Key Points:
- Diagenesis can impart large artifacts to coral Sr/Ca-based climate
reconstructions even with minor levels of alteration
- SIMS and other microscale analytical techniques can be used to bypass
diagenesis and recover Sr/Ca values from primary fossil coral material
- Transects of SIMS Sr/Ca analyses from an altered section of fossil
coral are significantly correlated to SST in the early
20th century
Abstract
Coral Sr/Ca ratios provide quantitative estimates of past sea surface
temperatures (SST) that allow for the reconstruction of changes in the
mean state and climate variations, such as the El Nino-Southern
Oscillation, through time. However, coral Sr/Ca ratios are highly
susceptible to diagenesis, which can impart artifacts of 1-2˚C that are
typically on par with the tropical climate signals of interest.
Microscale sampling via Secondary Ion Mass Spectrometry (SIMS) for the
sampling of primary skeletal material in altered fossil corals,
providing much-needed checks on fossil coral Sr/Ca-based
paleotemperature estimates. In this study, we employ a set modern and
fossil corals from Palmyra Atoll, in the central tropical Pacific, to
quantify the accuracy and reproducibility of SIMS Sr/Ca analyses
relative to bulk Sr/Ca analyses. In three overlapping modern coral
samples, we reproduce bulk Sr/Ca estimates within ±0.3% (1σ). We
demonstrate high fidelity between 3-month smoothed SIMS coral Sr/Ca
timeseries and SST (R = -0.5 to -0.8; p<0.5). For
lightly-altered sections of a young fossil coral from the
early-20th century, SIMS Sr/Ca timeseries reproduce
bulk Sr/Ca timeseries, in line with our results from modern corals.
Across a moderately-altered section of the same fossil coral, where
diagenesis yields bulk Sr/Ca estimates that are 0.6mmol too high
(roughly equivalent to -6˚C artifacts in SST), SIMS Sr/Ca timeseries
track instrumental SST timeseries. We conclude that 3-4 SIMS analyses
per month of coral growth can provide a much-needed quantitative check
on the accuracy of fossil coral Sr/Ca-derived estimates of
paleotemperature, even in moderately altered samples.
1. Introduction
Surface coral skeletons are one of the few marine archives capable of
providing absolutely-dated, sub-annually resolved records of past
tropical climate and oceanic variability. Cores from living coral
colonies are increasingly used to reconstruct past variability in
surface temperature (SST), salinity, and other oceanic parameters across
recent centuries (e.g. Nurhati et al. , 2009; 2011;Vásquez-Bedoya et al. , 2012; Sanchez et al. , 2016;Murty et al. , 2017; Jimenez et al. , 2018; Rodriguez
et al. , 2019), and cores from fossil corals can extend such
reconstructions through the Holocene and beyond (e.g. Beck et
al. , 1997; Linsley , 2000; Tudhope et al. , 2001;Cobb et al. , 2003; DeLong et al. , 2010; Cobb et
al. , 2013; McGregor et al. , 2013; Felis et al. , 2014;Toth et al. , 2015; Grothe et al. , 2019; Felis ,
2020). While fossil corals provide unique snapshots into past
seasonal-to-decadal climate variability, estimates of mean temperature
change from glacial-aged or older fossil corals are often
~2-3˚C cooler than those from other marine archives
(Gagan et al. , 2012). Analyses of similarly-aged corals reveal
that post-depositional alteration of the coral skeleton, or diagenesis
can easily introduce cool artifacts in coral-based reconstructions (e.g.Cohen and Hart , 2004; Allison , 2005). As the number of
fossil coral-based mean climate reconstructions increases, there is a
pressing need to better constrain their accuracy, especially if they are
to be included in large multi-proxy reconstruction and data assimilation
efforts (e.g. Emile-Geay et al. , 2013a; 2013b; Hakim et
al. , 2016; Tardif et al. , 2019; Sanchez et al. , 2021).
Diagenesis typically occurs as dissolution and/or precipitation of
secondary cements, both of which can produce significant artifacts in
coral-based paleoclimate reconstructions. A majority of existing
coral-based climate reconstructions utilize either oxygen isotope ratios
(δ18O), which tracks both changes in SST and the
hydrologic budget (Epstein et al. , 1953; Weber and
Woodhead , 1972; Hasson et al. , 2013; Conroy et al. , 2014;
2017), and/or strontium-to-calcium ratios (Sr/Ca), which primarily
reflect changes in SST (Weber , 1973; Smith et al. , 1979;Beck et al. , 1992). Secondary aragonite cements, which have more
enriched δ18O and higher Sr/Ca than coral aragonite,
produce cool artifacts in paleo-SST reconstructions (Enmar et
al. , 2000; Müller et al. , 2001; Ribaud-Laurenti et al. ,
2001; Quinn and Taylor , 2006; Hendy et al. , 2007). On the
other hand, secondary calcite cements are typically lower in
δ18O and Sr/Ca relative to coral aragonite, producing
warm artifacts in paleo-SST reconstructions (e.g. McGregor and
Gagan , 2003). The effects of skeletal dissolution on coral-based
paleoclimate records remain poorly characterized. However, evidence
suggests that some skeletal elements, such as the centers of
calcification (COCs), which are higher in Sr/Ca (Cohen et al. ,
2001), dissolve more easily, producing warm artifacts in paleo-SST
reconstructions (e.g Hendy et al. , 2007).
A variety of tools such as x-ray imaging, x-ray diffraction (XRD),
scanning electron microscopy (SEM), and light microscopy, are used to
screen coral samples for diagenetic alteration. With the exception of
x-ray imaging, these techniques are destructive, and as such, screening
is conducted a few millimeters to centimeters away from the section of
the core used for geochemical analysis. This strategy works well in
heavily altered corals, where the alteration is more consistent across
large sections of a core. In light to moderately altered corals,
however, alteration is more heterogeneously distributed (e.g.Hendy et al. , 2007; Sayani et al. , 2011) and can escape
detection, leading to diagenetic material being inadvertently included
in coral powders drilled for conventional bulk or “mm-scale” Sr/Ca or
δ18O analyses. The inclusion of even 1% of secondary
cements in coral powders can introduce warm/cool artifacts of 1-2˚C in
coral-based temperature reconstructions (Allison et al. , 2007;Sayani et al. , 2011). While fossil corals are routinely screened
for diagenesis, modern corals seldom are even though alteration can
occur while colonies are still living (e.g. Nothdurft et al. ,
2005; Hendy et al. , 2007; Nothdurft et al. , 2007;Nothdurft and Webb , 2008). The prevalence of diagenesis in both
modern and fossil corals highlights the need for both rigorous screening
and the development of techniques capable of extracting reliable
constraints on mean climate from altered corals.
As a microscale analytical technique, secondary ion mass spectrometry
(SIMS) has the potential to bypass diagenetic phases by targeting only
better-preserved areas of the coral skeleton for geochemical analyses.
These techniques capitalize on the fact that secondary cements and/or
low levels of dissolution leave much of the original coral skeleton
geochemically intact (e.g. Cohen and Hart , 2004; Allison ,
2005; Sayani et al. , 2011). Early application of SIMS technique
to altered fossil corals from the western tropical Pacific demonstrate
that targeted ion-microprobe analyses of pristine material in altered
fossil corals yield mean temperature estimates that are more consistent
with other reconstructions from the region (Cohen and Hart , 2004;Allison , 2005). Despite these early successes, microscale
analytical techniques like SIMS have remained underutilized in
paleoclimate reconstruction for several reasons. For one, few SIMS
facilities exist, given the cost and complexity associated with the
equipment. Second, SIMS coral Sr/Ca analyses are roughly 100 times more
time-consuming and expensive than standard ICP-OES analyses. Moreover,
the composition of biogenic carbonates is highly variability at
microscales (Allison , 1996; Hart and Cohen , 1996;Sinclair et al. , 1998; Meibom , 2003; Rollion-Bard et
al. , 2003; Gagnon et al. , 2007; Allison and Finch ,
2010b), potentially obscuring the climate signals of interest. In
corals, these large microscale fluctuations in composition likely
reflect biological processes rather than environmental variability (e.g.Meibom , 2003; Meibom et al. , 2008), limiting the utility
of SIMS for reconstructing daily- or weekly-resolved climate records.
Averaging or smoothing multiple SIMS measurements can resolve monthly
SST variability (e.g. Allison and Finch , 2004; Sinclair ,
2005; Allison and Finch , 2009), however, additional work is
required to establish best practices for applying SIMS to fossil corals
and quantify the uncertainties of the resulting reconstructions.
In this study, we measure SIMS Sr/Ca in three modern corals that overlap
in time and one ~100yr-old fossil coral from Palmyra
Atoll (6˚N, 162˚W) in the central tropical Pacific. We use the modern
corals to devise a SIMS sampling strategy to yield monthly-resolved
timeseries, and then test their reproducibility across all three modern
corals and their fidelity to SST. Guided by SIMS Sr/Ca sampling
protocols developed in our modern corals, we analyze SIMS Sr/Ca data
from three altered sections of a relatively young fossil coral that grew
during the early-20th century. We compare the fossil
coral SIMS Sr/Ca timeseries to both bulk Sr/Ca analyzed in the same
coral, and to instrumental SST, to assess the accuracy of SIMS coral
Sr/Ca across a gradient of diagenesis. Lastly, we synthesize our
findings into a discussion of the strengths and weaknesses of employing
SIMS-based coral Sr/Ca analyses for paleo-SST reconstruction in fossil
corals.
2. Methods
We present new SIMS Sr/Ca records from three Porites spp. modern
corals, as well as bulk and SIMS Sr/Ca records from a Poritesspp. fossil coral, collected from Palmyra Atoll (5°53’N, 162°5’W).
Modern corals PM, PM1, and PM5 are used to test and refine our SIMS
methodology as they’ve been previously vetted for diagenesis and used to
develop monthly coral Sr/Ca and oxygen isotope records (Cobb et
al. , 2001; Nurhati et al. , 2009; 2011; Sayani et al. ,
2019). We also present new bulk Sr/Ca timeseries and micro-scale SIMS
Sr/Ca timeseries from Palmyra fossil coral NB9, which extends from 1908
to 1935 (Cobb et al., 2003).
Bulk Sr/Ca analyses and
diagenesis screening
Coral cores were cut into ~1-cm thick slices and
prepared for geochemical analyses using standard procedures. For bulk
Sr/Ca analyses, coral powders were drilled using a 1mm bit at 1mm
intervals along a transect parallel to the primary growth axis. Coral
powders were digested in 2% trace-metal grade HNO3 and
analyzed using a Horiba Jobin-Yvon Ultima 2C Inductively Coupled Plasma
Optical Emission Spectrometer (ICP-OES) located at Georgia Tech using
procedures outlined in Sayani et al, (2019). Analytical
precisions for bulk Sr/Ca measurements are < ±0.1% or
±0.09mmol/mol (1σ).
Potentially altered horizons in core NB9 were identified through visual
screening of coral xrays for density anomalies and the existence of
exceptionally noisy intervals in the bulk Sr/Ca record, and flagged for
subsequent analysis via Scanning Electron Microscopy (SEM). Samples for
SEM imaging were prepared by extracting small (2-4mm) coral chunks near
the bulk Sr/Ca transect and gold coating. SEM images were obtained using
either a Hitachi S-800 field emission gun SEM or a LEO 1530
thermally-assisted field emission SEM located at Georgia Tech.
Coral sample preparation
for SIMS
Ion microprobe measurements were made on a series of continuous,
doubly-polished, thin-section rounds. Thin sections were prepared by
cutting 4-6cm long segments from each coral slab, typically within 1-3
cm from the transect used for bulk Sr/Ca analyses. Each segment was
lightly scored on the back and snapped by hand into ~1cm
pieces to preserve continuity across the sampling surface. Coral
segments were then embedded in epoxy, sectioned at similar depths,
polished and mounted onto 1-inch rounds, and then polished again to a
final thickness of at least ~30 µm. Prior to SIMS
analyses, each thin section was gold coated and then imaged using both a
transmitted and a reflected light microscope. Transmitted light
microscope images were used to identify the location of COCs, secondary
cements, and any imperfections in the thin section. Paired reflected
light images were used to map continuous sampling paths and navigate
across the thin section when loaded onto the SIMS.
SIMS coral Sr/Ca
analyses
SIMS Sr/Ca measurements were made using a Cameca IMS-1280 ion
microprobe, located at the Northeast National Ion Microprobe Facility
(NENIMF) at the Woods Hole Oceanographic Institute, equipped with a
~6nA O- ion beam, accelerated at
10keV. SIMS analyses were made using an ~20µm diameter
beam, targeting pristine sections spots on aragonite fibers located
between the COCs and the edge of the trabeculae. Following a
pre-analysis burn time of 90s to remove gold coating,42Ca and 88Sr were measured using a
-80eV energy filter across 10 cycles lasting 10s and 15s each,
respectively. As measurements were made using a single collector, we use
dead-time interpolation scheme to correct for drift or loss in signal
intensity across the 10 cycles. Analytical uncertainty for SIMS Sr/Ca
analyses, estimated as the standard error of88Sr/42Ca measurements across 9
dead-time corrected cycles, ranges from ±0.01 to ±0.05mmol/mol (1σ).
Mass 42Ca and 88Sr intensities were
converted to concentration using calibration curves built by measuring
three well-characterized standards (Gaetani and Cohen , 2006;Holcomb et al. , 2009; Gaetani et al. , 2011) at the
beginning and end of each day. Standards include an OKA Carbonatite
crystal, a calcite crystal (Blue-0875) and an aragonite crystal (AG1)
whose Sr/Ca ratios span 0.56-19.3mmol/mol. The average reproducibility
of Sr/Ca measurements on each standard, across four sessions between
2012 and 2016, are ±1.4% for OKA, ±2.4% for AG1, and ±2.3% for
Blue-0875 per session.
For SIMS Sr/Ca measurements, we primarily targeted skeletal material
between the centers of calcification and the edge of each trabecula. At
µm-scales, coral skeletal elements (e.g. the thecal wall, septa,
dissepiments) are composed of dense bundles of crystalline aragonite
fibers which radiate out from granular, sub-micron “centers of
calcification” (COCs) (see review in Rabier et al. , 2008). While
both the COCs and aragonite fibers likely track environmental
variability (Allison and Finch , 2004), they have distinct
geochemical compositions with COCs generally being more enriched in
Sr/Ca and other trace elements (e.g. Cohen et al. , 2001;Allison and Finch , 2004; Meibom et al. , 2008). As the COCs
are too small to be discretely measured with ion beam sizes
>10µm, we focus our analyses on the aragonite fibers
>95% of the coral skeleton.
To generate smoothed SIMS Sr/Ca time series, we remove any data points
that are ±2 standard deviations outside the mean of all analyses from
each sample, and then apply an 11-point running mean filter. The 2σ
filter primarily serves to remove measurements that may have
inadvertently included material from COCs (visually confirmed by
examining thin sections under a light microscope following SIMS
analyses), which are systematically higher in Sr/Ca and can bias the
resulting time series. The 11-month running mean filter averages SIMS
Sr/Ca measurements across a ~2.5month time period that
consistently provides the best match with bulk Sr/Ca time series from
each coral. Uncertainty in smoothed SIMS Sr/Ca is estimated by
calculating the standard error of all points used to calculate the
running mean at each time step. Initial age models were derived by
mapping the SIMS measurements to core depth and then interpolating ages
using the age-depth relationship previously developed for Sr/Ca records
from each core. To account for changes in extension rate changes between
the SIMS and ICP-OES transections, and sample loss between contiguous
thin sections, we slightly finetuned age models for SIMS data from PM
and PM1 by shifting some points by up to 1mm (equivalent to 2-3 weeks in
time).
3. Results
3.1. Sampling strategy for
deriving SIMS Sr/Ca records
Centers of calcification (COCs) contain significantly more Sr/Ca than
aragonite fibers, which make up >90% of the coral
skeleton, and are difficult to detect on gold-coated thin sections. To
develop a filter for removing any SIMS datapoints that might include a
COC, we measured spots with COCs and adjacent aragonite fibers at 7
locations in core PM. SIMS measurements on aragonite fibers yield a mean
Sr/Ca value of 8.95±0.02 mmol/mol (1σ, n=14), consistent with the
average bulk Sr/Ca values across this horizon (8.95±0.04mmol/mol, 1σ,
n=3). In contrast, SIMS analyses that include COCs yield an average
Sr/Ca value of 9.15±0.03mmol/mol (1σ; n=7), two standard deviations
greater than the mean Sr/Ca of the adjacent aragonite fibers. As such,
we exclude any SIMS Sr/Ca measurements that are ±2 standard deviations
outside the mean Sr/Ca value of each coral from subsequent analyses.
To determine the minimum number of SIMS analyses needed to resolve a
seasonal cycle, we measured SIMS Sr/Ca at ~200µm
intervals across an 18mm-long transect in core PM, targeting skeletal
structures where the location of COCs were more apparent in thin
sections. We observe large point-to-point variability of
~0.6mmol/mol across this initial SIMS time series,
consistent with results from previous studies measuring coral Sr/Ca at
microscales (Sinclair et al. , 1998; Cohen et al. , 2001;Meibom et al. , 2008; Allison and Finch , 2009). However,
the average of these SIMS Sr/Ca measurements (8.99±0.02mmol/mol, n=80)
is indistinguishable from that of bulk Sr/Ca measurements across the
same interval (8.97±0.02mmol/mol, n=20). Applying an 11-point running
mean filter, equivalent to ~2.5 months, yields a
timeseries with seasonal variability similar to that of bulk Sr/Ca. By
subsampling this initial dataset at coarser resolutions, we find that a
minimum sampling resolution of ~400µm is needed to
resolve seasonal variability in our SIMS data. Given core PM’s extension
rate of 16-18mm/year, this translates to roughly one SIMS Sr/Ca analysis
for every week of coral growth.
3.2. Reproducibility of
SIMS Sr/Ca analyses across overlapping modern corals
To assess the reproducibility of SIMS analyses between corals, we
measured Sr/Ca between 1985-1989 in three modern corals from Palmyra
atoll. Similar to observations from core PM, SIMS Sr/Ca measurements
from the 1986-1989 sections of cores PM5 (Figure 3B) and PM1 (Figure 3C)
are also characterized by high-amplitude fluctuations of
~0.6mmol/mol to ~0.7mmol/mol,
respectively. No significant correlations are observed between raw SIMS
Sr/Ca timeseries from cores PM, PM1, and PM5. However, SIMS analyses do
yield an average Sr/Ca value that is within error of the average bulk
Sr/Ca from each core. SIMS Sr/Ca measurements from the 1986-1989 section
of core PM1 yield an average value of 9.02±0.01mmol/mol (n=114),
consistent with the 9.04±0.01mmol/mol (n=45) average of bulk Sr/Ca from
across the same time period. Similarly, SIMS analyses from the 1986-1989
horizon of PM5 yield an average Sr/Ca value (8.89±0.01mmol/mol; n=84)
that is consistent with bulk Sr/Ca from this core (8.86±0.01mmol/mol;
n=46). The average SIMS Sr/Ca values of these cores are offset by
±0.07mmol/mol (1σ), similar the observed ±0.08mmol/mol (1σ) offsets in
bulk Sr/Ca records among these cores (Sayani et al. , 2019).
Smoothed SIMS Sr/Ca timeseries resolve seasonal variability observed in
both bulk Sr/Ca from each core and instrumental SST at our site.
Smoothed SIMS Sr/Ca timeseries from cores the 1985-1989 sections of PM
and PM1 (Figure 3) are well correlated with monthly SST (R= -0.5 and
-0.7, P<0.05, respectively) and their bulk Sr/Ca timeseries (R
= 0.4 and 0.6, P<0.05, respectively). In contrast, smoothed
SIMS Sr/Ca from core PM5 timeseries diverges considerably from both bulk
Sr/Ca (R = -0.4, P<0.05) and SST (R = 0.5, P<0.05)
between 1986-1987 (Figure 3B). Inspection of thin sections from PM5
after the SIMS session reveals that several spots sampled across this
horizon included COCs, which bias the smoothed timeseries towards higher
Sr/Ca values. However, smoothed PM5 SIMS data from the year before and
after this interval (1986-1987 and 1988-1989) reproduce variability
observed in bulk Sr/Ca and SST bulk Sr/Ca. We also tested out this
methodology to a second section of PM1 that covers 1995-1997 (Figure
3D), just before the 1997/98 El Niño event. Across this horizon,
smoothed SIMS Sr/Ca reproduces both bulk Sr/Ca (R = 0.5,
P<0.05) and SST (R = -0.8, P<0.05).
3.3. Bulk vs. targeted SIMS
Sr/Ca analyses in an altered fossil coral
Bulk Sr/Ca from fossil coral NB9 generally tracks instrumental SST
(R=-0.6, p<0.05) and overlapping bulk Sr/Ca from modern core
PM (R=0.5, p<0.05) between 1917-1935. We do not observe any
large departures of coral Sr/Ca from SST variations across this horizon
of NB9, however, there are several intervals (e.g. around 1918, 1922,
1928, and 1931) where individual bulk Sr/Ca datapoints from both NB9 and
PM deviate from each other and observed SST by up to
~1˚C. SEM images from these sections of NB9 reveal
smooth skeletal surfaces, such as those seen in modern coral PM (Figure
5A), with a few patches of small (<5µm-long) secondary
aragonite cements and minor dissolution (Figure 5B, C). In contrast, the
bulk NB9 Sr/Ca record from 1911-1917 is characterized by large
excursions of up to -0.6mmol/mol away from the baseline defined by SST
variations during this time. Across this ~6yr period,
bulk NB9 Sr/Ca suggests temperatures were 1-6˚C cooler than instrumental
observations. SEM images from this horizon of the core reveal more
continuous coverage of larger (~10µm-long) secondary
aragonite cements (Figure 5D), which are ~30% more
enriched in Sr/Ca relative to coral aragonite (Sayani et al. ,
2011) and produce cool artifacts in Sr/Ca-based temperature
reconstructions.
We measured SIMS Sr/Ca across three horizons of NB9 with increasing
levels of alteration. We first applied the methodology developed using
the modern corals above to two lightly-altered sections of NB9: (i) from
1931-1933, where the skeleton is well preserved and bulk Sr/Ca matches
SST, and (ii) from 1920-1922, where the skeleton contains trace
alteration and bulk Sr/Ca contains cool excursions of up to
~1˚C. SIMS Sr/Ca analyses across the 1931-1933 section
of NB9 produce an average value of 9.10±0.02mmol/mol (n=78), equivalent
to the average of bulk NB9 Sr/Ca (9.08±0.01mmol/mol; n=33). Both bulk
and SIMS Sr/Ca from NB9 yield similar temperature estimates of
27.4±0.05˚C and 27.2±0.2˚C, respectively, consistent with the
instrumental SST average of 27.4˚C across this interval. Similarly, SIMS
analyses from the 1920-1922 section of NB9 yield an average value of
9.09±0.02mmol/mol (n=73), consistent with bulk NB9 Sr/Ca
(9.07±0.01mmol/mol; n=31). Once again, bulk and SIMS Sr/Ca yield
temperature estimates (27.3±0.2˚C and 27.5±0.1˚C, respectively) that are
consistent with average instrumental SST across this period (27.7˚C).
Smoothed SIMS coral Sr/Ca time series across 1931-1933 and 1920-1922
from NB9 generally track overlapping bulk Sr/Ca and instrumental SST,
albeit with a few discrepancies. We measured Sr/Ca across these two
sections of NB9 using a sampling resolution of ~400µm,
which worked well for our faster growing modern corals (16-18mm/yr), but
did not provide enough analyses per month from the slower (10-14mm/yr)
growing NB9 to fully capture seasonal variability. For subsequent SIMS
analyses from NB9, we measure 3-4 spots per month of coral growth (i.e.
between consecutive dissepiments; DeCarlo & Cohen, 2017) instead of
using a fixed sampling interval to account for the slower extension
rate. Nonetheless, SIMS and bulk Sr/Ca from NB9, as well as bulk Sr/Ca
from PM, suggest temperatures in early-1922 that are much cooler than
instrumental observations. However, agreement between average SIMS and
bulk Sr/Ca measurements across both of these sections of NB9 suggests
that the low levels of diagenetic alteration observed here do not have a
noticeable impact on bulk Sr/Ca measurements.
Finally, we apply SIMS to the moderately-altered 1911-1914 section of
NB9, where SEM images show more continuous diagenesis (Figure 6) and
bulk Sr/Ca contains excursions of up to -6˚C. SIMS Sr/Ca analyses across
this horizon yield an average value of 9.06±0.02mmol/mol (n=119) or
27.3˚C, consistent with the average instrumental SST of 27.6˚C observed
at Palmyra across this interval. In contrast, bulk Sr/Ca measurements
yield an average value of 9.27±0.02mmol/mol (n=46), ~3˚C
cooler than both SIMS Sr/Ca-derived and instrumental SST, likely
reflecting contamination by high-Sr/Ca secondary precipitates. Moreover,
by sampling only preserved sections of the coral skeleton between
1911-1914, we derive a smoothed SIMS Sr/Ca timeseries that reproduces
seasonal SST variability during this interval (R=-0.9, p<0.05;
Figure 4).
4. Discussion
4.1. Reproducibility of
SIMS analyses in modern corals
Early application of SIMS to altered fossil corals highlighted that
microscale analytical techniques could circumvent analysis of diagenetic
phases (e.g. Cohen and Hart , 2004; Allison , 2005;Sayani et al. , 2011), but none of these studies thoroughly
benchmarked modern coral SIMS Sr/Ca measurements against SST. Here, SIMS
coral Sr/Ca timeseries from three time-overlapping modern corals
demonstrate that SIMS provides mean Sr/Ca values and monthly-resolved
time-series that are reproducible across coral colonies and coherent
with instrumental SST. Consistent with previous studies featuring
microscale trace-element and isotope measurements in corals, our SIMS
Sr/Ca measurements are characterized by large, high-amplitude
fluctuations of 0.6 to 0.7mmol/mol in SIMS Sr/Ca that are not correlated
between time-overlapping samples. These fluctuations exceed diurnal
variability at our site (1.5˚C or 0.1 mmol/mol) and are an order of
magnitude too large to be explained by analytical error (±0.01 to ±0.03;
1σ). As such, it is likely that they reflect either non-linear skeletal
accumulation (Barnes et al. , 1995; Cohen and Sohn , 2004;Sinclair , 2005) or non-environmental factors that impact the
biomineralization process (e.g. Juillet-Leclerc et al. , 2009;Allison and Finch , 2010a). Notably, we also find that a 2.5-month
smoothing of the SIMS coral Sr/Ca data isolates a seasonal cycle that
mirrors seasonal SST variability at the site, consistent with findings
from other studies (e.g. Sinclair et al. , 1998; Allison and
Finch , 2004; 2009). Thus, while individual SIMS Sr/Ca analyses may not
yield meaningful daily- or weekly-resolved SST records, smoothing SIMS
measurements across several months of coral growth can provide robust
sub-seasonal SST reconstructions.
4.2. Application of SIMS in
altered fossil corals
We apply the SIMS methodology developed using modern corals to a fossil
coral with varying degrees of alteration to assess the impact of low
alteration levels on the fidelity of bulk Sr/Ca analyses. Targeted SIMS
analyses of preserved coral aragonite across two lightly-altered
sections of fossil coral NB9 demonstrate that low-levels of alteration
have a negligible impact on the fidelity of bulk Sr/Ca records. Across
two lightly-altered horizons of fossil coral NB9 (1931-1933 and
1920-1922), targeted SIMS Sr/Ca analyses reproduce the mean and seasonal
variability observed in bulk Sr/Ca from both NB9 and Palmyra modern
coral PM (Figure 4). This consistency among timeseries generated by two
different Sr/Ca analytical techniques in two different corals implies
that the low levels of secondary cement coverage observed in one of the
corals have no impact on bulk Sr/Ca across this horizon. As such, the
relatively small ~1˚C discrepancies between bulk
Sr/Ca-derived SST and instrumental SST in 1922 may reflect vital
effects. Where both bulk Sr/Ca records show similar deviations from SST,
the discrepancies may reflect the fact that gridded SST products contain
large uncertainties in the early-20th century due to
limited SST observations across much of the tropical Pacific (e.g.Deser et al. , 2010; Tokinaga et al. , 2012).
Targeted SIMS analyses across the moderately-altered horizon of NB9
allow us to completely bypass secondary cements present in this section
of the coral, yielding Sr/Ca-derived temperatures that agree with
instrumental observations. These results are consistent with previous
studies that have used SIMS to derive more realistic mean temperature
estimates from ancient altered fossil corals (e.g. Cohen and
Hart , 2004; Allison , 2005; Sayani et al. , 2011). However,
building on this previous work, we use SIMS to derive the first,
realistic monthly-resolved SST timeseries from an altered section of a
fossil coral. Smoothed SIMS Sr/Ca data from this more altered horizon of
NB9 agrees with both published modern coral Sr/Ca data from Palmyra
(Nurhati et al. , 2011) and instrumental SST. Thus, while
diagenesis can produce significant artifacts in temperature
reconstructions based on bulk Sr/Ca measurements, our results highlight
that SIMS is a powerful tool that can be used to verify the accuracy of
bulk Sr/Ca measurements and derive more accurate paleo-SST
reconstructions.