Nitrous oxide (N2O) in the north-eastern Arabian Sea and its formation pathways, a stable isotope and isotopomeric perspective

Nitrous oxide (N2O) is a biogenic trace gas in the atmosphere having significant role in global climate change and stratospheric chemistry. Its concentration in the ambient air has increased to the current value of 325 ppbv from 275ppbv in last hundred years. Oceans account for 25-30 % of global N2O emissions. Tropical and coastal regions contributes to more than 50 % of oceanic emissions; however, the biogeochemical pathways leading to its formations are poorly constrained. Nitrification and denitrification dominates N2O production with their N2O source product varying with oxygen availability. This paper describes the dissolved nitrous oxide concentration and its changes with apparent oxygen utilisation (AOU) along the Northeast Arabian Sea waters during South West Monsoon. Stable isotopic study of N2O including d15Nbulk, d18O, d15Nα and its site preference (SP) which is the difference between d15N α and d15N β were also conducted for the first time in this oceanic regions. The N2O were highly concentrated at the surface as compared to the other oceanic waters especially in the upwelling regions and the concentration increased to a maximum value in the oxygen less bottom waters. From sea to air N2O flux measurements these water masses appeared as a source for N2O with an average flux of 0.001-0.5 Tg N2O per year. The N2O vs AOU and Nitrate relationship suggested the importance of nitrification in the formation of N2O. The dual isotopic signatures of d 15Nbulk-N2O and d 18O were suggestive of more than one mechanism responsible for production with a nitrification dominated pathway in N2O cycling. The d18O of O2 confirms that mostly N2O is derived through nitrification process through the oxidation of NH2OH/NO. The SP values confirmed the existence of nitrification and nitrifier denitrification throughout the stations along the depth. This results suggest the need for more detailed study in the Gulf waters from where the highest N2O emissions were reported in 1991 for the first time.