Assessing Nitrous Oxide Cycling in the Coastal Waters of the Eastern Tropical South Pacific Using Stable Isotopes

Nitrous oxide (N2O) is a climatically relevant greenhouse gas that can be produced in the ocean and emitted to the atmosphere, where it can later lead to ozone depletion. Previous studies show high accumulations of N2O above the oxygen minimum zone (OMZ) of the Eastern Tropical South Pacific (ETSP), but the relative roles of the microbial pathways that dictate N2O cycling vary regionally. The purpose of this study is to identify the primary pathways controlling N2O cycling in the coastal waters of the ETSP. In this study, N2O concentrations and its isotopes, as well as O2 concentrations and the isotopic compositions of nitrate and nitrite, were analyzed at three stations between the shallow coastal shelf and the shelf slope along the GEOTRACES GP16 Zonal Transect. Our results demonstrated a rapid decline in O2 concentrations coinciding with accumulation of N2O. This N2O accumulation is estimated to support significant fluxes of N2O to the atmosphere at our stations. N2O isotopocule results illustrate intense N2O cycling, driven by a combination of incomplete denitrification and hybrid N2O production in the oxycline. In the anoxic waters of the ODZ N2O isotopes indicate concurrent N2O production and consumption through changes in 𝛿15N⍺, 𝛿18O, and 𝛿15Nβ. Overall, N2O cycling in the coastal ETSP waters appears to be mainly driven by denitrification, with a smaller role from hybrid formation.