Nitrous oxide in the Arabian Sea

Abstract

Measurements of nitrous oxide (N₂O) at 16 stations in the Arabian Sea reveal high degrees of surface saturation (186 ± 37%) and consequently large atmospheric fluxes of N₂O (4.46 ± 2.60 μmol m^-2 day^-1). Unusually high for a non-upweling period, these values reflect the large vertical gradients of N₂O just below the mixed layer as the vertical exchange coefficients (0.55 ± 0.32 cm² s^-1) required to sustain the observed fluxes are not very high. Vertical distribution of N₂O in the Arabian Sea conforms to the trends seen in other oxygen-deficient environments, with minimal values occurring within the secondary nitrite maximum and the maxima in the N₂O profiles located at the peripheries of this layer. Outside the denitrification zone, only one broad maximum is observed at mid-depth. The mechanisms proposed for the production of N₂O are re-evaluated in the light of recent data on the nitrogen isotopic composition of N₂O from the western North Pacific Ocean. It is proposed that a nitrification-denitrification couple represented by the pathway NH₄⁺ → NO → N₂O may be a potentially dominant mechanism for N₂O production in the ocean. This can explain the observed enrichment of ¹⁵N in N₂O as compared to NO₃^- while nitrification or denitrification alone cannot. Appreciable horizontal (east-west) gradients in N₂O concentrations are observed at intermediate depths close to the continental margin off the west coast of India suggesting a possible sink of N₂O within the marginal sediments. The diffusive losses of N₂O to the denitrifying waters and marginal sediments are estimated as >2 Tg N y^-1. The Arabian Sea as a whole appears to be a net source of N₂O, suggesting that the gross production of oceanic N₂O h as been underestimated.