Some estimates of the budget of bioavailable nitrogen (fixed-N) suggest that the oceanic nitrogen cycle is grossly out of balance. We use observations of dissolved N2/Ar ratios along abyssal flow paths in the ocean to investigate fixed-N loss by benthic denitrification, one of the largest uncertainties. Dissolved N2/Ar in the deep-sea increases from the North Atlantic to the North Pacific. At depths >3500 m, this can be explained by the mixing of low N2/Ar source waters from the North Atlantic with high N2/Ar source waters from the Southern Ocean. Benthic denitrification in sediments bathed by these abyssal waters is below the detection limit. However, measureable increases in N2/Ar at depths of 2000–3000 m between the subtropical and subarctic North Pacific, regions that share the same source water, must be caused by benthic denitrification. The Cascadia Basin, with high denitrification rates and connection to the open North Pacific, is a likely source.