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

• Published in: Biogeosciences
• Main Authors: E. Ryabenko, A. Kock, H. W. Bange, M. A. Altabet, D. W. R. Wallace
• Format: Article in Journal/Newspaper
• Language: English
• Published: Copernicus Publications 2012
• Subjects: Ecology; QH540-549.5; Life; QH501-531; Geology; QE1-996.5; Pacific; North Atlantic
• Online Access: https://doi.org/10.5194/bg-9-203-2012; https://doaj.org/article/c20e940af6e04d4eb7578c4b52dde5ad

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 δ 15 N distributions. Pacific surface waters are marked by strongly positive values for δ 15 N-NO 3 – ) reflecting fractionation associated with subsurface N-loss and partial NO 3 – utilization. This contrasts with negative values in NO 3 – depleted surface waters of the Atlantic which are lower than can be explained by N supply via N 2 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 N 2 O 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 N 2 O vs. AOU for waters with oxygen concentrations ([O 2 ] < 5 μmol l −1 ) in both regions reflect a nitrification source. Sharp decrease in N 2 O concentrations is observed in the Pacific OMZ due to denitrification under oxygen concentrations O 2 < 5 μmol l −1 .