Table 1. Sunbird-1 LA-ICP-MS Mg/Ca derived SST using the
approach of Anand et al. (2003) without a pH correction. Minimum
and maximum age refer to the age range of the pooled samples
(Supplementary Table S7). Maximum and Minimum temperatures refer to the
full range of absolute temperatures derived incorporating the analytical
and calibration uncertainty, whereas Analytical Error Only Maximum and
Minimum temperatures refer to the range of temperatures derived from the
analytical uncertainty only.
2.8 δ18O paleo-sea surface temperature calculations
Due to the limited sampling resolution of the trace metal data, SST is
also calculated using foraminiferal δ18O.
Foraminiferal δ18O
(δ18Ocalcite) is converted to
temperature (T) using the palaeotemperature equation of Bemis et
al. (1998) (Equation 4), changes in global ice volume being corrected
using the δ18Osw value from the
nearest 0.1 Myr time interval in the compilation of Cramer et al.(2011).
\begin{equation}
\mathbf{Equation\ 4:\ \ }\left(\mathbf{\delta}^{\mathbf{18}}\mathbf{O}_{\mathbf{\text{calcite}}}\mathbf{\ \ }\mathbf{\delta}^{\mathbf{18}}\mathbf{O}_{\mathbf{\text{sw}}}\mathbf{\ +0.27}\right)\mathbf{=\ -0.21\ \pm 0.003\ T\ +\ 3.10\ \pm 0.07}\nonumber \\
\end{equation}The absence of a robust, independent salinity proxy makes any
quantitative attribution of its influence on foraminiferal
δ18O challenging. Therefore, we incorporate potential
δ18O variability due to salinity into any temperature
estimate uncertainty. Salinity of the upper water column in a 0.75° x
0.75° grid square around the modern-day study site varies between 34.9
and 35.4 PSU (Boyer et al. , 2013). Using the Indian Ocean
δ18Osw-salinity relationship ofLeGrande and Schmidt (2006) (Equation 5) this equates to a
maximum δ18Osw uncertainty of ±0.091‰.
Using Equation 4 this equates to a 0.4 ⁰C uncertainty in the calculated
surface temperature.
\begin{equation}
\mathbf{Equation\ 5:\ \delta}^{\mathbf{18}}\mathbf{O}_{\mathbf{\text{sw}}}\left(\mathbf{\text{SMOW}}\right)\mathbf{=}\left(\mathbf{0.16\pm 0.004\ x\ Salinity}\right)\mathbf{-5.31\pm}\mathbf{0.135}\nonumber \\
\end{equation}We acknowledge the likelihood of variability in sea surface salinity in
this downcore record. We use the paleolatitude calculator of van
Hinsbergen et al. (2015) to calculate a paleolatitude for Sunbird-1 at
10 Ma of approximately 5.5 °S. The latitudinal correction ofZachos et al. (1994) gives a
δ18Osw of 0.1‰. The absence of a
significant offset from SMOW (0‰) suggests that this will have a
negligible influence on the isotopic SST reconstructions.
3 Results
3.1 Solution ICP-MS trace element chemistry
D. altispira Mg/Ca measured by solution ICP-MS ranges from 3.15 ±
0.1 to 40.2 ± 0.2 mmol/mol (Figure 4a), translating to unrealistically
high reconstructed sea surface temperatures. The high Mg/Ca ratios
strongly suggest the addition of magnesium from a secondary,
post-depositional source, prior to 11.75 Ma. The elevated Mg/Ca ratios
are associated with correspondingly high Mn/Ca, Al/Ca, and U/Ca (Figure
4b-d). Six of the sixteen Mn/Ca ratios are in excess of the proposed 200
µmol/mol threshold, from our LA-ICP-MS analysis, above which Mg/Ca
ratios are excluded due to contamination (Supplementary Figure S3).
Furthermore, every foraminiferal U/Ca ratio is considerably higher than
typical U/Ca ratios of primary foraminiferal calcite, which range from
~3-23 nmol/mol (Chen et al. , 2017; Raitzsch
et al. , 2011; Russell et al. , 2004). In addition, foraminiferal
Al/Ca exceeds the commonly applied 100 µmol/mol threshold in all but the
four youngest samples.