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RATIONALE:The denitrifier method allows for highly sensitive measurement of the 15 N/ 14 N (δ 15 N value) and 18 O/ 16 O (δ 18 O value) of nitrate dissolved in natural waters and for highly sensitive δ 15 N measurement of other N forms (e.g., organic N) that can be converted into nitrate. Here, updates to instrumentation and protocols are described, and improvements in data quality are demonstrated. METHODS: A 'heart cut' of the N 2 O was implemented in the extraction system to (1) minimize introduction of contaminants into the mass spectrometer, reducing isotopic drift and (2)

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We report wintertime nitrogen and oxygen isotope ratios (δ 15 N and δ 18 O) of seawater nitrate in the Southern Ocean south of Africa. Depth profile and underway surface samples collected in July 2012 extend from the subtropics to just beyond the Antarctic winter sea ice edge. We focus here on the Antarctic region (south of 50.3°S), where application of the Rayleigh model to depth profile δ 15 N data yields estimates for the isotope effect (the degree of isotope discrimination) of nitrate assimilation (1.6-3.3‰) that are significantly lower than commonly observed in the summertime Antarctic (5-8‰). The δ 18 O data from the same depth profiles and lateral δ 15 N variations within the mixed layer, however, imply O and N isotope effects that are more similar to those suggested by summertime data. These findings point to active nitrification (i.e., regeneration of organic matter to nitrate) within the Antarctic winter mixed layer. Nitrite removal from samples reveals a low δ 15 N for nitrite in the winter mixed layer (À40‰ to À20‰), consistent with nitrification, but does not remove the observation of an anomalously low δ 15 N for nitrate. The winter data, and the nitrification they reveal, explain the previous observation of an anomalously low δ 15 N for nitrate in the temperature minimum layer (remnant winter mixed layer) of summertime depth profiles. At the same time, the wintertime data require a low δ 15 N for the combined organic N and ammonium in the autumn mixed layer that is available for wintertime nitrification, pointing to intense N recycling as a pervasive condition of the Antarctic in late summer.

We report nitrogen (N) isotopic measurements of nitrate, total dissolved nitrogen, and particulate nitrogen from Antarctic pack ice in early and late spring. Salinity-normalized concentrations of total fixed N are approximately twofold higher than in seawater, indicating that sea ice exchanges fixed N with seawater after its formation. The production of low-δ 15 N immobile organic matter by partial nitrate assimilation and the subsequent loss of high-δ 15 N nitrate during brine convection lowers the δ 15 N of total fixed N relative to the winter-supplied nitrate. The effect of incomplete nitrate consumption in sea ice is thus similar to that in the summertime surface ocean, but the degree of nitrate consumption is greater in ice, leading to a higher δ 15 N for organic N (~3.9‰) than in the open Antarctic Zone (~0.6‰). Relative to previous findings of very high-δ 15 N organic matter in sea ice (up to 41‰), this study indicates that it would be difficult for sea ice to explain the high δ 15 N of ice age Antarctic sediments. The partitioning of N isotopes between particulate and dissolved forms of reduced N suggests that primary production evolved from new to regenerated production from early to late spring. Even though nitrate assimilation raises the δ 15 N of nitrate, the δ 15 N of sea ice nitrate is frequently lower than that of seawater, providing direct evidence that the regeneration of reduced N in the ice includes nitrification, with mass and isotopic balances suggesting that nitrification supplies a substantial fraction (up to~70%) of nitrate assimilated within Antarctic spring sea ice.