Nitrous oxide cycling in the Black Sea inferred from stable isotope and isotopomer distributions

The low-oxygen regions of the world's oceans have been shown to be major sources of nitrous oxide, a trace gas in the atmosphere that contributes to both greenhouse warming and the destruction of stratospheric ozone. Nitrous oxide can be produced as a by-product of nitrification or an intermediate of denitrification; low oxygen conditions enhance the yield of nitrous oxide from both pathways. We measured the concentration and isotopic composition of dissolved nitrous oxide at several stations in the Black Sea, an anoxic basin with a well-defined suboxic layer that separates the ventilated surface waters from the sulfidic deep waters. Our data show that in contrast to other low-oxygen marine regions, nitrous oxide does not accumulate in the Black Sea at significant levels. Moreover, whereas the reduction of nitrous oxide by denitrification usually yields residual gas that is enriched in both stable isotopes, in the Black Sea declining nitrous oxide concentrations are accompanied by enrichment in O-18-N2O but depletion in N-15-N2O. We measured a minimum delta N-15-N2O value of -10.8 +/- 0.8 parts per thousand vs. air N-2, by far the lowest measured to date for seawater. Measurements of the distribution of N-15 within the linear nitrous oxide molecule reveal that this unusual isotopic signal is most pronounced in the end-position nitrogen, and that site preference, or the tendency for N-15 to be found in the center-position nitrogen, co-varies positively with O-18-N2O. We surmise that the highly unusual isotopic composition of Black Sea nitrous oxide is the result of two processes: production of N-15-depleted nitrous oxide by ammonium oxidation followed by its reduction by denitrification, which causes enrichment in 180 and enhancement of N-15-site preference. Bottle incubation experiments with N-15-ammonium and N-15-nitrite reveal that both oxidation and reduction pathways to nitrous oxide are active in the Black Sea suboxic zone. (c) 2006 Elsevier Ltd. All rights reserved.