Nitrate isotope dynamics in the Southern Ocean

The isotope dynamics of nitrate ( NO-3 ) promise to provide novel insights into the modern and past ocean nitrogen cycle. In high latitude surface waters such as those of the Southern Ocean, phytoplankton preferentially incorporate 14N- and 16O-bearing nitrate into phytoplankton biomass, leaving the nitrate pool enriched in 15N and 18O. The amplitude of this discrimination, the isotope effect of nitrate assimilation, or epsilon, is a key parameter in nitrate isotope dynamics. Through measurements and models, I investigate the isotope effect for nitrate assimilation in three major zones in the Southern Ocean: the most equatorward Subantarctic Zone (SAZ), the Polar Antarctic Zone (PAZ) close to the Antarctic continent, and the Open Antarctic Zone (OAZ) between them.;Isotopic analyses of samples collected from the SAZ during both winter and summer are assimilated into a time-dependent model of the productive SAZ summer, indicating a relatively high epsilon value of 8--9‰, a summertime export production rate of ≥ 3.5 mmol N m-2 d-1, and significant summertime nitrate supply during the summer from the Antarctic surface. In the PAZ, uncertainty in epsilon due to nitrification is constrained with dual N and O isotope measurements of nitrate. The observations are consistent with an epsilon of 5‰ and a very low rate of mixed layer nitrification (< 6% of NO-3 assimilated). The isotope data generated from across the Southern Ocean indicate lateral nitrate exchange between the PAZ and OAZ that answer previous questions and call for the revision of epsilon estimates from previously published OAZ profiles. Compilation of our estimates of epsilon and other published estimates show highest values in the SAZ, decreasing to the South. A correlation of epsilon is found with mixed layer depth. Based on others' culture work, this relationship with mixed layer depth is hypothesized to result from physiological adaptation of phytoplankton nitrate uptake to light limitation.;In a less directly related study, data and a model are used to estimate of N2 fixation in the Atlantic basin, with important constraints on the model calculation deriving from my Southern Ocean nitrate isotope data.