Nitrous oxide distribution and its origin in the central and eastern South Pacific Subtropical Gyre

Abstract

The mechanisms of microbial nitrous oxide (N₂O) production in the ocean have been the subject of many discussions in recent years. New isotopomeric tools can further refine our knowledge of N₂O sources in natural environments. This study compares hydrographic, N₂O concentration, and N₂O 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° S; 114° W) to the upwelling zone (St. UPX) off the central Chilean coast (~34° S). Although AOU/N₂O and NO₃^− trends support the idea that most of the N₂O (mainly from intermediate water (200–600 m)) comes from nitrification, N₂O isotopomeric composition (intramolecular distribution of ¹⁵N isotopes) expressed as SP (site preference of ¹⁵N) shows low values (10 to 12permil) that could be attributed to the production through of microbial nitrifier denitrification (reduction of nitrite to N₂O mediated by ammonium oxidizers). The coincidence of this SP signal with high – stability layer, where sinking organic particles can accumulate, suggests that N₂O 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 O₂ depletion suitable for nitrifier denitrication. As lateral advection seems to be relatively insignificant in the gyre, in situ nitrifier denitrification could account for 40–50% of the N₂O produced in this layer. In contrast, coastal upwelling system is characterized by O₂ deficient condition and some N deficit in a eutrophic system. Here, N₂O accumulates up to 480% saturation, and isotopic and isotopomer signals show highly complex N₂O production processes, which presumably reflect both the effect of nitrification and denitrification at low O₂ levels on N₂O production, but net N₂O consumption by denitrification was not observed.