Foraminifera-bound and bulk sediment nitrogen isotope records of ODP Site 1241

We measured foraminifera-bound δ15N (FB-δ15N) on species Neogloboquadrina dutertrei and Globorotalia menardii and bulk sediment nitrogen isotopes (δ15Nbulk) in sediment core ODP Site 1241 (6°N, 86°W, 2027 m) from the eastern Tropical North Pacific. The datafile contains FB-δ15N data measured on the two foraminifera species back to ~265 ka, δ15Nbulk measured back to ~63 ka, as well as an updated age model for ODP Site 1241. The primary age model for Site 1241 comes from alignment of benthic δ18O measurements in the core (Lalicata and Lea, 2011) to the LR04 stack (Lisiecki and Raymo, 2005) using PaleoDataView visual alignment software (Langner and Mulitza, 2019). To improve the timing of events during the last glacial-interglacial transition, radiocarbon measurements were conducted to add 8 tie points from ~1-25 ka (Extended Data Table 1). Measurements for the three youngest ages were duplicated using the planktonic foraminifera N. dutertrei and G. menardii at the National Ocean Sciences AMS facility of the Woods Hole Oceanographic Institution, where our age model incorporates the interspecies average ages for a given depth interval. Measurements for the five oldest ages were made using the planktonic foraminifera N. dutertrei at the AMS facilities of ETH Zürich. Conversion from radiocarbon age to calendar age was done using Calib 8.20 Marine20 (Stuiver and Reimer, 1993), with R = 0 yrs. FB-δ15N was measured with the “persulfate-denitrifier” method (Ren et al., 2009; Straub et al., 2013). In brief, 4-7 mg of the planktonic foraminifera G. menardii and N. dutertrei were picked from the >250 µm size fraction. Two to four replicates of a coral and or foraminiferal calcite standard were processed alongside each sample set. Foraminifera samples were gently crushed before undergoing chemical cleaning. The organic N bound within calcite was then released by dissolution with HCl and total sample organic N was converted to nitrate in a basic potassium persulfate solution. The nitrate concentration of the solution was determined by chemiluminescence, and an aliquot of the nitrate solution equivalent to 5 nmol N was converted to nitrous oxide (N2O) by denitrifying bacteria. The N isotopic composition of the N2O was measured with a custom continuous-flow system for N2O extraction and purification on-line to a Thermo MAT253 stable isotope mass spectrometer and referenced to air N2 using the international nitrate standards IAEA-N3 and USGS-34. The FB-δ15N data was then corrected for the contribution of the oxidation procedural blank with the average δ15N and [NO3-] of two sets of 3-4 mL persulfate solution used for the oxidation of samples and organic standards. δ15Nbulk was also measured with the “persulfate-denitrifier” method, an approach that has been validated previously (Kast et al., 2019). Approximately 0.5-1.0 mg of dried and ground sediment material was weighed and added into 4 mL glass vials. The following procedures for δ15Nbulk are identical to those used for FB-δ15N, including the addition of HCl to free organic N bound within CaCO3. Previous work has demonstrated comparable results between this approach for δ15Nbulk analysis and standard on-line combustion to N2, except that, in opal-rich samples, there can be an additional N blank in the latter approach due to adsorbed atmospheric N2 (Robinson et al., 2004).