Figure 2: Representative LA-ICP-MS Mg, Al, and Mn profiles demonstrating the selection of background (grey panel) and sample (blue panel) signals for a profile with an authigenic coating (A, B) and for a profile without an authigenic coating (C, D). Both examples are shown in raw isotopic counts (A, C), and ratios mode (B, D) where the isotopes of interest are relative to 43Ca, the internal standard. In both examples the x axis is analysis time (seconds), and the y axis is the raw intensity of the isotopes or ratios on a log scale. The sample interval is selected to avoid the elevated Mg/Ca, Mn/Ca, and Al/Ca at the outer surface of the test.
Individual depth profiles were corrected by first subtracting the mean background signal (determined from ~15 seconds of data acquired when the laser was turned off prior to ablation). The repeated analysis of the NIST 610 standard reference material was used to linearly correct for any instrumental drift. Typically, this is small, <2%, because of the good counting statistics and stable data acquisition during ablation. The ablation profiles were normalized to43Ca as the internal standard and elemental concentrations (TM/Ca) were calculated, assuming 40 wt % for CaCO3.
Following data processing, rigorous screening of the Mg/Ca ratios for the influence of intratest contamination was conducted. It is important to recognise that Mn/Mg and Al/Mg of contaminant phases vary greatly, such that there is no single universal threshold for these elements that can be applied in every situation (Lear et al ., 2015). For the Sunbird-1 samples we examined co-variation of Mg/Ca and Mn/Ca and chose to exclude all samples above a Mn/Ca threshold of 200 µmol/mol (Supplementary Figure S3). Consideration of Al/Ca was more complex, as some samples with extremely high Al/Ca (>1000 µmol/mol) was not associated with markedly elevated Mg/Ca. This result demonstrates that aluminum is sporadically present in foraminiferal tests in variable phases (with differing Al/Mg). We therefore used a dual-pronged approach, considering both Al/Ca and intra-sample heterogeneity. We excluded profiles where two conditions were met: (i) Al/Ca was >100 µmol/mol, and (ii) the associated Mg/Ca was substantially elevated relative to the other depth profiles from the same sample.
2.6 Determination of mean foraminiferal test Mg/Ca by laser ablation
Geochemical heterogeneity exists both within an individual foraminiferal test and between foraminiferal tests from the same sample (Eggins et al. , 2004; Fehrenbacher and Martin , 2014; Sadekov et al. , 2008; Sadekov et al. , 2005). Therefore, several laser ablation profiles are required to produce a consistent Mg/Ca ratio for temperature reconstructions. Here we analyzed ten depth profiles through each of ten individual D. altispira tests from the 1551-1554 m (11.74 Ma) sample to determine representative inter-specimen variability for these samples (Figure 3). Approximately one third (n=28) of the 100 depth profiles were excluded during screening for elevated Al/Ca and Mn/Ca indicative of diagenetic contamination. The Mg/Ca value of individual depth profiles in D. altispira from the 1551-1554 m sample ranges from 2.67 mmol/mol to 5.23 mmol/mol, with a mean of 3.63 ± 0.14 mmol/mol (n=72) (Figure 3a; Supplementary Table S5). The mean Mg/Ca value from four specimens, a total of 28 profiles, is 3.41 ± 0.18 mmol/mol (Figure 3a). Averaging profiles from ten individual tests did therefore not produce significantly better accuracy or precision than averaging profiles from four individual tests (Figure 3b). Therefore, for a Mg/Ca ratio to be considered representative it must represent an average of at least 28 laser ablation profiles, from at least four specimens, with the analytical uncertainty (2 SE) indicating the intra- and inter-specimen variability this incorporates. To account for depth profiles excluded due to contamination, where possible the number of measurements per sample was increased to 36, six depth profiles per specimen and six specimens per sample. This result is in line with other LA-ICP-MS studies (Rathmann et al. , 2004; Sadekov et al. , 2008). Future studies are advised to conduct similar testing to determine the number of measurements required for a mean sample Mg/Ca to be representative, as this will likely be site dependent.