Dataset: Water column nitrate plus nitrite d15N measurements from seawater collected in November 2019 and November 2020 in the Western Tropical South Pacific

Sampling and Analytical Procedures:
Duplicate water column samples were collected by Niskin or GoFlo rosette on one of the following types of casts:

'CTD',
'TMC' (Trace Metal Clean), or
'TWO' (small classical CTD with 12 x Niskin bottles) cast.

Nitrogen isotopic analysis (d15N of NO3-+NO2-) was 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, 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 N2O, which was then purged from the vial using a helium carrier gas. The d15N of N2O in samples was calibrated with international isotopic reference materials.

Calibration:
NO3-+NO2- d15N analyses were calibrated with IAEA N3 and USGS 34 NO3- d15N isotopic reference materials as described in McIlvin and Casciotti, 2011.

Precision:
The average precision of NO3-+NO2- d15N measurements was <0.2 per mil.