribution of 15 N within the linear NNO molecule) results for N 2 O and oxygen stable isotopic data for dissolved O 2 from Station ALOHA in the subtropical North Pacific gyre near Hawaii. The results indicate shallow (∼100–300 m) in situ N 2 O production. Results of isotope mass balance models constrain the rate of N 2 O production and the sea‐to‐air flux of N 2 O. Results of an isotope mass balance model that takes into account the ratios of the vertical gradients in the isotopic abundances of N, O, N α (central N) and N β (terminal N) of N 2 O and the measured gradients of N 2 O concentration through the thermocline indicate that shallow in situ production contributed 40% to 75% of the sea‐to‐air flux of N 2 O. This model also indicates that the net sea‐to‐air flux of N 2 O 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 N 2 O 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 N 2 O. These results can be used to better constrain the global N 2 O budget.