Marine oxygen deficient zones are hotspots of nitrous oxide (N₂O) production, but the drivers and mechanisms of this production remain poorly understood. Here we use the stable nitrogen and oxygen isotopes of nitrate, nitrite, and N₂O to constrain nitrogen cycle processes and their rates. N₂O is particularly rich in isotopic information since we can measure the isotopic content of each nitrogen atom individually (referred to as “isotopomers”) and their difference (“site preference”). We used these measurements to constrain the rates of N₂O cycle processes in a regional, process-based model (Nitrogen cycling in Oxygen Minimum Zones, or NitrOMZ) of the eastern tropical North Pacific oxygen deficient zone, which includes N₂O production via nitrification, denitrification, and hybrid N₂O production. We found that the model can reproduce the observed high near-surface concentrations of N₂O — and corresponding site preference minimum — with a combination of N₂O production via nitrification, denitrification, and N₂O production from nitrate via denitrification, with a nitrite “shunt” within the cell. In permanently anoxic waters, the model was only able to reproduce isotopic observations when denitrification is allowed to produce N₂O with a novel, non-zero site preference. In oxygen-deficient waters, isotopomer profiles also indicated a shift from N₂O production from nitrite to N₂O production from nitrate — and thus a potential shift between denitrifying communities. Sensitivity tests with increasing bottom boundary dissolved oxygen and decreasing top boundary particulate organic carbon flux indicate that the N₂O cycle is highly sensitive to both, with contrasting responses that are recorded in the isotopomer profiles.

Primary Presenter: Colette Kelly, Stanford University (clkelly@stanford.edu)

Authors:

Daniele Bianchi, University of California Los Angeles  (dbianchi@atmos.ucla.edu)

Daniel McCoy, University of California Los Angeles  (demccoy@atmos.ucla.edu)

Simon Yang, University of California Los Angeles  (yangsi@atmos.ucla.edu)

Bonnie Chang, Vesta, PBC  (bonniexchang@gmail.com)

Karen Casciotti, Stanford University  (kcasciotti@stanford.edu)