Extreme N2O accumulation in the coastal oxygen minimum zone off Peru

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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 100nM 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.

Figures

  • Figure 1. Station maps of the sampled N2O stations from cruises (a) M77-3, December 2008–January 2009 (•) and M77-4, January–February 2009 ( ), (b) M90, November 2012 (•) and M91, December 2012 ( ), (c) M92, January 2013 (•) and M93, February–March 2013 ( ). Section annotations in (a) and (b) correspond to the vertical sections shown in Figs. 2 and 3.
  • Figure 2. Spatial distributions of oxygen (a, b), nitrite (c, d) and N2O (e, f) along 86 ◦W during M77-4 (2009, a, c, e) and M90 (2012, b, d, f). Small dots indicate location and depth of the discrete samples. Data gridding: ODV/DIVA.
  • Figure 3. Cross-shelf distribution of (a) oxygen, (b) phosphate, (c) nitrate, (d) N′, (e) nitrite and (f) N2O during M91 (section F).
  • Figure 4. Selected depth profiles of oxygen (black dots, dotted line), potential density (σθ , grey line) and N2O (red line, open circles; left panel) and nitrate (grey line), nitrite (black circles, dotted line), ammonium (blue diamonds, straight line) and N′ (red line, small dots; right panel) from selected open-ocean and shelf stations during M90-93. Depth profiles of oxygen and σθ were taken from the CTD sensors, whereas the other parameters were taken from discrete samples. The locations of the respective stations are shown in the map. Red signals denote stations classified as “coastal” stations whereas blue signals denote “offshore” stations. Please note the changes in the scales for N2O, σθ , nitrite and ammonium.
  • Figure 5. Temperature–salinity diagrams with 1N2O colour coded for (a) the offshore stations and (b) the onshore stations. Grey symbols denote the T –S properties of (a) the onshore and (b) the offshore data. The approximate location of the different water masses annotated in the figure is given by black dots or lines. Different symbols denote different cruises: M77-3; 3 M77-4; © M92; left pointing triangle: M90; right pointing triangle: M91; open hexagonal star:M93.
  • Figure 6. 1N2O /AOU relationship from (a) offshore stations and (b) coastal stations. Samples from the upper OMZ and oxycline (sample depth < 350 m) are colour coded with N′, whereas samples from below 350 m are shown in grey. Different symbols for different cruises are denoted the same as in Fig. 5. The black line denotes the 1N2O /AOU relationship from the offshore data for samples with O2 > 50 µM and depth < 350 m (y = 0.13x+3.73; r2 = 0.83). Please note the change in the scaling for 1N2O values of 0–100 and 100–1000 nM (dotted line).

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CITATION STYLE

APA

Kock, A., Arevalo-Martinez, D. L., Loscher, C. R., & Bange, H. W. (2016). Extreme N2O accumulation in the coastal oxygen minimum zone off Peru. Biogeosciences, 13(3), 827–840. https://doi.org/10.5194/bg-13-827-2016

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