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In this study, we compare stable isotopic compositions of di-oxygen (O 2 ) and nitrous oxide (N 2 O) in two depth profiles at the well-characterized deep water station ALOHA (A Long-term Oligotrophic Habitat Assessment) in the subtropical North Pacific gyre to attain an understanding of the mechanisms of N 2 O production. The δ 18 O of O 2 varied from values indicative of an atmospheric origin near the surface ( 24.7‰ ), to minimum values reflective of a predominance of photosynthesis over respiration between the surface and 200 m (as low as 22.2 ‰ ), to maximum values as high as 36.6‰ in association with the O 2 minimum near 800 m. A similar pattern of isotopic variation was evident in the δ 18 O of N 2 O, however, values were enriched by approximately 20‰ . The similar pattern of variation in δ 18 O with depth is consistent with an origin of O in N 2 O from dissolved O 2 via the nitrification of intermediate compounds NH 2 OH or NO. Between the depths of 350 and 500 m, however, the distinction in the isotopic composition of N 2 O and O 2 was reduced to as little as 12‰ . This decrease in the difference between the δ 18 O of N 2 O and that of O 2 with depth indicates either a reduction in the magnitude of isotopic discrimination during nitrification or a contribution of O in N 2 O from water via the reduction of NO 2 − . Two mechanisms of N 2 O production via nitrification may, therefore, occur in the subtropical Pacific; release from the nitrification of NH 2 OH or NO at most depths and reduction of NO 2 − between 350 and 500 m. In that, the carbon flux decreases markedly over a similar depth interval at this locale (Karl, D.M., Knauer, G.A., Martin, J.H., 1988. Downward flux of particulate organic matter in the ocean: A particle decomposition paradox. Nature 332, 438–441), this distinct mechanism of N 2 O production between 350 and 500 m may be associated with the mineralization of organic matter from sinking particles. Low O 2 or anoxic micro-environments within particles within this depth interval may be maintained by lower ambient O 2 than at the surface and high rates of microbial activity supported by the mineralization of organic matter. Such conditions may facilitate an environment conducive to N 2 O production via NO 2 − reduction. |
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