Figure 4: Downcore solution ICP-MS (a) Mg/Ca, (b) Mn/Ca, (C)
U/Ca, and (d) Al/Ca records for D. altispira in the Sunbird-1
core, distinguishing between sample that were reductively cleaned (red
circles) and those that were not (blue squares).
The presence of elevated foraminiferal Mn/Ca, Al/Ca, and U/Ca ratios
does not necessarily mean that the Mg/Ca ratios are contaminated.
However, the downcore, point to point correlation (Figure 4) and
covariance (Supplementary Figure S7) between Mg/Ca and contaminant
indicators suggest a strong association. This downcore association
between Mg/Ca and contaminant indicators, despite a rigorous chemical
cleaning protocol, suggests one of two things; (i) the chemical cleaning
protocol is not fully effective at removing contaminant coatings, and/or
(ii) an Mg-rich contaminant phase is pervasive throughout the calcite
test.
Including the reductive cleaning step lowers Mg/Ca, Mn/Ca, and U/Ca
ratios in the post 11.8 Ma portion of the record, but has a negligible
effect on Al/Ca. Neither cleaning protocol is effective at removing the
authigenic coatings on the Sunbird-1 foraminifera in the pre 11.8 Ma
portion of the record (Figure 4). For this reason, we also analyzed
Sunbird-1 planktic foraminifera by laser ablation ICP-MS.
3.2 Downcore Laser Ablation ICP-MS Mg/Ca
Our laser ablation profiles clearly demonstrate that the metal-rich
contaminant is present as an authigenic surface coating on the glassy
foraminifera (e.g., Figure 2a-b). Because the alteration is not
pervasive throughout the calcite test, laser ablation ICP-MS is an ideal
approach to determine primary test Mg/Ca on these coated samples
(section 2.6). D. altispira Mg/Ca determined by laser-ablation
ICP-MS ranges from 3.03 to 5.07 mmol/mol, with an average value of 4.18
± 0.40 mmol/mol, and errors (±2SE) range from 0.10 to 1.04 mmol/mol
(Supplementary Table S6). However, due to elevated Al/Ca and Mn/Ca
ratios, only 14 of the 44 samples are represented by at least 28 laser
profiles. To alleviate this problem, adjacent samples have been combined
into longer time slices to ensure that the absolute mean Mg/Ca
measurements are robust (Supplementary Table S7). Samples comprising the
mean of at least 28 laser profiles are termed “un-pooled samples”.
Samples pooled to achieve a minimum of 28 laser profiles are termed
“pooled samples”. It is acknowledged that combining adjacent samples,
which span up to 420 kyr, could incorporate orbital scale climatic
variability into these pooled samples. However, we do not infer climatic
variability on orbital timescales because the coarse sampling resolution
could incorporate aliasing of any precessional or obliquital periodicity
into longer term eccentricity cycles (Pisias and Mix , 1988).
Combining adjacent samples to generate a representative mean Mg/Ca for a
longer time-slice could smooth orbital scale variability, could reduce
uncertainty and assist the interpretation of longer-term climatic
trends.
The mean Mg/Ca of representative samples after incorporating the nine
pooled Mg/Ca samples with the 14 un-pooled samples ranges from 3.08 to
4.70 mmol/mol, with an average value of 4.04 ± 0.29 mmol/mol, and errors
(±2SE) range from 0.14 to 0.48 mmol/mol (Supplementary Table S8). These
values are in good agreement with the reductively cleaned solution
ICP-MS data for the post-11.8 Ma portion of the record (Supplementary
Figure S2), coinciding with the interval when contaminant indicators
(Mn/Ca, Al/Ca, and U/Ca) are substantially lower (Figure 4b-d). This
agreement between Mg/Ca values obtained by LA-ICP-MS and solution ICP-MS
following effective reductive-cleaning supports the suitability of the
LA-ICP-MS analyses. Because we can be more confident that the laser
ablation data are not biased by authigenic coatings, the laser-ablation
approach has the advantage that we can also determine original test
Mg/Ca in the older part of the record.
There is no obvious long-term trend in Mg/Ca through the interval
(Figure 5a). Between 11.8 Ma and 11.7 Ma there is a 0.7-0.8 mmol/mol
decrease in Mg/Ca followed by a recovery to approximately previous
values at 11.5-11.4 Ma. There is a Mg/Ca decrease of similar magnitude
from between 10.7 Ma and 10.36 Ma, recovering by 9.85 Ma. We acknowledge
that the coarse sampling frequency, and the combining of samples could
be obscuring similar variability through the rest of the record.
3.3 G. obliquus δ18O
G. obliquus δ18O ranges from -3.63‰ to -2.34‰
with a mean value of -2.92‰. The δ18O record shows
very little variability, values remaining stable at -3.4‰ prior to a
positive 0.6‰ shift at ~12.5 Ma, and -2.7‰ after (Figure
5b). The low variability translates to a stable δ18O
SST record, temperatures ranging between 27°C and 31°C with the only
distinctive trend being a ~3°C decrease between
~12.7 Ma and 12.0 Ma. The coeval positive 0.3‰ shift in
seawater δ18O (Cramer et al. , 2011) dampens the
influence on the SST estimate of the positive 0.6‰ shift in G.
obliquus δ18O at ~12.5 Ma.