Water samples were collected at discrete depths using Niskin bottle type rosette samplers and an SBE9plus conductivity-temperature-depth (CTD) sensor package (SeaBird Electronics, Bellevue, WA). This dataset's samples were collected from Niskin bottles into 160-milliliter (mL) glass serum vials, killed with saturated mercuric chloride, and crimp sealed with gray butyl stoppers. Prior to sealing, ~1 mL of sample was removed to allow room for sample expansion and to prevent the sample vessel from shattering upon warming.

In all cases, samples were extracted and analyzed using a custom automated purge and trap inlet system and normalized to an injection of calibrated pure N2O reference gas introduced prior to the elution of each sample peak (McIlvin & Casciotti, 2010). Isotope ratios were referenced initially to the calibrated N2O reference tank to create a set of 'ratio of ratios' (³¹Rsample/³¹Rreference, ⁴⁵Rsample/⁴⁵Rreference, ⁴⁶Rsample/⁴⁶Rreference). Next, the data were size corrected in reference to a calibrated 20 volt-second (Vs) peak area for a mass-to-charge ratio of 44 (m/z 44). The size-corrected data were then corrected using a log-scaled normalization based on three isotopically distinct reference materials with known isotopocule compositions that were analyzed alongside the seawater samples (Kelly et al. 2023). Finally, 'scrambling coefficients' were applied to the isotopomer data to correct the measured 15Ra and 15Rb for the rearrangement of nitrogen atoms in N2O when the gas is ionized in the mass spectrometer ion source (Frame et al., 2014; Frame & Casciotti, 2010; Kelly et al., 2021). The isotope ratios of N and O atoms in N2O, ¹⁵Rsample or ¹⁸Rsample, respectively, are expressed in delta notation (δ), where the δ15N and δ18O are defined relative to the isotope ratios of certified standards: δ15N or δ18O = (Rsample/Rstandard - 1) x1000. The Rstandard values used for δ15N and δ18O are the ratios of 15N/14N and 18O/16O in atmospheric N2 and Vienna Standard Mean Ocean Water (VSMOW), respectively.