N-loss isotope effects in the Peru oxygen minimum zone studied using a mesoscale eddy as a natural tracer experiment

Abstract

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 (ε; 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 (NO3-), nitrite (NO2-), and biogenic N2 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 NO3- was nearly exhausted, we measured the highest δ15N values for both NO3- and NO2- (up to ∼70‰ and 50‰) ever reported for an OMZ. Correspondingly, N deficit and biogenic N2-N concentrations were also the highest near the eddy's center (up to ∼40 μmol L-1). δ15N-N2 also varied with biogenic N2 production, following kinetic isotopic fractionation during NO2- reduction to N2 and, for the first time, provided an independent assessment of N isotope fractionation during OMZ N-loss. We found apparent variable ε for NO3- reduction (up to ∼30‰ in the presence of NO2-). However, the overall ε 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 (NO3- + NO2-) or from the appearance of N2 and changes in isotopic composition. Further, we calculated the separate ε values for NO3- reduction to NO2- and NO2- reduction to N2 of ∼16-21‰ and ∼12‰, respectively, when the effect of NO2- oxidation could be removed. These results, together with the relationship between N and O of NO3- isotopes and the difference in δ15N between NO3- and NO2-, confirm a role for NO2- oxidation in increasing the apparent ε associated with NO3- reduction. The lower ε 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.