Potential use of the N2/Ar ratio as a constraint on the oceanic fixed nitrogen loss

Using a global ocean biogeochemical model, we investigate the suitability of the N-2/Ar supersaturation ratio (N-2/Ar) as a tracer of marine nitrogen fixation and denitrification, i.e., the main biological processes that add or remove fixed nitrogen to or from the ocean. In a series of factorial simulations, we demonstrate that, in regions away from the oxygen minimum zones (OMZs), the N-2/Ar characteristics are mostly determined by benthic denitrification occurring in the deep ocean with minor contributions from benthic and water column denitrification at shallower depths. In the OMZs, the subsurface maxima of N-2/Ar are mainly determined by water column denitrification. In contrast, nitrogen fixation has little impact on N-2/Ar owing to the rapid loss of the N-2 supersaturation signal through air-sea exchange. We thus conclude that N-2/Ar can act as a powerful constraint on water column and benthic denitrification occurring in intermediate to deep waters, but it cannot be used to estimate nitrogen fixation. A comparison between the currently very limited observations of the N-2/Ar with our model results shows an acceptable level of agreement, suggesting that the model's prescribed rates and distributions of benthic and water column denitrification (i.e., 140 and 52TgNyr(-1), respectively) are reasonable and confirm the results derived from other constraints.