Extreme N<sub>2</sub>O accumulation in the coastal oxygen minimum zone off Peru

Abstract

Abstract. Depth profiles of nitrous oxide (N 2 O) 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. N 2 O 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 N 2 O concentrations in the open ocean were about 80 nM. A linear relationship between N 2 O 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, N 2 O profiles close to the shelf revealed a much higher variability, and N 2 O concentrations higher than 100 nM were often observed. The highest N 2 O concentration measured at the shelf was ∼ 850 nM. Due to the extremely sharp oxygen gradients at the shelf, N 2 O maxima occurred in very shallow water depths of less than 50 m. In the coastal area, a linear relationship between N 2 O and AOU could not be observed as extremely high N 2 O values were scattered over the full range of oxygen concentrations. The data points that showed the strongest deviation from a linear N 2 O / 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 N 2 O 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 N 2 O production.

Influence of mesoscale eddies on the distribution of nitrous oxide in the eastern tropical South Pacific

“…Both our maximum and minimum of N 2 O concentrations along the vertical O 2 gradients were well within the range of previous observations in the ETSP (Farías et al, 2007(Farías et al, , 2009Ryabenko et al, 2012;Kock et al, 2016), and no statistically significant differences (two-sample t test, p values > 0.05, α = 0.05) were found between our N 2 O values at the centre of the three eddies and a mean open-ocean profile along the 86 • W section (Fig. S2).…”

“…We showed that the main vertical distribution of N 2 O within eddies A, B, and C fits well to the typical open-ocean distribution reported in previous works in the same area (Löscher et al, 2012;Ryabenko et al, 2012, Kock et al, 2016. Thus, elevated concentrations of N 2 O at the upper and lower boundaries of the OMZ could be attributed to production via nitrification and partial denitrification (as inferred from high amoA and nirS abundances, respectively), whereas denitrification-driven depletion of N 2 O was clearly observed at the core of the OMZ (O 2 ∼ < 5 µmol L −1 ).…”

“…In order to obtain background N 2 O concentrations for the ETSP at the time of sampling, we used data from Kock et al (2016). For this, we computed a mean profile by averaging water column concentrations of N 2 O from stations located in the open ocean (86 • W section) between 6 and 16 • S. We did not include near-coastal stations for the background profile calculation due to the high variability that can be observed, since this hampers any attempts of obtaining a "typical" profile (see Kock et al, 2016). Our designation of particular stations to eddy cores and edges was based on the SSHA data and follows the criteria defined by Stramma et al (2013).…”

“…The fact that OMZ core waters (which act as a sink for N 2 O) are transported over long distances offshore also suggests a potentially higher relevance of this region as a sink for N 2 O. Nevertheless, given the extremely high production of N 2 O in the ETSP (Kock et al, 2016) and our current lack of understanding of the overall, long-term impact of mesoscale eddies in the biogeochemistry of N 2 O, it is, at this point, difficult to assess how relevant this sink term might be.…”

“…S2). Hence, although the influence of eddies on the N 2 O distribution in the ETSP as a whole seems to be masked by the high variability of the large-scale distribution of N 2 O (see Kock et al, 2016), the negative N 2 O anomalies within the core of the OMZ at the centre of anticyclonic eddies suggest a locally enhanced sink for this gas.…”