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 un-dersaturated (about 94% saturation), and the calculated mean sea-air flux rate was −3.68±2.57 μmolm−2 d−1. The vertical distributions tested exhibited N2O maxima at around 150–300m (ÉN2O, 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 15Nbulk and +43.5 to +46.2‰ for 18O associated with the coupling of nitrification and denitrification production mechanisms. Site preference decreased from an average 17‰ at the surface to the ÉN2O 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±5.3Mg N2O-N d−1, which represents the amount that can escape to the atmosphere and thus contribute to emissions into the world’s oceans.