Planktic foraminifer shells were picked from Oceanic Flux Program (OFP) sediment traps at 500 m, 1500 m, and 3200 m water depth (Conte et al., 2001; Conte and Weber, 2014). After crushing and the appropriate cleaning steps (Ren et al., 2012; Ren et al., 2015), shell-bound N was released by dissolution with hydrochloric acid, and organic N converted to nitrate using a basic persulfate oxidising reagent (POR; a potassium persulfate/sodium hydroxide solution) (Nydahl, 1978; Knapp et al., 2005). The "denitrifier method" was used to convert sample nitrate to N₂O gas for isotope analysis (Sigman et al., 2001; Casciotti et al., 2002; Weigand et al., 2016). See methods section of Smart et al. (2018) for more details of sample collection, isotope analysis and data quality.

The isotopic composition of N₂O was measured at Princeton University by GC-IRMS using a Thermo MAT 253 mass spectrometer with a purpose-built on-line N₂O extraction and purification system. See methods section of Smart et al. (2018) for more details of sample collection, isotope analysis and data quality.