Budgetary and biogeochemical implications of N_2O isotope signatures in the Arabian Sea

摘要

Nitrous oxide (N_2O) 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 N_2O 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 ~(15)N/~(14)N and ~(18)O/~(16)O ratios) N_2O occurs in the upper ocean through nitrification process, and that the flux of this light N_2O from sea to air isotopically counters the flux of heavy N_2O from the stratosphere to the troposphere. But eastern-boundary ocean-upwelling zones, which contain oxygen-depleted waters and are sites of intense N_2O efflux, have not been adequately studied. We show here, using new isotope data, that in spite of huge denitrification-related enrichments of ~(15)N and ~(18)O in N_2O at mid-depths in the Arabian Sea, N_2O emitted from upwelled waters is only slightly enriched in ~(18)O, and moderately depleted in ~(15)N, relative to air. These opposing isotopic signatures and modest departures from the isotopic composition of tropospheric N_2O indicate that air-sea exchange cannot—given the heavy isotopic signature of N_2O derived from the stratosphere—allow the tropospheric budget of N_2O to be closed without invoking hitherto-unknown N_2O 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_2O production.