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We present new data for the stable isotope ratio of inorganic nitrogen speciesfrom the contrasting oxygen minimum zones (OMZs) of the Eastern Tropical NorthAtlantic, south of Cape Verde, and the Eastern Tropical South Pacific off Peru.Differences in minimum oxygen concentration and corresponding N-cycle processesfor the two OMZs are reflected in strongly contrasting δ15N distributions. Pacificsurface waters are marked by strongly positive values for δ15N-NO3–) reflectingfractionation associated with subsurface N-loss and partial NO3– utilization.This contrasts with negative values in NO3– depleted surface waters of the Atlanticwhich are lower than can be explained by N supply via N2 fixation. We suggest thenegative values reflect inputs of nitrate, possibly transient, associated withdeposition of Saharan dust. Strong signals of N-loss processes in the subsurfacePacific OMZ are evident in the isotope and N2O data, both of which are compatiblewith a contribution of canonical denitrification to overall N-loss. However theapparent N isotope fractionation factor observed is relatively low (ϵd=11.4 ‰)suggesting an effect of influence from denitrification in sediments. Identicalpositive correlation of N2O vs. AOU for waters with oxygen concentrations ([O2] < 5 μmol l−1) inboth regions reflect a nitrification source. Sharp decrease in N2O concentrationsis observed in the Pacific OMZ due to denitrification under oxygen concentrations O2 < 5 μmol l−1

Contrasting biogeochemistry of nitrogen in the Atlantic and Pacific Oxygen Minimum Zones