Figure 1: Location of the Sunbird-1 study site (black square).
Other sites for which there are mid to late Miocene sea surface
temperature reconstructions from Mg/Ca (red circles),
ẟ18O (blue circles), unsaturated alkenones (green
circles) and TEX86 (pink diamonds) are shown. Figure
produced using Ocean Data Viewer (Schlitzer, R., 2018) using
modern-day mean annual sea surface temperature data from the World Ocean
Database.
Micropaleontological and calcareous nannoplankton assemblages for
Sunbird-1 were analyzed by Haydon Bailey and Liam Gallagher of Network
Stratigraphic Consulting. Biostratigraphic datums, correlated with the
astronomical timescale of Raffi et al. (2020), are based on the
planktic foraminifera zonations of Wade et al. (2011) and
calcareous nanofossil zonations of Backman et al. (2012). An age
model was constructed by linear interpolation between these
biostratigraphic datums (Supplementary Figure S1). Sedimentation rates
were ~3 cm/kyr immediately following the middle Miocene
Climate Transition (MMCT), and subsequently increased to
~17 cm/kyr between 11.8 and 11.5 Ma, before decreasing
to ~8 cm/kyr until 9.5 Ma.
2.2 Foraminiferal stable isotope analysis
Up to 12 individual tests of the planktic foraminiferGlobigerinoides obliquus showing glassy preservation were used.G. obliquus is an extinct, symbiont-bearing species with a
tropical to subtropical paleogeographical distribution, and is
interpreted as a surface mixed-layer dweller (Aze et al. , 2011;Keller , 1985). The assertion that G. obliquus inhabits and
calcifies in the surface mixed layer (Aze et al. , 2011;Keller , 1985) is supported by multispecies analyses from a 10.0
Ma sediment sample from the Indian Ocean offshore Tanzania showingG. obliquus to have the most negative δ18O
(-2.5‰) of all species (Paul Pearson, personal communication, 2019).
Tests were crushed between two glass plates ensuring all chambers were
opened. Any visible infill was removed using a fine paintbrush under a
binocular microscope. Fine clays and other detrital material on the
outer surface of the test were removed by rinsing three times in 18.2 MΩ
DI water, ultrasonicating for 5-10 seconds in analytical grade methanol,
and finally rinsing a further time in 18.2 MΩ DI water. Samples were
analyzed at Cardiff University on a ThermoFinnigan MAT253 with online
sample preparation using an automated Kiel IV carbonate device. Results
are reported relative to Vienna Pee Dee Belemnite, and long‐term
uncertainty based on repeat analysis of NBS‐19 is ±0.08 ‰ (n=469, 2
standard deviations) and on repeat analysis of BCT63 is ±0.07 ‰ (n=310,
2 standard deviations). Data is available in Supplementary Table S2.
2.3 Solution ICP-MS trace metal analysis
Between 10 and 15 individuals of the planktic foraminiferDentoglobigerina altispira from the 250 – 355 µm size fraction
were picked and weighed on a six-decimal-place balance to determine
average test weight. Individual tests were then crushed between two
glass plates ensuring all chambers were opened. Due to the low
foraminiferal abundance it was not possible to analyze the same species
for stable isotope and trace metal composition. Any visible infill was
removed using a fine paintbrush under a binocular microscope. Fragments
were cleaned to remove clays and organic matter following the standard
protocol (Barker et al. , 2003; Boyle and Keigwin , 1985).
Due to the clay-rich nature of the sediment the clay removal procedure
was conducted twice. To test for the possible presence of metal oxides
half of the samples were reductively cleaned between the clay removal
and oxidative cleaning steps. Samples were dissolved in trace metal pure
0.065 M HNO3 and diluted with trace metal pure 0.5M
HNO3 to a final volume of 350 µl. Samples were analyzed
at Cardiff University on a Thermo Element XR ICP‐MS using standards with
matched calcium concentrations to reduce matrix effects (Lear et
al. , 2010; Lear et al. , 2002). Together with Mg/Ca, several
other ratios (Al/Ca, Mn/Ca, and U/Ca) were analyzed to screen for
potential contaminant phases. Data are available in Supplementary Table
S3. Long-term analytical precision for Mg/Ca throughout the study is
better than 2%.
2.4 Laser ablation-ICP-MS analysis
Direct sampling of solid phase material via laser ablation (LA-) allows
for geochemical analyses through individual foraminiferal tests at the
sub-micron scale when coupled to an inductively-coupled-plasma mass
spectrometer (ICP-MS) (Detlef et al. , 2019; Eggins et al. ,
2004; Evans et al. , 2015a; Fehrenbacher et al. , 2015;Hines et al. , 2017; Petersen et al. , 2018; Reichart
et al. , 2003). A key advantage of analyzing the trace element
composition of foraminifera using LA-ICP-MS over the more traditional
solution-based ICP-MS is the ability to recognize the diagenetically
altered portions of the tests, allowing identification of the primary
calcite (Creech et al. , 2010; Hasenfratz et al. , 2016;Pena et al. , 2005). The elemental composition of this primary
calcite can provide important information about palaeotemperature
(Nooijer et al. , 2017; Eggins et al. , 2003; Pena et
al. , 2005) and other paleo-environmental conditions such as pH
(Mayk et al. , 2020; Thil et al. , 2016) and oxygenation
(Koho et al. , 2015; Petersen et al. , 2018).
Up to six specimens of D. altispira per sample were selected from
44 depth intervals through the Sunbird-1 core for LA-ICP-MS analysis.
Foraminiferal sample preparation included the removal of fine clays and
other detrital material on the outer surface of the test using DI water
and methanol, but the more aggressive oxidative and reductive steps
(Barker et al. , 2003; Boyle and Keigwin , 1985), were not
required for laser ablation analysis (Vetter et al. , 2013). The
cleaned tests were mounted onto glass slides using double sided carbon
tape and were allowed to dry before being mounted into the sample cell
(Evans et al. , 2015b; Fehrenbacher et al. , 2015;Hines et al. , 2017).
Analyses were performed using an ArF excimer (193nm) LA- system with
dual-volume laser-ablation cell (RESOlution S-155, Australian Scientific
Instruments) coupled to a Thermo Element XR ICP-MS. Optimized ablation
parameters and analytical settings determined for analyzing foraminifera
in the Cardiff University CELTIC laboratory (Supplementary Table S4;
(Detlef et al. , 2019; Nairn , 2018)) were used for this
study. Three cleaning pulses to remove any contaminant on the outer
~0.5 µm of the test surface were included prior to
analysis. We analyzed 25Mg, 27Al,43Ca, 55Mn and88Sr, each isotope having a constant 50 ms dwell time.
Typically, intervals with elevated Mn and Al in concert with elevated Mg
are interpreted as being contaminant phases (e.g., Fe-Mn
oxides-hydroxides or clays), and are commonly found on the inner and
outer test surface (Barker et al. , 2003; de Nooijer et
al. , 2014; Hasenfratz et al. , 2016; Koho et al. , 2015;Pena et al. , 2005).
Where possible, three laser spot depth profiles were collected on each
of the penultimate (f-1) and previous (f-2) chambers by ablating with
100 consecutive laser pulses in one position on the test. Assuming that
each laser pulse only ablates a ~0.1 µm layer of calcite
(Eggins et al. , 2003), we estimate the profile to represent a
transect through the test wall approximately 15 µm long. However, in
some cases older chambers were required to ensure six laser profiles per
specimen were analyzed (Nairn , 2018). NIST SRM 610 glass standard
was measured between every six laser profiles, and NIST SRM 612 at the
beginning and end of analyses from each sample depth. The reference
values for elemental concentrations in both silicate glass standards are
taken from the GEOREM website
(http://georem.mpch-mainz.gwdg.de/sample_query_pref.asp),
updated from Jochum et al. (2011a). NIST SRM 612 was used to
determine long term external reproducibility using NIST SRM 610. For
Mg/Ca, NIST 612 (n=90) had an accuracy of 12.0% and a precision of
3.7% relative to the reported value. A similar ~12%
negative offset relative to the reported value of NIST 610-calibrated
NIST 612 has been observed over a much longer period of data collection
(Evans and Müller , 2018). To supplement this assessment, we also
conducted accuracy tests using the GOR-132 and KL-2 MPI-DING glasses
(Jochum et al. , 2011b). For this, GOR-132 and KL2 were treated as
unknowns, with both NIST 610 and NIST 612 as calibration standards. For
Mg/Ca, GOR-132 (n=25) had an accuracy of 1.1% and a precision of 3.2%
relative to the reported value, and KL-2 (n=25) had an accuracy of 0.6%
and a precision of 2.6% relative to the reported value when calibrated
using NIST 610. These values increased to 10.9% and 9.4% for GOR-132,
and 8.2% and 5.4% for KL-2 when calibrated using NIST 612. The NIST
610-calibrated data presented here supports the determination ofEvans et al., (2015a) that the Mg values for NIST 612 requires
reassessment.
An important issue related to accuracy is that because a
well-characterized, homogenous calcite reference material is not
currently available for laser ablation use, the glass standards we used
have a different matrix to the calcite foraminifera tests (Evans
et al. , 2015a; Evans and Müller , 2018; Fehrenbacher et
al. , 2015). Therefore, while we have high confidence in the accuracy of
the intra-and inter-specimen geochemical variability described in
Section 2.6, we must consider the possibility of an analytical bias in
the absolute geochemical composition of foraminiferal tests determined
by laser ablation ICP-MS. One way to assess the magnitude of such
potential bias is to analyse foraminiferal samples by both solution and
laser ablation ICP-MS. However, it is important to note that the
corrosive cleaning protocol for solution analysis tends to slightly
lower primary test calcite Mg/Ca, an issue that is routinely
circumvented by employing the same cleaning on calibration samples for
paleotemperature reconstructions (Barker et al., 2003). For the
purpose of this study, it is therefore important that our LA-ICP-MS
technique gives values that are consistent with our samples analysed by
solution ICP-MS. We are able to make a direct comparison of our youngest
samples in this way, because these do not have significant authigenic
coatings biasing the solution analyses. For these samples, our solution
and laser ablation results are in excellent agreement, which gives us
confidence in the LA-ICP-MS values for the older samples, where we know
the solution ICP-MS results are compromised by authigenic coatings
(Supplementary Figure S2). Furthermore, we note that if future work
indicates a consistent offset between laser ablation Mg/Ca analyses of
carbonates and silicate glasses, owing to their differing matrices, our
standard values reported above will allow our data to be corrected to
obtain an accurate composition of the uncleaned foraminiferal calcite.
2.5 LA-ICP-MS data processing and screening
Each individual laser ablation profile was carefully inspected and
processed using the SILLS data reduction software package
(Guillong et al. , 2008) following the established protocol
outlined in Longerich et al. (1996). Profiles generally followed
one of two patterns: (i) a rise from background values to a transient
peak, followed by a somewhat lower plateau, or (ii) a rise from
background values to a general plateau (Figure 2). There are two likely
explanations for the initial transient peak in some isotope profiles:
ablation of authigenic coatings enriched in some trace metals, or laser
ablation induced isotope fractionation (so-called “pit effects”). We
favor the first explanation because we used the same operating
parameters on every profile, and would therefore expect any “pit
effects” to be consistent among the profiles. Furthermore, profiles
containing the transient peaks were more prevalent in the older part of
the record, where our solution Mn/Ca analyses demonstrate the presence
of authigenic coatings. Therefore, we assume the transient peaks
represent contaminated portions of the test and exclude those regions.
The integration interval for the profile was selected based upon the
following three criteria: (i) stable 43Ca counts,
indicating ablation of calcite, (ii) stable Mg/Ca signal, indicating a
consistent primary calcite phase, (iii) flat Mn/Ca and Al/Ca signals,
avoiding any peaks indicating intervals of contamination (Figure 2).