Budgetary and biogeochemical implications of N2O isotope signatures in the Arabian Sea

Abstract

Nitrous oxide (N2O) 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-quantified1,2,3,4,5. However, multi-isotope characterization of N2O 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 15N/14N and 18O/16O ratios) N2O occurs in the upper ocean through nitrification process, and that the flux of this light N2O from sea to air isotopically counters the flux of heavy N2O from the stratosphere to the troposphere1,2. But eastern-boundary ocean-upwelling zones, which contain oxygen-depleted waters and are sites of intense N2O efflux6,7,8,9,10, have not been adequately studied. We show here, using new isotope data, that in spite of huge denitrification-related enrichments of 15N and 18O in N2O at mid-depths in the Arabian Sea, N2O emitted from upwelled waters is only slightly enriched in 18O, and moderately depleted in 15N, relative to air. These opposing isotopic signatures and modest departures from the isotopic composition of tropospheric N2O indicate 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 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 N2O production.