Contrasting biogeochemistry of nitrogen in the Atlantic and Pacific oxygen minimum zones

We present new data for the stable isotope ratio ofinorganic 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-NO−3) reflecting fractionation associated withsubsurface 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 N2 fixation. We suggest the negative 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 ofwhich are compatible with a contribution of canonical denitrification to overall N-loss. However the apparent N isotope fractionation factor observed is relatively low (ɛd=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

Tracing the Fate of Enhanced Organic Carbon Production during a Southern Ocean Fe Fertilization Experiment using Natural Variations in Carbon and Nitrogen Isotopic Composition

This project focused on the N and C natural stable isotope response during SOFeX--a purposeful iron (Fe) addition experiment in the Fe limited Southern Ocean. One purpose of the study was to determine if relief of phytoplankton Fe stress would increase productivity sufficiently to enhance C export from surface to deep waters. We proposed that N and C stable isotopes would be useful for tracing this export. Iron was added to waters north and south of the Antarctic Polar Front in waters to the southwest of New Zealand. While both sites have high-nutrient, low chlorophyll conditions (HNLC) typical of Fe limitation, [SiO4] a required nutrient for diatoms was low at the northerly site and high at the southern location. The most extensive coverage occurred at the southern site. Here, FeSO4 was added four different times over an {approx}two week period. We found that: (1) Particulate organic nitrogen and carbon in the mixed layer increased by a factor of 2-3 in response to the Fe addition in the southern patch. (2) PN accumulation and NO3- drawdown were both 1-2 {micro}M during the occupation of the bloom, suggesting retention of particulates within the mixed layer of the southern patch. (3) {sub 15}N of PN and of NO{sub 3}{sup -} increased by 1-2{per_thousand} as [NO{sub 3}{sup -}] decreased, and there is a clear contrast between in- and out-patch stations with respect to particulate {sub 15}N. The isotopic fractionation factor for NO{sub 3}{sup -} was near 5-6{per_thousand} and appears to have been unaffected by Fe fertilization. In contrast, there was little change in {delta}{sup 13}C. (4) The &gt; 54 {micro}m size fraction was typically lighter than the 1-54 {micro}m size fraction by about 0.5 {per_thousand} in {delta}{sup 13}C. In the south patch, this difference increased as the bloom progressed, and with increasing PN concentration. This result may have been caused by large chain-forming diatoms responded to the Fe addition and were likely isotopically lighter than smaller flagellates. Similar observations were made for {delta}{sup 13}C

Contribution of Southern Ocean surface-water stratification to low atmospheric CO2 concentrations during the last glacial period

The nitrogen-isotope record preserved in Southern Ocean sediments, along with several geochemical tracers for the settling fluxes of biogenic matter, reveals patterns of past nutrient supply to phytoplankton and surface-water stratification in this oceanic region. Areal averaging of these spatial patterns indicates that reduction of the CO2 'leak' from ocean to atmosphere by increased surface-water stratification south of the Polar Front made a greater contribution to the lowering of atmospheric CO2 concentration during the Last Glacial Maximum than did the increased export of organic carbon from surface to deep waters occurring further north