Nitrogen and oxygen isotopic composition of N sub(2)O from suboxic waters of the eastern tropical North Pacific and the Arabian Sea - measurement by continuous-flow isotope-ratio monitoring

Abstract

Nitrous oxide (N sub(2)O) is a trace gas that is increasing in the atmosphere. It contributes to the greenhouse effect and influence the global ozone distribution. Recent reports suggest that regions such as the Arabian Sea may be significant sources of atmospheric N sub(2)O. In the ocean, N sub(2)O is formed as a by-product of nitrification and as an intermediary of denitrification. In the latter process, N sub(2)O can be further reduced to N sub(2). These processes, which operate on different source pools and have different magnitudes of isotopic fractionation, make separate contributions to the sup(15)N and sup(18)O isotopic composition of N sub(2)O. In the case of nitrification in oxic waters, the isotopic composition of N sub(2)O appears to depend mainly on the sup(15)N/ sup(14)N ratio of NH sub(4) sup(+) and the sup(18)O/ sup(16)O ratio of O sub(2) and H sub(2)O. In suboxic waters, denitrification causes progressive sup(15)N and sup(18)O enrichment of N sub(2)O as a function of degree of depletion of nitrate and dissolved oxygen. Thus the isotopic signature of N sub(2)O should be a useful tool for studying the sources and sinks for N sub(2)O in the ocean and its impact on the atmosphere. We have made observations of N sub(2)O concentrations and of the dual stable isotopic composition of N sub(2)O in the eastern tropical North Pacific (ETNP) and the Arabian Sea. The stable isotopic composition of N sub(2)O was determined by a new method that required only 80-100 nmol of N sub(2)O 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 sup(15)N and delta sup(18)O increased linearly with one another and with decreasing N sub(2)O concentrations, presumably reflecting the effects of denitrification. Our results suggest that the ocean could be an important source of isotopically enriched N sub(2)O to the atmosphere