| dc.description.abstract | Nitrous oxide (N sub(2)O) is an important greenhouse gas that also plays a role in the chemistry of stratospheric ozone depletion, but its atmospheric budget has yet to be well-quantified sup(1-5). However, multi-isotope characterization of N sub(2)O emitted from various natural sources is a potentially powerful tool for providing the much-needed constraints. It is generally believed that production of isotopically light (low sup(15)N/ sup(14)N and sup(18)O/ sup(16)O ratios) N sub(2)O occurs in the upper ocean through nitrification process, and that the flux of this N sub(2)O from sea to air isotopically counters the flux of heavy N sub(2)O from the stratosphere to the troposphere sup(1,2). But eastern-boundary ocean-upwelling zones, which contain oxygen-depleted waters and are sites of intense N sub(2)O efflux sup(6-10), have not been adequately studied. We show here, using new isotope data, that in spite of huge denitrification-related enrichments of sup(15)N and sup(18)O in N sub(2)O at mid-depths in the Arabian Sea, N sub(2)O emitted from upwelled waters is only slightly enriched in sup(18)O, and moderately depleted in sup(15)N, relative to air. These opposing isotopic signatures and modest departures from the isotopic composition of tropospheric N sub(2)O indicate that air-sea exchange cannot-given the heavy isotopic signature of N sub(2)O derived from the stratosphere-allow the troposheric budget of N sub(2)O to be closed without invoking hitherto-unknown N sub(2)O sources and sinks. Our oceanic data cannot be explained through either nitrification or denitrification alone, such that a coupling between the two processes may be an important mechanism of N sub(2)O production | |
