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 δ<sup>15</sup>N distributions. Pacific surface waters are marked by strongly positive values for δ<sup>15</sup>N-NO<sub>3</sub><sup>−</sup> reflecting fractionation associated with subsurface N-loss and partial NO<sub>3</sub><sup>−</sup> utilization. This contrasts with negative values in NO<sub>3</sub><sup>−</sup> depleted surface waters of the Atlantic which are lower than can be explained by N supply via N<sub>2</sub> 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<sub>2</sub>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<sub>d</sub> = 11.4 ‰) suggesting an effect of influence from denitrification in sediments. Identical positive correlation of N<sub>2</sub>O vs. AOU for waters with oxygen concentrations [O<sub>2</sub>] > 50 μmol l<sup>−1</sup> in both regions reflect a nitrification source. Sharp decrease in N<sub>2</sub>O concentrations is observed in the Pacific OMZ due to denitrification under oxygen concentrations O<sub>2</sub> < 5 μmol l<sup>−1</sup>

Silicon and nitrogen cycling in the upwelling area off Peru: A dual isotope approach

“…For samples within the OMZ, the relationship between δ 15 and δ 18 O‐ along the 10°S transect is indistinguishable from a slope of 1 : 1 implying that is reduction is the main process. Applying a Rayleigh‐type model an enrichment factor for N‐loss of −19.2‰ was estimated within the OMZ, which is in agreement with published data from Casciotti () and Ryabenko et al (). However, a clear distinction if anammox or denitrification is the dominating process cannot be made.…”

“…Aliquots of these samples were treated in the laboratory with sufficient sulfanic acid to remove prior to 15 analysis. Acidified samples do not show any storage effects given that repeat measurements of 15 N were constant over years (see also Ryabenko et al, ). The stable isotope composition of dissolved was measured using Cd‐reduction to followed by reaction with azide to produce N 2 O.…”

Close coupling of N‐cycling processes expressed in stable isotope data at the redoxcline of the Baltic Sea

“…The stability of interfaces in the water column exerts considerable influence on nutrient dynamics because these interfaces separate the water body from rapid vertical exchange processes and from the oxygen supply [ Lam and Kuypers , ; Dalsgaard et al ., ]. In the world's largest ODZs, the eastern tropical North Pacific, the eastern tropical South Pacific, and the Arabian Sea, apparent enrichment factors for denitrification are as high as ε = 30‰ [ Brandes et al ., ; Voss et al ., ; Ryabenko et al ., ] (Table ), similar to the intrinsic/organismal isotope effect reported for heterotrophic denitrifying cultures [ Barford et al ., ; Fuchsman et al ., ; Granger et al ., , and references therein]. In the hypoxic zone of the Baltic Sea, the apparent enrichment factor of ε 15N , estimated using the Rayleigh closed‐system approach, was 4.7‰, whereas the observed δ 15 N NO3 was 35.1‰ (similar to values in other anoxic basins, Table ).…”

Differential N2 O dynamics in two oxygen-deficient lake basins revealed by stable isotope and isotopomer distributions

“…a,b). This feature has also been observed in oceanic oxygen minimum zones, where accumulation and N 2 O consumption co‐occur (Ryabenko et al ; Babbin et al ), and may be linked to electron donor‐limited (i.e., organic matter, H 2 S, or Fe 2+ ) denitrification in this zone. A second N 2 O sink is most likely located within the sediments below the depth of benthic N 2 O accumulation, where denitrification consumes , , and N 2 O to completion.…”