Nitrous oxide in the Arabian Sea

Naqvi, S. W. A. ; Noronha, R. J. (1991) Nitrous oxide in the Arabian Sea Deep Sea Research Part A. Oceanographic Research Papers, 38 (7). pp. 871-890. ISSN 0198-0149

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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.

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