Samples were collected from a standard Niskin bottle rosette.
Nitrate isotope analyses were conducted by denitrifier method (Sigman et al., 2001; Casciotti et al., 2002) with updates (McIlviin and Casciotti, 2011). Nitrite was removed from samples containing more than 0.1 uM nitrite, using sulfuric acid (Granger and Sigman, 2009). Standards include USGS32, USGS34, and USGS35.
Nitrite isotope analyses were conducted by azide method (McIlvin and Altabet, 2005). Standards include N23, N7373, and N10219.
Nitrous oxide isotope analyses were conducted as described in McIlvin and Casciotti, 2010. N2O isotope measurements were calibrated against a laboratory N2O working gas.
The water mass analysis for the oxygen deficient zone waters was conducted as described in Peters et al., 2018, Deep Sea Research II, Estimating fixed nitrogen loss and associated isotope effects using concentration and isotope measurements of NO3-, NO2-, and N2 from the Eastern Tropical South Pacific oxygen deficient zone.