Dataset: N2 excess and N species isotopic composition

Sampling at sea was done using a standard SeaBird CTD/Rosette system. Hydrographic data was processed using SeaBird software and standard procedures. O2 was measured by an SBE43 sensor and final data were calibrated against Winkler O2 determinations

NO3- and NO2- concentrations were measured in the laboratory on frozen samples using a SmartChem autoanalyzer using standard chemical methods

N2 excess were determined from N2/Ar ratios measured using the procedures described by Charoenpong et al. (2014) with the exception that the measurements were made at sea using a Pfieffer 400 series quadrupole mass spectrometer system. Data acquisition and initial data processing used custom LabView software. Biogenic N2 concentrations were determined by subtraction of estimated background excess N2 as described in Chang et al. (2012). The d15N of N2 was determined in the laboratory using similar methodology but using an IsoPrime Isotope Ratio Mass Spectrometer (IRMS) as described by Charoenpong et al. (2014) using IonVantage software. 

The isotopic composition of nitrate and nitrite was determined on samples returned to the laboratory using procedures described by McIlvin and Casciotti (2011) and McIlvin and Altabet (2005). Samples for nitrate isotope analysis were preserved by mild acidification and addition of sulfamic acid to remove nitrite. Nitrite samples were preserved at high pH with NaOH to retain its d18O signature. An IsoPrime IRMS running IonVantage was used to make these measurements.

Final data reduction and organization was done using Microsoft Excel.