Fixed nitrogen loss from the eastern tropical North Pacific and Arabian Sea oxygen deficient zones determined from measurements of N₂:Ar

[1] Previous work estimating the N₂excess above background due to denitrification has suggested that nitrate deficit-type methods may be an underestimate of fixed nitrogen (N) loss in the major oxygen deficient zones of the ocean. The N₂excess approach has the advantage over nitrate deficit-type methods in that it does not depend on stoichiometric assumptions of fixed N to phosphate or oxygen utilization and avoids any uncertainly regarding the pathway of N loss. Measurements of N₂:Ar from two stations within the eastern tropical North Pacific and from one station within the Arabian Sea oxygen deficient zones were used to determine the N₂ excess due to denitrification. In both of the regions, the N₂ excess was comparable in shape and magnitude to the concurrent fixed nitrogen deficit. In the eastern tropical North Pacific oxygen deficient zone, the N₂ excess was near zero at the surface and rose to maxima of 13.7 ± 1.8 and 10.8 ± 1.9 μM N, compared to maximum fixed N deficits of 13.5 ± 1.5 and 12.3 ± 1.5 μM N, respectively. In the Arabian Sea oxygen deficient zone, the maximum N₂ excess was 11.1 ± 1.5 μM N, compared to a maximum deficit of 12.5 ± 1.0 μM N. These results suggests that previous estimates of fixed N loss based on fixed N deficit calculations in these regions are likely reasonable, given the same considerations of volume and residence time of the water of the oxygen deficient zone.