Production and consumption mechanisms of N₂O in the Southern Ocean revealed from its isotopomer ratios

Abstract. The distribution of dissolved N₂O in the Southern Ocean at 140° E was measured during the austral summer (February–March 2002) in the framework of the 43rd Japanese Antarctic Research Expedition (JARE-43). Surface-dissolved N₂O was undersaturated (about 94% saturation), and the calculated mean sea-air flux rate was −3.68±2.57 μmol m⁻² d⁻¹. The vertical distributions tested exhibited N₂O maxima at around 150–300 m (ΔN₂O, 7.90–8.51 nM) below the chlorophyll-rich layer, which coincided with the layer of minimum oxygen. These observations strongly suggest that N₂O production and consumption are related to apparent oxygen utilization (AOU). In the deeper layer, the presence of anoxic microsites within particles, together with the horizontal and vertical movement of cold water around Antarctica, is one of the parameters that govern the intramolecular distribution of the isotopic composition of N₂O. The N₂O isotopic compositions in the maximum layer were +7.3 to +8.2‰ for δ¹⁵N_bulk and +43.5 to +46.2‰ for δ¹⁸O associated with the coupling of nitrification and denitrification production mechanisms. Site preference decreased from an average 17‰ at the surface to the ΔN₂O maximum and slightly increased with depth up to 24‰ at the deeper region. The influence of deep Southern Ocean N₂O on the global N₂O budget is estimated to be about 46.2±5.3 Mg N₂O-N d⁻¹, which represents the amount that can escape to the atmosphere and thus contribute to emissions into the world's oceans.