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-quantified. 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 troposphere. But eastern-boundary ocean-upwelling zones, which contain oxygen-depleted waters and are sites of intense N₂O efflux 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.