Dataset: Natural abundance N2O isotopomers measured from seawater samples collected in the Eastern Tropical North Pacific during R/V Sally Ride (SR1805) cruise from March to April 2018

Seawater samples for dissolved nitrate, nitrite, and nitrous oxide isotope analysis were collected from either a 30-liter, 12-bottle rosette or a 12-liter, 24-bottle rosette. Samples for nitrate and nitrite isotopic analysis were collected, unfiltered, into 500 mL Nalgene polypropylene bottles following three rinses of the bottle, caps, and threads of at least 10% of the bottle volume. After collection from the rosette, samples for nitrate isotopic analysis were syringe-filtered with a 60 mL syringe through a 0.22 µM pore size Sterivex filter into 60-mL high density polyethylene bottles, then frozen at –20ºC

For nitrite isotopic analysis, samples were preserved within two hours of collection for δ15N-NO2- and δ18O-NO2- using the azide method (McIlvin and Altabet, 2005), along with nitrite isotope reference materials (Casciotti et al., 2007) in different amounts. Briefly, seawater samples were added to 20 mL vials in volumes targeted to achieve 10 nmol nitrite, based on shipboard colorimetric nitrite analysis (Grasshoff et al., 1999), then capped with a gray butyl septum (National Scientific) and sealed with an aluminum crimp seal. Where [NO2–] > 0.25 µM (limit of detection for these analyses, Figure S1 in Kelly et al, 2020) but < 2 µM, the maximum volume allowable for analysis (10 mL) seawater was subsampled, regardless of actual nitrite concentration. Reference materials (Table S1, Kelly et al., 2020) were diluted into nitrite-free seawater and prepared in 5 nmol and 10 nmol amounts to bracket low-nitrite samples. Vials were purged with N2 gas for 15 minutes to remove background N2O, then treated with a sparged sodium azide/acetic acid solution to chemically convert dissolved nitrite into N2O. The reaction was halted after 30 minutes with the addition of 6 M sodium hydroxide solution (McIlvin and Altabet, 2005).

For nitrous oxide isotopic analysis, samples were collected into 160 mL glass serum bottles (Wheaton), following standard gas-sampling procedures: gas-tight tubing (Tygon) was used to overflow each serum bottle with sample three times, after which a ~1 mL air headspace was introduced, and the bottle was capped with a gray butyl septum (National Scientific). Given the trace amount of N2O in the atmosphere (NOAA Global Monitoring Laboratory) and complete flushing of the bottle during analysis, the effect of this headspace and N2O partitioning between the gas and liquid phase falls within the analytical uncertainty for N2O concentration measurements. Samples for N2O isotopic analysis were promptly preserved by adding 100 µL mercuric chloride (HgCl2) to each 160 mL bottle, then sealed with an aluminum crimp seal and stored at lab temperature (20-22ºC).