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

Abstract

Abstract. The mechanisms of microbial nitrous oxide (N 2 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 2 O sources in natural environments. This study compares hydrographic, N 2 O concentration, and N 2 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 • S; 114 • W) to the upwelling zone (St. UPX) off the central Chilean coast (∼34 • S). Although AOU/N 2 O and NO − 3 trends support the idea that most of the N 2 O (mainly from intermediate water (200-600 m)) comes from nitrification, N 2 O isotopomeric composition (intramolecular distribution of 15 N isotopes) expressed as SP (site preference of 15 N) shows low values (10 to 12‰) that could be attributed to the production through of microbial nitrifier denitrification (reduction of nitrite to N 2 O mediated by ammonium oxidizers). The coincidence of this SP signal with high -stability layer, where sinking organic particles can accumulate, suggests that N 2 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 2 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 N 2 O produced in this Correspondence to: J. Charpentier (jcharpentier@profc.udec.cl) layer. In contrast, coastal upwelling system is characterized by O 2 deficient condition and some N deficit in a eutrophic system. Here, N 2 O accumulates up to 480% saturation, and isotopic and isotopomer signals show highly complex N 2 O production processes, which presumably reflect both the effect of nitrification and denitrification at low O 2 levels on N 2 O production, but net N 2 O consumption by denitrification was not observed.

Nitrous oxide dynamics in low oxygen regions of the Pacific: insights from the MEMENTO database

“…Information on their N 2 O production rates is still very limited. N 2 O production rates from anammox (Kartal et al, 2007) and nitrifier denitrification (e.g., Charpentier et al, 2007) are also not well constrained.…”

“…2). Most of these N 2 O datasets have been comprehensively described previously (Cohen and Gordon, 1978;Pierotti and Rasmussen, 1980;Friederich et al, 1992;Dore and Karl, 1996;Dore et al, 1998;Popp et al, 2002;Nevison et al, 2003;Charpentier et al, 2007;Farías et al, 2007;Ryabenko et al, 2012), with the exception of two cruises in the ETP (Kock and Löscher, unpublished data; Bange, unpublished data). N 2 O values for all cruises were obtained by gas chromatography coupled with electron capture detectors.…”

“…Precision for N 2 O measurements for all datasets was < 3.4 %. For most datasets, O 2 was determined by modified Winkler method (the exception being that of Charpentier et al (2007), where O 2 was determined by CTD measurements calibrated with the Winkler method). Measurements of NO…”

Vertical segregation among pathways mediating nitrogen loss (N&lt;sub&gt;2&lt;/sub&gt; and N&lt;sub&gt;2&lt;/sub&gt;O production) across the oxygen gradient in a coastal upwelling ecosystem

“…This leads to an active removal of fixed N and greenhouse gas production (Farías et al, 2009;Galán et al, 2014). Likewise, the dense upwelled waters cause increased stratification in the water column, decelerating the downward flux of organic particles formed at the surface (Charpentier et al, 2007), facilitating their accumulation, and thus creating anoxic microbial hotspots within the oxycline that favour the N-based anaerobic microbial metabolisms (Klawonn et al, 2015;Stief et al, 2016).…”

Toward High-Resolution Vertical Measurements of Dissolved Greenhouse Gases (Nitrous Oxide and Methane) and Nutrients in the Eastern South Pacific

“…At the subtropical gyre (station C-ST26; Figure 8D), CH 4 concentrations were relatively low and varied from 3.8 to 5.0 nmol L −1 , with a slight peak at the subsurface layer (150-250 m depth). This peak was observed within the most stable layer, where particle accumulation occurred (Charpentier et al, 2007). Subsequently, CH 4 levels decreased in intermediate waters.…”