BUDGETARY AND BIOGEOCHEMICAL IMPLICATIONS OF N2O ISOTOPE SIGNATURES IN THE ARABIAN SEA

Nitrous oxide (N2O) is an important greenhouse gas that also plays a r ole in the chemistry of stratospheric ozone depletion, but its atmosph eric budget has yet to be well-quantified(1-5). However, multi-isotope characterization of N2O emitted from various natural sources is a pot entially powerful tool for providing the much-needed constraints. It i s generally believed that production of isotopically light (low N-15/N -14 and O-18/O-16 ratios) N2O occurs in the upper ocean through nitrif ication process, and that the flux of this light N2O from sea to air i sotopically counters the flux of heavy N2O from the stratosphere to th e troposphere(1,2). But eastern-boundary ocean-upwelling zones, which contain oxygen-depleted waters and are sites of intense N2O efflux(6-1 0), have not been adequately studied. We show here, using new isotope data, that in spite of huge denitrification-related enrichments of N-1 5 and O-18 in N2O at mid-depths in the Arabian Sea, N2O emitted from u pwelled waters is only slightly enriched in O-18, and moderately deple ted in N-15, relative to air. These opposing isotopic signatures and m odest departures from the isotopic composition of tropospheric N2O ind icate that air-sea exchange cannot-given the heavy isotopic signature of N2O derived from the stratosphere-allow the tropospheric budget of N2O to be closed without invoking hitherto-unknown N2O sources and sin ks. Our oceanic data cannot be explained through either nitrification or denitrification alone, such that a coupling between the two process es may be an important mechanism of N2O production.