Pathways of Nitrous Oxide Production in the Eastern Tropical South Pacific Oxygen Minimum Zone

Oceanic emissions of nitrous oxide (N2O) account for roughly one-third of all natural sources to the atmosphere. Hot-spots of N2O outgassing occur over oxygen minimum zones (OMZs), where the presence of steep oxygen gradients surrounding anoxic waters leads to enhanced N2O production from both nitrification and denitrification. However, the relative contributions from these pathways to N2O production and outgassing in these regions remains poorly constrained, in part due to shared intermediary nitrogen tracers, and the tight coupling of denitrification sources and sinks. To shed light on this problem, we embed a new, mechanistic model of the OMZ nitrogen cycle within a three-dimensional eddy-resolving physical-biogeochemical model of the Eastern Tropical South Pacific (ETSP), tracking contributions from remote advection, atmospheric exchange, and local nitrification and denitrification. The model indicates that net N2O production from denitrification is approximately one order of magnitude greater than nitrification within the ETSP OMZ. However, only similar to 32% of denitrification-derived N2O production ultimately outgasses to the atmosphere in this region (contributing similar to 36% of the air-sea N2O flux on an annual basis), while the remaining is exported out of the domain. Instead, remotely produced N2O advected into the OMZ region accounts for roughly half (similar to 57%) of the total N2O outgassing, with smaller contributions from nitrification (similar to 7%). Our results suggests that, together with enhanced production by denitrification, upwelling of remotely derived N2O contributes the most to N2O outgassing over the ETSP OMZ.