N‐loss isotope effects in the Peru oxygen minimum zone studied using a mesoscale eddy as a natural tracer experiment. Issue 6 (6th June 2015)

• Record Type: Journal Article
• Authors: Bourbonnais, Annie; Altabet, Mark A.; Charoenpong, Chawalit N.; Larkum, Jennifer; Hu, Haibei; Bange, Hermann W.; Stramma, Lothar
• Is Part Of: Global biogeochemical cycles. Volume 29:Issue 6(2015:Jun.)
• Page Start: 793
• Page End: 811
• Publication Date: 2015-06-06
• Subjects: Biogeochemical cycles -- Periodicals; Electronic journals; 577.1405
• DOI: 10.1002/2014GB005001
• Languages: English
• ISSNs: 0886-6236

Abstract

<abstract abstract-type="main" id="gbc20276-abs-0001"> <title>Abstract</title> <p id="gbc20276-para-0001">Mesoscale eddies in Oxygen Minimum Zones (OMZs) have been identified as important fixed nitrogen (N) loss hotspots that may significantly impact both the global rate of N‐loss as well as the ocean's N isotope budget. They also represent "natural tracer experiments" with intensified biogeochemical signals that can be exploited to understand the large‐scale processes that control N‐loss and associated isotope effects (<italic>ε</italic>; the ‰ deviation from 1 in the ratio of reaction rate constants for the light versus heavy isotopologues). We observed large ranges in the concentrations and N and O isotopic compositions of nitrate (NO<sub>3</sub><sup>−</sup>), nitrite (NO<sub>2</sub><sup>−</sup>), and biogenic N<sub>2</sub> associated with an anticyclonic mode‐water eddy in the Peru OMZ during two cruises in November and December 2012. In the eddy's center where NO<sub>3</sub><sup>−</sup> was nearly exhausted, we measured the highest <italic>δ</italic><sup>15</sup>N values for both NO<sub>3</sub><sup>−</sup> and NO<sub>2</sub><sup>−</sup> (up to ~70‰ and 50‰) ever reported for an OMZ. Correspondingly, N deficit and biogenic N<sub>2</sub>‐N concentrations were also the highest near the eddy's center (up to ~40 µmol L<sup>−1</sup>). <italic>δ</italic><sup>15</sup>N‐N<sub>2</sub> also varied with biogenic N<sub>2</sub> production, following kinetic isotopic fractionation during NO<sub>2</sub><sup>−</sup> reduction to N<sub>2</sub> and, for the first time, provided an independent assessment of N isotope fractionation during OMZ N‐loss. We found apparent variable <italic>ε</italic> for NO<sub>3</sub><sup>−</sup> reduction (up to ~30‰ in the presence of NO<sub>2</sub><sup>−</sup>). However, the overall <italic>ε</italic> for N‐loss was calculated to be only ~13–14‰ (as compared to canonical values of ~20–30‰) assuming a closed system and only slightly higher assuming an open system (16–19‰). Our results were similar whether calculated from the disappearance of DIN (NO<sub>3</sub><sup>−</sup> + NO<sub>2</sub><sup>−</sup>) or from the appearance of N<sub>2</sub> and changes in isotopic composition. Further, we calculated the separate <italic>ε</italic> values for NO<sub>3</sub><sup>−</sup> reduction to NO<sub>2</sub><sup>−</sup> and NO<sub>2</sub><sup>−</sup> reduction to N<sub>2</sub> of ~16–21‰ and ~12‰, respectively, when the effect of NO<sub>2</sub><sup>−</sup> oxidation could be removed. These results, together with the relationship between N and O of NO<sub>3</sub><sup>−</sup> isotopes and the difference in <italic>δ</italic><sup>15</sup>N between NO<sub>3</sub><sup>−</sup> and NO<sub>2</sub><sup>−</sup>, confirm a role for NO<sub>2</sub><sup>−</sup> oxidation in increasing the apparent <italic>ε</italic> associated with NO<sub>3</sub><sup>−</sup> reduction. The lower <italic>ε</italic> for N‐loss calculated in this study could help reconcile the current imbalance in the global N budget if representative of global OMZ N‐loss.</p> </abstract>