Two-isotope characterization of N20 in the Pacific Ocean and constraints on its origin in deep water

Abstract

THREE decades after the first study of nitrous oxide (N₂O) in the ocean¹, the marine geochemistry of this compound is of exceptional interest for two reasons: first, the need to understand the role of N₂O in both nitrification and denitrification in the sea^2–5, and second, the importance of N₂O in the stratospheric ozone cycle⁶ and as a greenhouse trace gas that, molecule for molecule, is 200 times more effective than carbon dioxide⁷. Related questions concern the role of the ocean as a source or sink of tropospheric N₂O and the processes responsible for its production and consumption in the sea. Our approach to these problems is one that has been successful in studies of H₂O geochemistry⁸ and sources of atmospheric CO (ref. 9): the simultaneous study of two isotopic components of a molecule, here the ¹⁵N/¹⁴N and ¹⁸O/¹⁶O ratios. We have measured these ratios in N₂O in three Pacific Ocean profiles and in Pacific marine air. Relative to atmospheric N₂O, dissolved N₂O is slightly depleted in both heavy isotopes down to a depth of ∼600m, but in deep and bottom water, N₂O is everywhere enriched in ¹⁵N and ¹⁸O. Thus, contrary to recent predictions^10,11, N₂O production in deep water is characterized by heavy-isotope enrichment in both nitrogen and oxygen. These enrichments may be due to microbial reduction of N₂O produced by nitrification, or they may reflect a prior ¹⁵N, ¹⁸O enrichment in an intermediate compound (for example, NH₂OH) from which the light isotopes are preferentially channelled into nitrite and nitrate during nitrification.