Identifying the Sources and Drivers of Nitrous Oxide Accumulation in the Eddy‐Influenced Eastern Tropical North Pacific Oxygen‐Deficient Zone
Patrick J. Monreal, Colette L. Kelly, Nicole M. Travis, Karen L. Casciotti
Abstract
Abstract Nitrous oxide (N 2 O) is a powerful greenhouse gas, and oceanic sources account for up to one third of the total natural flux to the atmosphere. In oxygen‐deficient zones (ODZs) like the Eastern Tropical North Pacific (ETNP), N 2 O can be produced and consumed by several biological processes. In this study, the concentration and isotopocule ratios of N 2 O from a 2016 cruise in the ETNP were analyzed to examine sources of and controls on N 2 O cycling across this region. Along the north‐south transect, three distinct biogeochemical regimes were identified: background, core‐ODZ, and high‐N 2 O stations. Background stations were characterized by smaller variations in N 2 O concentration and isotopic profiles relative to the other regimes. Core‐ODZ stations were characterized by co‐occurring N 2 O production and consumption at anoxic depths, indicated by high δ 18 O‐N 2 O (>90‰) and low δ 15 N 2 O β (<−10‰) values, and confirmed by a time‐dependent model, which indicated that N 2 O production via denitrification was significant and may occur with a nonzero site preference. High‐N 2 O stations, located at the periphery of a mesoscale eddy, were defined by N 2 O reaching 126.07 ± 12.6 nM and low oxygen concentrations expanding into near‐surface isopycnals. At these stations, model results indicated significant N 2 O production from ammonia‐oxidizing archaea and denitrification from nitrate at the N 2 O maximum within the oxycline, while bacterial nitrification and denitrification from nitrite were insignificant. This study also represents the first in the ETNP to link N 2 O production mechanisms to a mesoscale eddy through isotopocule measurements, suggesting the importance of eddies to spatiotemporal variability in N 2 O cycling and emissions from this region.