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 (> 90‰) and low δ15Nβ (< -10‰)
values, 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.