6 research outputs found
Nitrous oxide distribution and its origin in the central and eastern South Pacific Subtropical Gyre
The mechanisms of microbial nitrous oxide (N<sub>2</sub>O) production in the ocean have been the subject of many discussions in recent years. New isotopomeric tools can further refine our knowledge of N<sub>2</sub>O sources in natural environments. This study compares hydrographic, N<sub>2</sub>O concentration, and N<sub>2</sub>O isotopic and isotopomeric data from three stations along a coast-perpendicular transect in the South Pacific Ocean, extending from the center (Sts. GYR and EGY) of the subtropical oligotrophic gyre (~26&deg; S; 114&deg; W) to the upwelling zone (St. UPX) off the central Chilean coast (~34&deg; S). Although AOU/N<sub>2</sub>O and NO<sub>3</sub><sup>&minus;</sup> trends support the idea that most of the N<sub>2</sub>O (mainly from intermediate water (200&ndash;600 m)) comes from nitrification, N<sub>2</sub>O isotopomeric composition (intramolecular distribution of <sup>15</sup>N isotopes) expressed as SP (site preference of <sup>15</sup>N) shows low values (10 to 12permil) that could be attributed to the production through of microbial nitrifier denitrification (reduction of nitrite to N<sub>2</sub>O mediated by ammonium oxidizers). The coincidence of this SP signal with high &ndash; stability layer, where sinking organic particles can accumulate, suggests that N<sub>2</sub>O could be produced by nitrifier denitrification inside particles. It is postulated that deceleration of particles in the pycnocline can modify the advection - diffusion balance inside particles, allowing the accumulation of nitrite and O<sub>2</sub> depletion suitable for nitrifier denitrication. As lateral advection seems to be relatively insignificant in the gyre, in situ nitrifier denitrification could account for 40&ndash;50% of the N<sub>2</sub>O produced in this layer. In contrast, coastal upwelling system is characterized by O<sub>2</sub> deficient condition and some N deficit in a eutrophic system. Here, N<sub>2</sub>O accumulates up to 480% saturation, and isotopic and isotopomer signals show highly complex N<sub>2</sub>O production processes, which presumably reflect both the effect of nitrification and denitrification at low O<sub>2</sub> levels on N<sub>2</sub>O production, but net N<sub>2</sub>O consumption by denitrification was not observed