Analytical methods, age model and calibrations
For δ18Ocoral and Sr/Ca analyses, the
slabs were microsampled at 0.4-mm intervals following a method developed
at Australian National University (Gagan et al., 1994). The analytical
procedure and the δ18Ocoral record for
the years 1970–2002 have been published previously (Liu et al., 2013).
We report here for the first time continuous
δ18Ocoral and Sr/Ca records from the
entire core. The δ18Ocoral values of
samples covering the period from 1890 to 2002 was analyzed using an
online system comprising an IsoPrime Isotope-ratio mass spectrometer (GV
Instruments Ltd.) coupled to a Multicarb automatic sample treatment
system at the Geological Survey of Japan (GSJ), whereas the
δ18Ocoral values of samples predating
1890 were determined by the Center for Advanced Marine Core Research at
Kochi University (KCC). All δ18Ocoraldata were normalized to Vienna Pee Dee Belemnite (V-PDB) using the
NBS-19 standard (δ18O = +2.2‰) from the U.S. National
Institute of Standards and Technology. The International Atomic Energy
Agency standard IAEA-603 (δ18O = –2.37 ± 0.04‰) was
also used for some measurements conducted at KCC. The standard
deviations of replicate δ18O measurements of the
standards during the mass spectrometer runs were 0.04‰ and
<0.10 ‰ at GSJ and KCC, respectively.
Concentrations of Ca and Sr were measured by inductively coupled plasma
(ICP) optical emission spectrometry using an IRIS Advantage system
(Thermo Electron Co., Ltd) at GSJ and an Agilent 720 system (Agilent
Technologies) at Okayama University. We used an analytical method that
involves the insertion of a reference solution after every third sample
and makes rapid determination of high-precision Sr/Ca ratios feasible
(Schrag, 1999). The reference solution was made from JCp-1, a coral
material with a known chemical composition provided by GSJ (Okai et al.,
2002). Each sample was first weighed (100 ± 10 µg) and then dissolved in
2% HNO3. Sr/Ca analyses of samples covering the period
after 1883 were conducted at GSJ, and samples predating 1890 were
analyzed at Okayama University, respectively, with a 6-year overlap to
confirm measurement consistency. The analytical precision of the
measurements made at both facilities in terms of the inferred SST was
0.55 °C (1σ).
An age model for the period before 1982 was constructed using the Sr/Ca
ratios, which reflect SST, with Sr/Ca maxima (indicating relatively cool
SSTs) corresponding to growth in January and February, the coolest
months in this region. For the period 1982–2002, the precise timing of
the coolest SSTs was determined using IGOSS-nmc SSTs. Other Sr/Ca ratios
in this time interval were then converted to the time domain by simple
linear interpolation. This age model was confirmed by counting annual
density bands on the X-radiographs of the coral core. Age–depth models
of the remaining core were developed using the annual density bands and
then fine-tuned using the seasonal Sr/Ca cycles. At around 1883, when
the Krakatau eruption occurred, geochemical data were disturbed for 1–2
years and the density bands were blurred for about 3 years; thus, the
1982–2002 age model may include an age error of 1–3 years before
around 1885. The temporal resolution of the Sr/Ca analyses for the most
recent 46 years of the record was approximately monthly, whereas for
earlier periods, the resolution was mostly bimonthly. The temporal
resolution of the δ18O analyses was bimonthly
throughout most of the core.
The SST calibration obtained by comparing Sr/Ca minima and maxima to SST
maxima and minima, respectively, is (Figure S2a):
Sr/Ca = –0.084SST + 11.137.
Seawater δ18O values
(δ18Osw) were calculated from the
paired proxy records as (Cahyarini et al., 2008):
δ18Osw =
(δ18Ocoral –
δ18Ocoral, m) –
γ1/β1 (Sr/Ca – Sr/Cam),
where δ18Ocoral is the measured coral
δ18O value;
δ18Ocoral, m is the mean
δ18Ocoral value; Sr/Ca is the measured
coral Sr/Ca ratio and Sr/Cam is its mean; and
γ1 and β1 are the slopes of the linear
regressions of δ18O (–0.143; Figure S2b) and Sr/Ca
(–0.084; Figure S2a), respectively, against IGOSS SSTs. Then the minima
(maxima)
of
δ18Osw and SSS were used to calibrate
SSS as (Figure S2c):
δ18Osw = 0.790SSS – 27.103.