ALBERT

Description: Individual species were identified and ~550 individuals per species were picked from the 〉250 µm fraction under dissecting microscope for each sample. Approximately 7 mg of picked and identified foraminifera shells were crushed between glass microscope slides and rinsed with MilliQ water. Samples were cleaned and prepared for δ¹⁵NFB measurement as in Smart et al. (2020, doi:10.1029/2019gc008440). Nitrate concentrations were measured by chemiluminescence on a Teledyne Instruments (Model 200E) chemiluminescence NO/NOx analyzer (Braman & Hendrix, 1989; doi:10.1021/ac00199a007). δ¹⁵NFB was measured by bacterial conversion of nitrate to nitrous oxide (Sigman et al., 2001; doi:10.1021/ac010088e), with measurement of the δ¹⁵N of the nitrous oxide by automated extraction and gas chromatography-isotope ratio mass spectrometry (Casciotti et al., 2002; doi:10.1021/ac020113w ) on a Thermo Delta V Plus IRMS. The potassium nitrate reference materials IAEA-N3 and USGS 34 (+4.7 ‰ and 1.8 ‰, respectively) were used to standardize results (Gonfiantini et al., 1995). Sample replicates and triplicates were analyzed when possible. Full procedural replicates were analyzed for 161 sample splits, representing 77 unique samples, when enough foraminifera were available for duplicate or triplicate analysis. Precision for full procedural replicates was 1.0 ‰, this is comparable with modern shell-bound measurements of the same species taken from net tows in this region (standard error = 0.9 ‰, n=10 & 1.1 ‰, n=6, of G. bulloides and G. inflata, respectively; Smart et al., 2020). In every batch, full operational blanks and amino acid standards (USGS 65 amino acid standard) were used to correct for the persulfate oxidation blank and to ensure complete conversion of N. The age model is based on the benthic oxygen isotope stratigraphy presented by Starr et al. (2021, doi:10.1038/s41586-020-03094-7).