Dataset: Nitrate plus nitrite isotopic compositions from Leg 1 of the Antarctic Circumnavigation Expedition (ACE) research cruise onboard the R/V Akademik Treshnikov from December 2016 to January 2017

Samples were collected during Leg 1 of the Antarctic Circumnavigation Expedition (ACE) research cruise onboard the R/V Akademik Treshnikov, which took place in the Indian Sector of the Southern Ocean from December 2016 to January 2017. Water column samples were collected by Niskin bottle on a CTD rosette. Surface samples were collected from the ship's underway intake (at ~5 meters depth). Nitrate and nitrite (NO3- + NO2-) d15N and d18O analyses were by the "denitrifier method" and followed the methods described by Sigman et al. (2001), Casciotti et al. (2002), McIlvin and Casciotti (2011), and Weigand et al. (2016). Briefly, NO3-+NO2- was quantitatively reduced to N2O by Pseudomonas aureofaciens and Pseudomonas chlororaphis, which was then cryogenically focused and analyzed on an isotope ratio mass spectrometer. A volume of sample was added to each bacterial vial to achieve a final quantity of 10 or 20 nanomoles (nmols) N2O, which was then purged from the vial using a helium carrier gas. The d15N and d18O of samples were calibrated with the international isotopic reference materials as described below.

The average precision of nitrate+nitrite d15N measurements was <0.2 per mil and for d18O was <0.3 per mil. NO3-+NO2- d15N and d18O analyses were calibrated with IAEA N3 and USGS 34 NO3- d15N isotopic reference materials as described in McIlvin and Casciotti (2011). NO3-+NO2- d18O was also calibrated with the USGS 35 isotopic reference material as described in McIlvin and Casciotti (2011).