Living planktic foraminifera were collected for tissue N analysis from the upper 200 m of the water column using a 1-m2, 200-um-mesh plankton net aboard the R/V Atlantic Explorer during ten cruises between July 2011 and November 2013. Approximately 90% of the tow material was preserved in a 5–10% pH-buffered formalin solution and stored at 4°C until picking (Ren et al., 2012). After crushing and the appropriate cleaning steps (Ren et al., 2012; Ren et al., 2015), organic N (tissue) samples were 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.