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).