Figure 1: Location of the Sunbird-1 study site (black square). Other sites for which there are mid to late Miocene sea surface temperature reconstructions from Mg/Ca (red circles), ẟ18O (blue circles), unsaturated alkenones (green circles) and TEX86 (pink diamonds) are shown. Figure produced using Ocean Data Viewer (Schlitzer, R., 2018) using modern-day mean annual sea surface temperature data from the World Ocean Database.
Micropaleontological and calcareous nannoplankton assemblages for Sunbird-1 were analyzed by Haydon Bailey and Liam Gallagher of Network Stratigraphic Consulting. Biostratigraphic datums, correlated with the astronomical timescale of Raffi et al. (2020), are based on the planktic foraminifera zonations of Wade et al. (2011) and calcareous nanofossil zonations of Backman et al. (2012). An age model was constructed by linear interpolation between these biostratigraphic datums (Supplementary Figure S1). Sedimentation rates were ~3 cm/kyr immediately following the middle Miocene Climate Transition (MMCT), and subsequently increased to ~17 cm/kyr between 11.8 and 11.5 Ma, before decreasing to ~8 cm/kyr until 9.5 Ma.
2.2 Foraminiferal stable isotope analysis
Up to 12 individual tests of the planktic foraminiferGlobigerinoides obliquus showing glassy preservation were used.G. obliquus is an extinct, symbiont-bearing species with a tropical to subtropical paleogeographical distribution, and is interpreted as a surface mixed-layer dweller (Aze et al. , 2011;Keller , 1985). The assertion that G. obliquus inhabits and calcifies in the surface mixed layer (Aze et al. , 2011;Keller , 1985) is supported by multispecies analyses from a 10.0 Ma sediment sample from the Indian Ocean offshore Tanzania showingG. obliquus to have the most negative δ18O (-2.5‰) of all species (Paul Pearson, personal communication, 2019). Tests were crushed between two glass plates ensuring all chambers were opened. Any visible infill was removed using a fine paintbrush under a binocular microscope. Fine clays and other detrital material on the outer surface of the test were removed by rinsing three times in 18.2 MΩ DI water, ultrasonicating for 5-10 seconds in analytical grade methanol, and finally rinsing a further time in 18.2 MΩ DI water. Samples were analyzed at Cardiff University on a ThermoFinnigan MAT253 with online sample preparation using an automated Kiel IV carbonate device. Results are reported relative to Vienna Pee Dee Belemnite, and long‐term uncertainty based on repeat analysis of NBS‐19 is ±0.08 ‰ (n=469, 2 standard deviations) and on repeat analysis of BCT63 is ±0.07 ‰ (n=310, 2 standard deviations). Data is available in Supplementary Table S2.
2.3 Solution ICP-MS trace metal analysis
Between 10 and 15 individuals of the planktic foraminiferDentoglobigerina altispira from the 250 – 355 µm size fraction were picked and weighed on a six-decimal-place balance to determine average test weight. Individual tests were then crushed between two glass plates ensuring all chambers were opened. Due to the low foraminiferal abundance it was not possible to analyze the same species for stable isotope and trace metal composition. Any visible infill was removed using a fine paintbrush under a binocular microscope. Fragments were cleaned to remove clays and organic matter following the standard protocol (Barker et al. , 2003; Boyle and Keigwin , 1985). Due to the clay-rich nature of the sediment the clay removal procedure was conducted twice. To test for the possible presence of metal oxides half of the samples were reductively cleaned between the clay removal and oxidative cleaning steps. Samples were dissolved in trace metal pure 0.065 M HNO3 and diluted with trace metal pure 0.5M HNO3 to a final volume of 350 µl. Samples were analyzed at Cardiff University on a Thermo Element XR ICP‐MS using standards with matched calcium concentrations to reduce matrix effects (Lear et al. , 2010; Lear et al. , 2002). Together with Mg/Ca, several other ratios (Al/Ca, Mn/Ca, and U/Ca) were analyzed to screen for potential contaminant phases. Data are available in Supplementary Table S3. Long-term analytical precision for Mg/Ca throughout the study is better than 2%.
2.4 Laser ablation-ICP-MS analysis
Direct sampling of solid phase material via laser ablation (LA-) allows for geochemical analyses through individual foraminiferal tests at the sub-micron scale when coupled to an inductively-coupled-plasma mass spectrometer (ICP-MS) (Detlef et al. , 2019; Eggins et al. , 2004; Evans et al. , 2015a; Fehrenbacher et al. , 2015;Hines et al. , 2017; Petersen et al. , 2018; Reichart et al. , 2003). A key advantage of analyzing the trace element composition of foraminifera using LA-ICP-MS over the more traditional solution-based ICP-MS is the ability to recognize the diagenetically altered portions of the tests, allowing identification of the primary calcite (Creech et al. , 2010; Hasenfratz et al. , 2016;Pena et al. , 2005). The elemental composition of this primary calcite can provide important information about palaeotemperature (Nooijer et al. , 2017; Eggins et al. , 2003; Pena et al. , 2005) and other paleo-environmental conditions such as pH (Mayk et al. , 2020; Thil et al. , 2016) and oxygenation (Koho et al. , 2015; Petersen et al. , 2018).
Up to six specimens of D. altispira per sample were selected from 44 depth intervals through the Sunbird-1 core for LA-ICP-MS analysis. Foraminiferal sample preparation included the removal of fine clays and other detrital material on the outer surface of the test using DI water and methanol, but the more aggressive oxidative and reductive steps (Barker et al. , 2003; Boyle and Keigwin , 1985), were not required for laser ablation analysis (Vetter et al. , 2013). The cleaned tests were mounted onto glass slides using double sided carbon tape and were allowed to dry before being mounted into the sample cell (Evans et al. , 2015b; Fehrenbacher et al. , 2015;Hines et al. , 2017).
Analyses were performed using an ArF excimer (193nm) LA- system with dual-volume laser-ablation cell (RESOlution S-155, Australian Scientific Instruments) coupled to a Thermo Element XR ICP-MS. Optimized ablation parameters and analytical settings determined for analyzing foraminifera in the Cardiff University CELTIC laboratory (Supplementary Table S4; (Detlef et al. , 2019; Nairn , 2018)) were used for this study. Three cleaning pulses to remove any contaminant on the outer ~0.5 µm of the test surface were included prior to analysis. We analyzed 25Mg, 27Al,43Ca, 55Mn and88Sr, each isotope having a constant 50 ms dwell time. Typically, intervals with elevated Mn and Al in concert with elevated Mg are interpreted as being contaminant phases (e.g., Fe-Mn oxides-hydroxides or clays), and are commonly found on the inner and outer test surface (Barker et al. , 2003; de Nooijer et al. , 2014; Hasenfratz et al. , 2016; Koho et al. , 2015;Pena et al. , 2005).
Where possible, three laser spot depth profiles were collected on each of the penultimate (f-1) and previous (f-2) chambers by ablating with 100 consecutive laser pulses in one position on the test. Assuming that each laser pulse only ablates a ~0.1 µm layer of calcite (Eggins et al. , 2003), we estimate the profile to represent a transect through the test wall approximately 15 µm long. However, in some cases older chambers were required to ensure six laser profiles per specimen were analyzed (Nairn , 2018). NIST SRM 610 glass standard was measured between every six laser profiles, and NIST SRM 612 at the beginning and end of analyses from each sample depth. The reference values for elemental concentrations in both silicate glass standards are taken from the GEOREM website (http://georem.mpch-mainz.gwdg.de/sample_query_pref.asp), updated from Jochum et al. (2011a). NIST SRM 612 was used to determine long term external reproducibility using NIST SRM 610. For Mg/Ca, NIST 612 (n=90) had an accuracy of 12.0% and a precision of 3.7% relative to the reported value. A similar ~12% negative offset relative to the reported value of NIST 610-calibrated NIST 612 has been observed over a much longer period of data collection (Evans and Müller , 2018). To supplement this assessment, we also conducted accuracy tests using the GOR-132 and KL-2 MPI-DING glasses (Jochum et al. , 2011b). For this, GOR-132 and KL2 were treated as unknowns, with both NIST 610 and NIST 612 as calibration standards. For Mg/Ca, GOR-132 (n=25) had an accuracy of 1.1% and a precision of 3.2% relative to the reported value, and KL-2 (n=25) had an accuracy of 0.6% and a precision of 2.6% relative to the reported value when calibrated using NIST 610. These values increased to 10.9% and 9.4% for GOR-132, and 8.2% and 5.4% for KL-2 when calibrated using NIST 612. The NIST 610-calibrated data presented here supports the determination ofEvans et al., (2015a) that the Mg values for NIST 612 requires reassessment.
An important issue related to accuracy is that because a well-characterized, homogenous calcite reference material is not currently available for laser ablation use, the glass standards we used have a different matrix to the calcite foraminifera tests (Evans et al. , 2015a; Evans and Müller , 2018; Fehrenbacher et al. , 2015). Therefore, while we have high confidence in the accuracy of the intra-and inter-specimen geochemical variability described in Section 2.6, we must consider the possibility of an analytical bias in the absolute geochemical composition of foraminiferal tests determined by laser ablation ICP-MS. One way to assess the magnitude of such potential bias is to analyse foraminiferal samples by both solution and laser ablation ICP-MS. However, it is important to note that the corrosive cleaning protocol for solution analysis tends to slightly lower primary test calcite Mg/Ca, an issue that is routinely circumvented by employing the same cleaning on calibration samples for paleotemperature reconstructions (Barker et al., 2003). For the purpose of this study, it is therefore important that our LA-ICP-MS technique gives values that are consistent with our samples analysed by solution ICP-MS. We are able to make a direct comparison of our youngest samples in this way, because these do not have significant authigenic coatings biasing the solution analyses. For these samples, our solution and laser ablation results are in excellent agreement, which gives us confidence in the LA-ICP-MS values for the older samples, where we know the solution ICP-MS results are compromised by authigenic coatings (Supplementary Figure S2). Furthermore, we note that if future work indicates a consistent offset between laser ablation Mg/Ca analyses of carbonates and silicate glasses, owing to their differing matrices, our standard values reported above will allow our data to be corrected to obtain an accurate composition of the uncleaned foraminiferal calcite.
2.5 LA-ICP-MS data processing and screening
Each individual laser ablation profile was carefully inspected and processed using the SILLS data reduction software package (Guillong et al. , 2008) following the established protocol outlined in Longerich et al. (1996). Profiles generally followed one of two patterns: (i) a rise from background values to a transient peak, followed by a somewhat lower plateau, or (ii) a rise from background values to a general plateau (Figure 2). There are two likely explanations for the initial transient peak in some isotope profiles: ablation of authigenic coatings enriched in some trace metals, or laser ablation induced isotope fractionation (so-called “pit effects”). We favor the first explanation because we used the same operating parameters on every profile, and would therefore expect any “pit effects” to be consistent among the profiles. Furthermore, profiles containing the transient peaks were more prevalent in the older part of the record, where our solution Mn/Ca analyses demonstrate the presence of authigenic coatings. Therefore, we assume the transient peaks represent contaminated portions of the test and exclude those regions. The integration interval for the profile was selected based upon the following three criteria: (i) stable 43Ca counts, indicating ablation of calcite, (ii) stable Mg/Ca signal, indicating a consistent primary calcite phase, (iii) flat Mn/Ca and Al/Ca signals, avoiding any peaks indicating intervals of contamination (Figure 2).