| Summary: | Changes in Southern Ocean nutrient consumption are hypothesized to alter the atmospheric concentration of carbon dioxide by varying the efficiency of the global biological pump, and are implicated in glacial cycles. In the modern Southern Ocean, nitrate (NO- \(_{3}\)) consumption increases surface NO-\(_{3}\)-d\(^{15}\)N and NO-\(_{3}\)-d\(^{18}\)O, and in the paleoceanographic record, nutrient consumption can be inferred from the d\(^{15}\)N of organic nitrogen in buried diatom microfossils. However, current interpretations of nitrogen and oxygen isotope data overlook the effects of seasonal changes in physical, chemical, and biological processes. This thesis explores the impacts of seasonality on Southern Ocean nitrogen and oxygen isotopes using both experimental observations and numerical modeling. The first chapter reports NO-\(_{3}\)-d\(^{15}\)N and NO-\(_{3}\)-d\(^{18}\)O measurements from seven profiles in the Southern Ocean Antarctic Zone, collected in late-summer during the 2014 CLIVAR P16S cruise and analyzed using the denitrifier method. We identify a region of low NO-\(_{3}\)-d\(^{15}\)N in the subsurface from 60oS to 64oS, which is coincident with elevated regenerated NO-\(_{3}\) concentration and likely reflects the remineralization of sinking low-d15N organic nitrogen in the core of the Ross Gyre. In the surface mixed layer, treatment with sulfanilamide reveals that nitrite (NO-\(_{2}\)) has an extremely low d\(^{15}\)N (-60h vs. air). This depletion is attributed to a previously unobserved isotopic equilibration between NO- \(_{3}\) and NO-\(_{2}\), possibly mediated through the nitrite oxidoreductase enzyme of nitrite oxidizers that have been excavated from the subsurface as the mixed layer deepens during the fall. The combined NO- \(_{3}\)+NO-\(_{2}\) isotope data indicate a nitrogen isotope effect for NO-\(_{3}\) assimilation of 4.9h to 6.3h, consistent with previous estimates from the region. The second chapter of this study describes a seasonally resolved six-box model of NO-\(_{3}\) consumption and nitrogen isotope fractionation in the upper water column of the Open Antarctic Zone. The model is calibrated using modern estimates and reproduces isotopic observations of the modern upper water column. The calibration reveals that deviations from Rayleigh dynamics in the remnant of the winter mixed layer result from late-summer nitrogen recycling and subsequent wintertime nitrification. The model is used to simulate glacial conditions, demonstrating that ice age elevation in diatom-d\(^{15}\)N results from increased summertime NO-\(_{3}\) consumption and not from an increase in winter mixed layer NO-\(_{3}\)-d\(^{15}\)N. Lastly, the model is used to investigate observed glacial to interglacial variations in the d\(^{15}\)N difference between centric and pennate diatoms. Our findings support the contention that the reversal of the centric to pennate d\(^{15}\)N difference during peak glacial conditions may result from diatom ammonium assimilation following the near-complete consumption of NO-\(_{3}\) in the ice age summer mixed layer.
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