Investigating heterogeneity in nitrous oxide cycling of the Eastern Tropical North Pacific through isotopocules

Abstract

Nitrous oxide (N2O) is a powerful greenhouse gas, and oceanic sources account for up to one third of total flux to the atmosphere. In oxygen-deficient zones (ODZs) like the Eastern Tropical North Pacific (ETNP), N2O can be produced and consumed by several biological processes that are controlled by a variety of oceanographic conditions. In this study, the concentration and isotopocule ratios of N2O from a 2016 cruise to the ETNP were analyzed to examine heterogeneity in N2O cycling across the region. Along the north-south transect, three distinct biogeochemical regimes were identified: background, core-ODZ, and high-N2O stations. Background stations were characterized by less dynamic N2O cycling. Core-ODZ stations were characterized by co-occurring N2O production and consumption at anoxic depths, indicated by high δ18O (> 90values, and confirmed by a time-dependent model, which indicated that N2O production via denitrification was significant and may occur with a non-zero site preference. High-N2O stations were defined by [N2O] reaching 126.07±12.6 nM, low oxygen concentrations expanding into near-surface isopycnals, and the presence of a mesoscale eddy. At these stations, model results indicated significant N2O production from ammonia-oxidizing archaea and denitrification from nitrate in the near-surface N2O maximum, while bacterial nitrification and denitrification from nitrite were insignificant. This study also represents the first in the ETNP to link N2O isotopocule measurements to a mesoscale eddy, suggesting the importance of eddies to the spatiotemporal variability in N2O cycling in this region.