ALBERT

Description: We examine a single interglacial during the late Pliocene (KM5c, ca. 3.205 +/- 0.01 Ma) when atmospheric CO2 concentrations were higher than pre-industrial, but similar to today and to the lowest emission scenarios for this century. As orbital forcing and continental configurations were also similar to today, we are able to focus on equilibrium climate system response to modern and near-future CO2. We have synthesised largely published datasets, scrutinised their age models, and generated mean absolute SSTs and their anomaly relative to the pre-industrial. Here we present those values for both alkenone and foraminifera-Mg/Ca proxies.

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: 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).