Extreme N2O accumulation in the coastal oxygen minimum zone off Peru

[摘要] Depth profiles of nitrous oxide (N₂O) were measured during six cruisesto the upwelling area and oxygen minimum zone (OMZ) off Peru in 2009 and2012/2013, covering both the coastal shelf region and the adjacent openocean. N₂O profiles displayed a strong sensitivity towards oxygenconcentrations. Open ocean profiles with distances to the shelf break largerthan the first baroclinic Rossby radius of deformation showed a transitionfrom a broad maximum close to the Equator to a double-peak structure southof 5° S where the oxygen minimum was more pronounced. MaximumN₂O concentrations in the open ocean were about 80 nM. A linearrelationship between ΔN₂O and apparent oxygen utilization (AOU)could be found for measurements within the upper oxycline, with a slopesimilar to studies in other oceanic regions. In contrast, N₂O profilesclose to the shelf revealed a much higher variability, and N₂Oconcentrations higher than 100 nM were often observed. The highest N₂Oconcentration measured at the shelf was  ∼  850 nM. Due to theextremely sharp oxygen gradients at the shelf, N₂O maxima occurred invery shallow water depths of less than 50 m. In the coastal area, a linearrelationship between ΔN₂O and AOU could not be observed asextremely high ΔN₂O values were scattered over the full rangeof oxygen concentrations. The data points that showed the strongestdeviation from a linear ΔN₂O ∕ AOU relationship also showedsignals of intense nitrogen loss. These results indicate that the coastalupwelling at the Peruvian coast and the subsequent strong remineralizationin the water column causes conditions that lead to extreme N₂Oaccumulation, most likely due to the interplay of intense mixing and highrates of remineralization which lead to a rapid switching of the OMZ watersbetween anoxic and oxic conditions. This, in turn, could trigger incompletedenitrification or pulses of increased nitrification with extreme N₂Oproduction.

[学科分类] 地球化学与岩石