2-ISOTOPE CHARACTERIZATION OF N2O IN THE PACIFIC-OCEAN AND CONSTRAINTS ON ITS ORIGIN IN DEEP-WATER

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 sea2-5, 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 predictions10,11, 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. © 1990 Nature Publishing Group.