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

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