Spatial Variations in N2O Concentration and Isotopomer Composition off the Peru Coast

Nitrous oxide (N₂O) is a potent greenhouse gas and stratospheric ozone depleting substance. The ocean is an important source of N₂O to the atmosphere, accounting for up to about 30% of total emissions. However, the factors controlling N₂O production and consumption in oceanic environments are still not understood nor constrained. N₂O is a by-product of aerobic nitrification, and is formed by two different pathways: 1) the decomposition of hydroxylamine, an intermediate during ammonium oxidation to nitrite, or 2) the reduction of nitrite to N₂O (nitrifier-denitrification). N₂O is also an intermediate during denitrification under anoxic conditions. In this study, we measured N₂O concentrations and isotopomer ratios, as well as O₂, nutrient and biogenic N₂ concentrations and the isotopic compositions of nitrate, nitrite and biogenic N₂ at several coastal stations during two cruises off the Peru coast (6-14°S, 75-81°W) in December 2012 and January 2013. [N₂O] varied from below equilibrium values in the OMZ to up to 238 nmol L^-1 at 11 m depth at one of the shallowest stations. The isotopic composition of N₂O (bulk δ¹⁵N: -5 to 30‰, δ¹⁸O: 41 to 95‰, and Site Preference: -3 to 65‰) also varied widely, with important differences between stations. Our results show a strong spatial heterogeneity in the mechanisms controlling N₂O production and consumption in coastal upwelling regions, which should be taken into account in oceanic N₂O models. We will discuss the contributions from different N₂O production processes responsible for the observed extreme N₂O accumulations.