Figure 5: (a) Mean D. altispira LA-ICP-MS Mg/Ca ratios (mmol/mol) for unpooled (black squares) and pooled (grey squares) samples from Sunbird-1. Error bars denote the age range for pooled samples, and the ± 2SE of Mg/Ca from all depth profiles in the sample. (b) G. obliquus δ18O from Sunbird-1. Solid line is a five-point moving average. (c) Sea surface temperature records at Sunbird-1 from planktic foraminiferal δ18O, and LA-ICP-MS Mg/Ca using our preferred approach that applies the calibration of Anand et al. (2003) without a pH correction. Symbols are the same as in (a) and (b). Error bars on the δ18O record denote the analytical uncertainty (± 2SD), and error bars on the Mg/Ca record denote the sample uncertainty (± 2SE). As in (a), pooled Mg/Ca samples also have horizontal error bars denoting the sample age range. Dashed blue and black lines denote the full uncertainty on the temperature estimates, including that derived from the calibration uncertainty, for δ18O and LA-ICP-MS Mg/Ca respectively. Supplementary Figure S8 provides LA-ICP-MS Mg/Ca sea surface temperatures using the alternative approach ofEvans et al. , (2016).
4 Discussion
4.1 Reconstructing sea surface temperature from diagenetically altered foraminifera using laser ablation ICP-MS
Robust paleotemperature reconstructions using foraminiferal Mg/Ca ratios are reliant upon the Mg/Ca ratio recording a primary environmental signal, unaltered by diagenetic alteration. Despite employing a thorough cleaning protocol (Barker et al. , 2003; Boyle and Keigwin , 1985), our Mg/Ca ratios from solution-based ICP-MS analysis in the >11.8 Ma portion of the record are clearly influenced by a diagenetic contaminant phase containing elevated magnesium (Figure 4). This finding demonstrates that foraminifera with a glassy appearance under the light microscope are not necessarily free from the influence of all modes of diagenetic alteration. We therefore emphasize the importance of complementary trace metal ratios indicative of contamination (i.e. Al/Ca, Mn/Ca, U/Ca) to assess the reliability of foraminiferal Mg/Ca ratios (Figure 4). The application of LA-ICP-MS to collect high resolution elemental profiles through the foraminiferal tests, excluding regions displaying diagenetic contamination, has facilitated the identification of what we interpret to be primary paleotemperatures from diagenetically altered foraminifera (Hines et al. , 2017; Hollis et al. , 2015).
The Sunbird-1 δ18OPF SST record fromG. obliquus reconstructs very similar absolute temperatures to the planktic foraminiferal Mg/Ca SST record (Figure 5c). Mean SST from the Sunbird-1 δ18OPF record (29⁰C) is 2⁰C higher than mean SST from the Mg/Ca record (27⁰C), although with the exception of the two transient decreases in Mg/Ca reconstructed SST initiating at 11.8 Ma and 10.7 Ma the records are within error. The similarity of the absolute SSTs reconstructed by the two proxies strengthens the case for the LA-ICP-MS Mg/Ca SST record recording a primary temperature signal, and that these absolute sea surface temperatures at Sunbird-1 should be considered primary.
The majority of the uncertainty in the absolute temperature estimates is derived from the uncertainties incorporated from the relevant calibrations, in particular the seawater Mg/Ca and seawater δ18O records (Figure 5c). This is true for both LA-ICP-MS Mg/Ca (Table 1 and Supplementary Table S9) and δ18O (Supplementary Table S10). Therefore, despite being appreciable, the uncertainty resulting from the geochemical heterogeneity both within an individual foraminiferal test and between foraminiferal tests from the same sample (Figure 3) is not the primary contributor to the final absolute temperature uncertainty.
4.2 Mid-late Miocene sea surface temperatures in the equatorial Indian Ocean
The results from Sunbird-1 indicate that SST in the equatorial Indian Ocean remained stable at ~27⁰C-29⁰C through the 13.3 Ma to 9.5 Ma interval (Figure 5c). This finding suggests that tropical climate was relatively stable following the global cooling associated with the expansion of the East Antarctic Ice Sheet across the MMCT. These records from Sunbird-1 supports the robustness of contemporaneous alkenone based studies which exhibit similar absolute tropical SST estimates (Herbert et al. , 2016; Huang et al. , 2007;Rousselle et al. , 2013; Seki et al. , 2012; Zhang et al. , 2014) (Figure 6a). The Uk37 SST calibration fails to reconstruct SST>29⁰C (Müller et al. , 1998) but these results using Mg/Ca paleo-thermometry suggest that outside the western Pacific warm pool this restriction does not apply to this time interval, unlike the preceding Miocene Climatic Optimum during which Mg/Ca temperature estimates are higher than those estimated with the Uk37 proxy (Badger et al. , 2013).