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 prior to δ¹⁵NFB measurement as described in Marcks et al., (2023). Once samples were clean, organic nitrogen was released into solution by acid dissolution of the foraminiferal calcite. Samples were acidified prior to measurement. Nitrate concentrations were measured by chemiluminescence on a Teledyne Instruments (Model 200E) chemiluminescence NO/NOx analyzer. δ15NFB samples, 10 nmol in size, were measured by bacterial conversion of nitrate to nitrous oxide, with measurement of the δ¹⁵N of the nitrous oxide by automated extraction and gas chromatography-isotope ratio mass spectrometry 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). Note, testing of a subset of 6 samples, each with full procedural triplicates, for a total of 18 samples, showed negligible differences in nitrogen content and δ¹⁵NFB values with and without a reductive cleaning step, and so it was omitted here to avoid unnecessary loss of sample material. Sample replicates and triplicates were analyzed when possible. Full procedural replicates were analyzed for 134 sample splits, representing 66 unique samples, when enough foraminifera were available for duplicate or triplicate analysis. The average standard deviation of procedural replicates is 0.4 ‰. Full operational blanks and amino acid standards (USGS 65 glycine) were measured in each batch. The average standard deviation of glycine standards measured in triplicate is 0.3 ‰. We estimated the δ¹⁵N value of the persulfate blank using a dilution series (5, 7.5, 10, and 20 μM of the glycine standard and the fraction of the blank in standards. We applied a blank correction to each sample based on the calculated mean δ¹⁵N value of all of the persulfate blanks for the dataset and the fraction of the blank in the N content of each sample. Data were subset to exclude N content outliers (〉2 s.d. from mean and where the blank was greater than 20 % of the sample N content, with significantly different δ¹⁵N values from other replicates). Full propagated analytical error associated with measurement and blank correction, following Higgins et al., (2009, doi:10.1021/ac8017185 ), was on average 0.6 ‰. Propagating the errors, including not only the procedural replicates and their variance, but the relative size of the blanks, the mean of the calculated blank δ¹⁵N values (5± 10 ‰). The mean value, 0.6 ‰, is used where procedural replicates were limited by sample availability. The age model is based on the benthic oxygen isotope stratigraphy presented by Starr et al. (2021, doi:10.1038/s41586-020-03094-7). This age model for Site 361-U1475 was generated with 12 radiocarbon dates and 33 benthic oxygen isotope tie points which were graphically aligned with a probabilistic stack of 180 globally distributed benthic oxygen isotope records (Starr et al., 2021, doi:10.1038/s41586-020-03094-7).

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).

Description: Neogloboquadrina pachyderma abundance was obtained by washing ~ 10 cm³ of sediment through a 150µm sieve and drying at ~ 50 ºC for 24 h. This dried fraction was split until a total of 300-400 individuals remained. From this amount, we identified the relative abundance of Neogloboquadrina pachyderma tests according to Kennett and Srinivasan (1983) and Loeblich and Tappan (1988, doi:10.1007/978-1-4899-5760-3).

Description: Biogenic silica accumulation was obtained by analyzing approximately 200 mg of homogenized and freeze dried sediment for each sample. Cleaning, chemical treatment, and measurement followed protocols outlined in (Mortlock & Froelich, 1989, doi:10.1016/0198-0149(89)90092-7). Samples were measured with a UV Vis spectrophotometer at 812 nm wavelength. Full procedural replicates were performed on 163 of the 435 samples yielding an average standard deviation of 0.2 %. Samples were referenced to RICCA VerSpec SiO32- in 1 % NaOH for intercomparison. Opal mass accumulation rates were calculated by multiplying the fraction of opal by dry bulk density and sedimentation rates from Starr et al. (2021, doi:10.1038/s41586-020-03094-7).

Description: The meridional variability of the Subtropical Front (STF) in the Southern Hemisphere, linked to expansions or contractions of the Southern Ocean, may have played an important role in global ocean circulation by moderating the magnitude of water exchange at the Indian-Atlantic Ocean Gateway, so called Agulhas Leakage. Here we present new biomarker records of upper water column temperature ( and ) and primary productivity (chlorins and alkenones) from marine sediments at IODP Site U1475 on the Agulhas Plateau, near the STF and within the Agulhas retroflection pathway. We use these multiproxy time-series records from 1.4 to 0.3 Ma to examine implied changes in the upper oceanographic conditions at the mid-Pleistocene transition (MPT, ca. 1.2?0.8 Ma). Our reconstructions, combined with prior evidence of migrations of the STF over the last 350 ka, suggest that in the Southwestern Indian Ocean the STF may have been further south from the Agulhas Plateau during the mid-Pleistocene Interim State (MPIS, MIS 23?12) and reached its northernmost position during MIS 34?24 and MIS 10. Comparison to a Globorotalia menardii-derived Agulhas Leakage reconstruction from the Cape Basin suggests that only the most extreme northward migrations of the STF are associated with reduced Agulhas Leakage. During the MPIS, STF migrations do not appear to control Agulhas Leakage variability, we suggest previously modeled shifting westerly winds may be responsible for the patterns observed. A detachment between STF migrations and Agulhas Leakage, in addition to invoking shifting westerly winds may also help explain changes in CO2 ventilation seen during the MPIS.