Extreme N₂O accumulation in the coastal oxygen minimum zone off Peru

Abstract. Depth profiles of nitrous oxide (N₂O) were measured during six cruises to the upwelling area and oxygen minimum zone (OMZ) off Peru in 2009 and 2012/2013, covering both the coastal shelf region and the adjacent open ocean. N₂O profiles displayed a strong sensitivity towards oxygen concentrations. Open ocean profiles with distances to the shelf break larger than the first baroclinic Rossby radius of deformation showed a transition from a broad maximum close to the Equator to a double-peak structure south of 5° S where the oxygen minimum was more pronounced. Maximum N₂O concentrations in the open ocean were about 80 nM. A linear relationship between ΔN₂O and apparent oxygen utilization (AOU) could be found for measurements within the upper oxycline, with a slope similar to studies in other oceanic regions. In contrast, N₂O profiles close to the shelf revealed a much higher variability, and N₂O concentrations higher than 100 nM were often observed. The highest N₂O concentration measured at the shelf was  ∼  850 nM. Due to the extremely sharp oxygen gradients at the shelf, N₂O maxima occurred in very shallow water depths of less than 50 m. In the coastal area, a linear relationship between ΔN₂O and AOU could not be observed as extremely high ΔN₂O values were scattered over the full range of oxygen concentrations. The data points that showed the strongest deviation from a linear ΔN₂O ∕ AOU relationship also showed signals of intense nitrogen loss. These results indicate that the coastal upwelling at the Peruvian coast and the subsequent strong remineralization in the water column causes conditions that lead to extreme N₂O accumulation, most likely due to the interplay of intense mixing and high rates of remineralization which lead to a rapid switching of the OMZ waters between anoxic and oxic conditions. This, in turn, could trigger incomplete denitrification or pulses of increased nitrification with extreme N₂O production.