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.