Nitrogen and oxygen isotopic composition of N2O from suboxic waters of the eastern tropical North Pacific and the Arabian Sea - Measurement by continuous-flow isotope-ratio monitoring

Nitrous oxide (N2O) is a trace gas that is increasing in the atmosphere. It contributes to the greenhouse effect and influences the global ozone distribution. Recent reports suggest that regions such as the Arabian Sea may be significant sources of atmospheric N2O. In the ocean, N2O is formed as a by-product of nitrification and as an intermediary of denitrification. In the latter process, N2O can be further reduced to N-2. These processes, which operate on different source pools and have different magnitudes of isotopic fractionation, make separate contributions to the N-15 and O-18 isotopic composition of N2O. In the case of nitrification in oxic waters, the isotopic composition of N2O appears to depend mainly on the N-15/N-14 ratio of NH4+ and the O-18/O-16 ratio of O-2 and H2O. In suboxic waters, denitrification causes progressive N-15 and O-18 enrichment of N2O as a function of degree of depletion of nitrate and dissolved oxygen. Thus the isotopic signature of N2O should be a useful tool for studying the sources and sinks for N2O in the ocean and its impact on the atmosphere. We have made observations of N2O concentrations and of the dual stable isotopic composition of N2O in the eastern tropical North Pacific (ETNP) and the Arabian Sea. The stable isotopic composition of N2O was determined by a new method that required only 80-100 nmol of N2O per sample analysis. Our observations include determinations across the oxic/suboxic boundaries that occur in the water columns of the ETNP and Arabian Sea. In these suboxic waters, the values of delta(15)N and delta(18)O increased linearly with one another and with decreasing N2O concentrations, presumably reflecting the effects of denitrification. Our results suggest that the ocean could be an important source of isotopically enriched N2O to the atmosphere.