![A: Stable carbon and oxygen isotope variability as a function of relative growth time across the shell. The shaded areas reflect the uncertainty (2 SD). The first year could not be sampled (see the section on preservation). The grey bars indicate position and width of the visible dark bands transferred to the time scale. B: Closeup of the section of 20 to 30 years with vertical lines indicating the middle of each year. δ 18 O Arag [VPDB] values show asymmetric yearly cycles while δ 13 C [VPDB]](https://www.researchgate.net/profile/Iris-Arndt-2/publication/387677509/figure/fig4/AS:11431281303408971@1736849316868/A-Stable-carbon-and-oxygen-isotope-variability-as-a-function-of-relative-growth-time_Q320.jpg)
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Invited Research Article
20,000 days in the life of a giant clam reveal late Miocene tropical
climate variability
Iris Arndt
a,b,*
, Miguel Bernecker
a
, Tobias Erhardt
a,b
, David Evans
b,c
, Jens Fiebig
a
,
Maximilian Fursman
a,b
, Jorit Kniest
a,b,1
, Willem Renema
d,e
, Vanessa Schlidt
a,b
,
Philip Staudigel
a
, Silke Voigt
a,b
, Wolfgang Müller
a,b
a
Institute of Geosciences, Goethe University Frankfurt, Frankfurt am Main, Germany
b
Frankfurt Isotope and Element Research Center (FIERCE), Goethe University Frankfurt, Frankfurt am Main, Germany
c
School of Ocean and Earth Science, University of Southampton, Southampton, United Kingdom
d
Marine Biodiversity Group, Naturalis Biodiversity Center, Leiden, the Netherlands
e
Institute for Biodiversity and Ecosystem Dynamics (IBED), University of Amsterdam, Amsterdam, the Netherlands
ARTICLE INFO
Editor: M Elliot
Keywords:
Elemental ratio analysis via LA-ICPMS
Sub-daily resolution
Internal age model
(Sub-seasonal) palaeoclimate reconstruction
ENSO
Dual clumped isotopes
ABSTRACT
Giant clams (Tridacna) are well-suited archives for studying past climates at (sub-)seasonal timescales, even in
‘deep-time’ due to their high preservation potential. They are fast growing (mm-cm/year), live several decades
and build large aragonitic shells with seasonal to daily growth increments. Here we present a multi-proxy record
of a late Miocene Tridacna that grew on the western margin of the Makassar Strait (Indonesia). By analysing daily
elemental cycle lengths using our recently developed Python script Daydacna, we build an internal age model,
which indicates that our record spans 20,916 ±1220 days (2 SD), i.e. ~57 ±3 years. Our temporally resolved
dataset of elemental ratios (El/Ca at sub-daily resolution) and stable oxygen and carbon isotopes (δ
18
O and δ
13
C
at seasonal to weekly resolution) was complemented by dual clumped isotope measurements, which reveal that
the shell grew in isotopic equilibrium with seawater. The corresponding Δ
47
value yields a temperature of 27.9
±2.4 ◦C (2 SE) from which we calculate a mean oxygen isotopic composition of late Miocene tropical seawater of
−0.43 ±0.50 ‰. In our multi-decadal high temporal resolution records, we found multi-annual, seasonal and
daily cycles as well as multi-day extreme weather events. We hypothesise that the multi-annual cycles (slightly
above three years) might reect global climate phenomena like ENSO, with the more clearly preserved yearly
cycles indicating regional changes of water inow into the reef, which together impact the local isotopic
composition of water, temperature and nutrient availability. In addition, our chronology indicates that twice a
year a rainy and cloudy season, presumably related to the passing of the ITCZ, affected light availability and
primary productivity in the reef, reected in decreased shell growth rates. Finally, we nd irregularly occurring
extreme weather events likely connected to heavy precipitation events that led to increased runoff, high
turbidity, and possibly reduced temperatures in the reef.
1. Introduction
Bivalve shells are important archives of (palaeo)climate and envi-
ronmental changes and can provide continuous decadal records at very
high (seasonal to sub-daily) time resolution (e.g. Ayling et al., 2015;
Batenburg et al., 2011; de Winter et al., 2020b; Elliot et al., 2009; Hori
et al., 2015; Nützel et al., 2010; Peharda et al., 2021; Sch¨
one et al., 2011;
Shao et al., 2020; Warter and Müller, 2017; Watanabe et al., 2004).
Their dense structure makes them less sensitive to diagenetic recrys-
tallisation compared to other high time-resolution environmental ar-
chives such as corals, which is especially relevant for ‘deep-time’, i.e.
pre-Pleistocene, palaeoenvironmental reconstructions (Grifths et al.,
2013; Veeh and Chappell, 1970; Welsh et al., 2011). ‘Deep-time’ tem-
perature seasonality has been studied using the oxygen isotopic (δ
18
O)
compositions of bivalve shells, e.g. for the Cretaceous (Steuber et al.,
2005; Walliser et al., 2018), Triassic (Nützel et al., 2010) or Permian
* Corresponding author at: Institute of Geosciences, Goethe University Frankfurt, Altenh¨
oferallee 1, 60438 Frankfurt am Main, Germany.
E-mail address: arndt@em.uni-frankfurt.de (I. Arndt).
1
Current address: GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany.
Contents lists available at ScienceDirect
Palaeogeography, Palaeoclimatology, Palaeoecology
journal homepage: www.elsevier.com/locate/palaeo
https://doi.org/10.1016/j.palaeo.2024.112711
Received 30 June 2024; Received in revised form 29 October 2024; Accepted 30 December 2024
Palaeogeography, Palaeoclimatology, Palaeoecology 661 (2025) 112711
Available online 2 January 2025
0031-0182/© 2025 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ).
(Ivany and Runnegar, 2010) alongside other archives and proxies (e.g.
Evans et al., 2013; Ivany and Judd, 2022). (Sub-)daily environmental
changes in the past have been revealed through element ratios e.g. for
the Neogene (Sano et al., 2012; Warter and Müller, 2017), or the
Cretaceous (de Winter et al., 2020b). Environmental parameters which
have been reconstructed sub-seasonally from bivalve shells include
temperature (e.g., Ayling et al., 2015; Batenburg et al., 2011; P¨
atzold
et al., 1991; Warter et al., 2015), nutrient availability (Arias-Ruiz et al.,
2017; Batenburg et al., 2011; Elliot et al., 2009), irradiance and diurnal
cycle lengths (de Winter et al., 2020a; Hori et al., 2015; Sano et al.,
2012; Warter et al., 2018) and short-term disturbances such as past
extreme weather events including storms and heavy precipitation events
(Komagoe et al., 2018; Yan, 2020).
(Sub)tropical marine giant clams (Tridacna) are long-lived, fast-
growing organisms (mm-cm/year, e.g. Arndt et al., 2023; Bonham,
1965; Elliot et al., 2009; Fousiya et al., 2024; Ma et al., 2020; Mills et al.,
2023; Rosewater, 1965) that can build large aragonitic shells of up to
one meter in size over some hundred years (Knop, 1996; Watanabe et al.,
2004). Therefore, their shells are ideally suited to record multi-decadal
environmental changes at sub-seasonal resolution in (sub-)tropical reefs,
ever since their emergence in the early Miocene (Harzhauser et al.,
2008).
The late Miocene (~10 Ma; see below) Tridacna specimen investi-
gated in this study was collected in East Kalimantan. It grew on the
western margin of the Makassar Strait, a key part of the Indonesian
Throughow (ITF), which affects global atmospheric climate patterns
(e.g., Gallagher et al., 2024; Schneider, 1998). The late Miocene was
characterised by pCO
2
values of ~400–500 ppm (Rae et al., 2021) with
global mean surface temperatures ~3–4 ◦C higher than modern (Evans
et al., 2024; LaRiviere et al., 2012; Pound et al., 2011; Zhang et al.,
2014). A considerable reduction in atmospheric CO
2
(Rae et al., 2021)
combined with eccentricity minima facilitated the growth and stabili-
sation of the Antarctic ice sheet during the Mid-Miocene Climate Tran-
sition (MMCT) at ~14.5 to 13 Ma (Holbourn et al., 2005), resulting in a
sea level decrease of ~50 m (John et al., 2011). These changes in sea
level and the tectonic closure of the Indonesian ocean gateway in the
late Oligocene/early Miocene (Kuhnt et al., 2004) inuenced the for-
mation and position of the proto-West Pacic Warm Pool. The formation
of the earliest proto-West Pacic Warm Pool in the late Miocene is
thought to have induced an ocean-atmospheric pattern similar to today
with variable Walker circulation strength, a La Ni˜
na main state and El
Ni˜
no-Southern Oscillation (ENSO)-like dynamics (Fox et al., 2021;
Gallagher et al., 2024; Sosdian and Lear, 2020). The cooling of the
Southern Ocean in the late Miocene led to a northward shift of the
intertropical convergence zone (ITCZ) in the austral summer (Holbourn
et al., 2010), from an equator-near position at 15 Ma to ~10◦N at 8 Ma
(Groeneveld et al., 2017). Additionally, a combination of tectonic
changes, such as the closure of the Tethys and the uplift of the Hima-
layan Plateau, reorganised ocean circulation and induced global cooling
which intensied the South Asian Monsoon at ~13 Ma (Betzler et al.,
2018 and refs. therein).
Because of these changes in ocean circulation and precipitation
patterns, the Miocene is a particularly interesting epoch to study in the
Indo-Pacic region. Through dual clumped isotope analysis (Fiebig
et al., 2019), we determine that the giant clam shell was precipitated
indistinguishably from isotopic equilibrium and gain a temperature as
well as a seawater δ
18
O value for this late Miocene Indo-Pacic reef.
Based on elemental ratios and stable isotopic composition, we present a
57-year long continuous multiproxy palaeoenvironmental record to
provide multidecadal information at up to daily time-resolution,
revealing multi-annual ENSO-like cycles, regional precipitation driven
seasonal patterns and extreme weather events during the Miocene. Such
(sub-)seasonal data help to constrain palaeoclimate reconstructions and
can further be utilized to test the skill of climate models (Carr´
e and
Cheddadi, 2017; Cauquoin et al., 2019; Schmidt et al., 2014; Tierney
et al., 2020).
2. Materials
A large Tridacna shell, referred to as TOBI (Fig. 1), was collected in
2013 in East Borneo, near Bontang (0◦08
′
15”N, 117◦26
′
21
″
E). The shell
was taken from a bioclastic packstone horizon within silty shales and in
close vicinity to a sandstone horizon (Renema et al., 2015).
Following external cleaning with a brush and tap water, the shell was
cut along the presumed maximum growth axis (Fig. 1 B). The resultant
slab was divided into six smaller rectangular slabs (~3 ×6 cm), each of
which in turn was sliced longitudinally. The mirror sides of each of these
halves were used to create powder for stable isotope analysis by
micromilling and thin sections (50
μ
m thick) used for laser ablation
inductively coupled plasma mass spectrometry (LA-ICPMS) analysis,
respectively. The thin sections were polished using a 3
μ
m diamond
suspension.
3. Methods
3.1. MC-ICPMS – Strontium isotopes
The strontium isotopic composition of the shell was analysed using
multi collector inductively coupled plasma mass spectrometry (MC-
ICPMS) in solution mode. For the purpose of Strontium Isotope Stra-
tigraphy (SIS), twelve powder samples (10 to 20 mg) were taken using a
handheld drill with a diamond tipped drill bit; four samples were
selected from optically pristine areas, four from the visibly altered rim
areas and four from recrystallised areas. The powder samples were pre-
leached for one hour in 0.1 M acetic acid. Subsequently, each sample
was centrifuged and the remaining solid phase was leached in 1 M acetic
acid at room temperature for another 17 h. For seasonally-resolved Sr
isotope data another set of twelve powder samples of 1 to 2 mg were
taken from the pristine inner part of the shell using a micromill equipped
with a conical, diamond coated drill bit (tip diameter: 0.3 mm). Six
samples were taken from areas with dark banding and six from the light
areas in between the bands (Fig. 2). The samples were leached in 0.1 M
acetic acid for one hour and subsequently centrifuged. The acetic acid
leachates of all samples were dried and re-dissolved in 3 M nitric acid
before chromatographic Sr purication using the Sr resin SR B50 S with
50–100
μ
m grain size. These Sr eluates were measured using a Thermo
Fisher Neptune Plus MC-ICPMS, equipped with an auto-sampler. The
samples were introduced into the plasma using a nebuliser and con-
ventional glass spray chamber. Masses (m/z) from 83 to 88, including Kr
and Rb interferences, were simultaneously monitored and mass bias was
corrected using the exponential mass fractionation law (Russell et al.,
1978) and
86
Sr/
88
Sr =0.1194 (Steiger and J¨
ager, 1977), after subtrac-
tion of on-peak baselines. NIST SRM987 was used as standard reference
material and for tuning. Using a 100 ppb SRM987 solution and a CeO
+
/
Ce
+
rate of 2 %, we achieved an
88
Sr sensitivity of ~13 V. For SIS
analysis, the corresponding SRM987 analyses yielded
87
Sr/
86
Sr of
0.710241 ±0.000007 (2 SD, n =10;
88
Sr signal ~40 V at 10
11
Ω), and
consequently no adjustment to the internationally adopted
87
Sr/
86
Sr of
0.710248 (McArthur et al., 2001) was necessary. Material from both
leaches were used for SIS analysis, excluding four (of twenty-four)
measurements for which we did not obtain sufciently high
88
Sr sig-
nals. In contrast, during later analysis of seasonally-resolved Sr isotope
data, the SRM987 analyses yielded
87
Sr/
86
Sr of 0.710283 ±0.000026 (2
SD, n =16;
88
Sr signal ~11 V at 10
11
Ω); as a result, the
87
Sr/
86
Sr ratios
of these samples were adjusted by subtracting 0.000035. Final
87
Sr/
86
Sr
uncertainties listed in tables S1 and S2 include the quadratically prop-
agated individual internal errors (2 SE) and external SRM987 re-
producibilities (2 SD).
3.2. LA-ICPMS – Elemental ratios
The thin sections were precleaned in an ultrasonic bath with ethanol.
Sub-daily resolved LA-ICPMS data were measured with a RESOlution LR
I. Arndt et al.
Palaeogeography, Palaeoclimatology, Palaeoecology 661 (2025) 112711
2
laser-ablation system (Applied Spectra Inc., USA) equipped with a two-
volume S155 cell (Laurin Technic; Müller et al., 2009) coupled to a
Thermo Fisher Element XR sector eld mass spectrometer. We used a 3
×33
μ
m rotatable rectangular slit for ablation to maximise spatial res-
olution while maintaining suitable sensitivity, following Warter and
Müller (2017). Pathways were set parallel to the maximum growth axis
and orthogonal to the daily bands. Laser precleaning was performed at a
scan speed of 7
μ
m/s and a repetition rate of 20 Hz (using the same laser
spot size), while measurements were conducted at 1
μ
m/s and 10 Hz
respectively. The isotopes
11
B,
23
Na,
24
Mg,
27
Al,
43
Ca,
89
Y,
88
Sr,
138
Ba
were monitored with a sweep time of 330 ms to achieve a sampling
interval of ~0.34
μ
m (distance per sweep time). Instrumental tuning was
performed on standard reference material NIST SRM612 with a 60
μ
m
spot at 1
μ
m/s scan speed and 5 Hz repetition rate, tuning the plasma to
minimise the oxide production rate to ≤0.4 % for ThO
+
/Th
+
, to achieve
a Th/U ratio of 0.97 and normalised argon index (Fietzke and Frische,
2016) of 1.1. The doubly-charged production rate was monitored via the
22/44 ratio and was ~1.5 %.
Sample measurements were bracketed with NIST SRM612 as
external standard, analysed in an identical manner as the samples, with
all reference values taken from Jochum et al. (2011) with the exception
of Mg, for which we follow the recommendation of Evans and Müller
(2018). The Ca concentration of the shell was assumed to be 40 wt%
based on aragonite stoichiometry (but not required for El/Ca).
Elemental ratio (El/Ca) quantication was conducted after Longerich
et al. (1996) using the software Iolite 4 (Paton et al., 2011) with
43
Ca as
internal standard. Accuracy and precision were determined by
measuring the MPI-DING glass KL2-G (Jochum et al., 2006) and the
nanopellet carbonate standard MACS-3NP (Garbe-Sch¨
onberg and
Müller, 2014). Accuracies are in the range of −2 to 5 % for KL2-G and 7
to 32 % for MACS-3NP, while precision ranged from 2 to 14 % 2 RSD for
KL2-G and 3 to 18 % 2 RSD for MACS-3NP; for full details see table S3.
Data processing and visualisation was conducted via Python and
beneted from the Python libraries and packages NumPy (Harris et al.,
2020), Pandas (Reback et al., 2022), Matplotlib (Caswell et al., 2022)
and PyCWT, based on Torrence and Compo (1998). For details, see
Arndt et al. (2023).
3.3. Gas MS – Stable oxygen and carbon isotopes
Powder samples of ~100
μ
g each were taken from the shell slabs
using a NewWave micromill along the entire shell length at increments
of 100–200
μ
m for the ontogenetically younger slabs (slabs 1–4) and at
300
μ
m for the ontogenetically older slabs (slabs 5–6), using a conical
diamond coated drill bit with a 300
μ
m tip. First, the outermost rims of
the sample path area were drilled, and the powder removed. Then the
subsequent samples were removed “quarry-like” in steps below the drill
bit’s width. The sampling depth was 200
μ
m in the z-direction and the
path length was between 500 and 1000
μ
m (x-y-direction). The drill bit
rotation was reduced to 70 % of maximum drill speed to reduce heating
and minimise recrystallisation to calcite while drilling (Moon et al.,
2021; Staudigel and Swart, 2016; Waite and Swart, 2015). Approxi-
mately 100
μ
g of sample powder was weighed and inserted into glass
exetainers. Samples were loaded into a Gas Bench II (Thermo Fisher)
connected to a MAT 253 gas source mass spectrometer (Thermo Fisher)
and analysed for their bulk carbon and oxygen isotope compositions. For
data correction (Sp¨
otl and Vennemann, 2003), an inhouse Carrara
marble standard with known isotopic composition was run along with
the samples. Final δ
13
C and δ
18
O values are reported relative to VPDB.
3.4. Gas MS – Dual clumped isotopes
A block was cut from the inner shell part, ground with an agate
mortar and sieved through a 100
μ
m mesh. Δ
47
, Δ
48
, δ
18
O and δ
13
C of
Fig. 1. A: Map of the Indo-Pacic region, with the sampling locality (0◦08
′
15”N 117◦26
′
21
″
E) marked by a red clam icon. B: Sample before cutting, the cutting
direction is indicated by the dashed line. C: Section across the shell with numbered cut slabs and red tracks indicating the sampling axis for the geochemical data. The
red dot indicates the location of the sample taken for dual clumped analysis. D: Thin section (50
μ
m thick) of slab 6 with a conspicuously regular double-banding
pattern at the 5 to 10 mm scale and visible alteration at the outer rim (lower left corner). The direction of growth (DOG) is from right to left for B, C and D. (For
interpretation of the references to colour in this gure legend, the reader is referred to the web version of this article.)
I. Arndt et al.
Palaeogeography, Palaeoclimatology, Palaeoecology 661 (2025) 112711
3
this sample were measured on a Thermo Scientic 253 Plus gas source
mass spectrometer using the setup and following the methodology of
Fiebig et al. (2019, 2021) and Bernecker et al. (2023). Acid digestion
was conducted with phosphoric acid (>108 wt%) at 90 ◦C on 8
replicates. For each replicate, 10.0 ±0.2 mg of the ne powder was
weighed into a silver capsule. Alongside the samples we measured car-
bonate standards ETH-1 and ETH-2 (Bernasconi et al., 2021), Carrara
marble, GU1, as well as CO
2
gases of varying bulk isotopic compositions
Fig. 2. A: Image of the shell showing the optically visible light-dark banding pattern. B: Corresponding El/Ca data from the shell section shown in A, which
demonstrate the covariation of the elemental ratio variability with the growth banding. Note that the positions of the green bars in panel B are dened on the basis of
the Mg/Ca and Na/Ca peaks (see Arndt et al., 2023, Fig. S16) and not directly determined by the position of the shell’s dark bands. C: Stable oxygen and carbon data
from the shell section, with optically determined dark bands (as in A) shown in grey. Unlike El/Ca ratios (Mg/Ca, Na/Ca), no direct correlation between the dark shell
bands (in grey) and the isotopic composition is observed. (For interpretation of the references to colour in this gure legend, the reader is referred to the web version
of this article.)
I. Arndt et al.
Palaeogeography, Palaeoclimatology, Palaeoecology 661 (2025) 112711
4
equilibrated at 25 ◦C and 1000 ◦C. All samples and secondary standards
were measured against a reference gas with known isotopic composi-
tions of δ
13
C
VPDB
= − 4.2 ‰ and δ
18
O
VSMOW
=25.26 ‰ (ISO-TOP, Air
Liquide, France). Raw intensity data were corrected for the pressure
baseline effect utilizing a m/z 47.5 half-mass cup and optimised scaling
factors (Bernecker et al., 2023; Fiebig et al., 2021), such that equili-
brated gases displayed slopes of zero in δ
47
vs Δ
47
and δ
48
vs Δ
48
space.
Standardisation was carried out relative to equilibrated CO
2
gases and a
reaction temperature of 90 ◦C (carbon dioxide equilibrium scale; CDES
90) using the pooled session mode of D47crunch (Da¨
eron, 2021). For
data processing, ETH-3 replicates were individually labelled since their
Δ
47
and Δ
48
values have been shown to be compromised by variable
amounts of NO
2
interferent (Fiebig et al., 2024). This way, ETH-3 is
excluded from the variance minimization algorithm of D47crunch
(Da¨
eron, 2021) which would otherwise affect Δ
47
and Δ
48
values of
unknown samples (Fiebig et al., 2024). Uncertainties in Δ
47
(CDES 90)
and Δ
48
(CDES 90) reect fully propagated 2 SE, both considering
autogenic and allogenic errors. The revised Δ
47
-T calibration of Fiebig
et al. (2024) was utilized to calculate temperatures. In order to recon-
struct seawater δ
18
O we used the equation based on molluscs of Gross-
man and Ku (1986). δ
18
O values of CO
2
evolved from phosphoric acid
digestion of aragonite was converted to δ
18
O
Arag
by applying the acid
fractionation equation of Kim et al. (2007) for calcite since Grossman
and Ku (1986) originally normalised their calibration data to NBS 19
calcite and, therefore, did not account for differences in acid fraction-
ation factors between aragonite and calcite.
3.5. Daydacna – Daily Mg/Ca cycle based internal age model
To determine the ontogenetic age of the clam and to obtain a daily
resolved internal age model we used the Python script Daydacna (see
Arndt et al., 2023). This script uses wavelet transformation on sub-daily
resolved El/Ca data to determine the main daily cycle wavelengths at a
given distance along the shell. With the information of daily cycle
lengths and the changes therein, an internal growth rate, and thus age
model, is calculated. The El/Ca data measured along the distance of the
shell are converted onto the time scale, facilitating the interpretation of
proxy data regarding the timing of events and cycles.
4. Results
4.1. Preservation and optical analysis of the shell
The specimen displays signs of alteration on its external shell parts.
The umbo is not well preserved, and it is possible that some outermost
parts of the shell may be missing, e.g. via minor dissolution (Fig. 1 B).
Therefore, it is not possible to analyse the earliest periods of the clam’s
growth or to determine the exact lifespan of the shell. Despite some
visible alteration on the outside (Fig. 1 B), the inner areas appear pris-
tine. Optical investigations of the thin sections conrm this observation,
as the outer rim (~1 cm) shows a clear change in colour and at times
visible recrystallisation, while the inner part of the shell has an optically
pristine appearance, with no signs of recrystallisation or dissolution
(Fig. 1 C,D). Additional tests for alteration were conducted using El/Ca
data and mineral phase analyses. These indicate that the aragonitic shell
mineralogy is preserved within the inner shell area (see Arndt et al.,
2023).
Along the entire cross-section of the shell there is a conspicuous light-
dark banding pattern (Fig. 1 C,D). In the ontogenically younger part of
the shell the banding pattern is irregular, while the ontogenetically older
area of the shell displays a more regular banding structure (Fig. 1D). The
daily growth increment structure is partially visible but not continuously
discernible throughout the shell.
4.2. SIS age
The
87
Sr/
86
Sr values of the pristine shell areas are 0.708883 ±
0.000016 (2 SD), whereas visually altered sections reach values of up to
0.708896 ±0.000019 (2 SD). Altered and pristine values are within
error of each other, with the mean
87
Sr/
86
Sr of all datasets equal to
0.708895 ±22 (2 SD) see table S1. Corresponding ages are based on an
approximate age from biostratigraphic analyses of the specimen’s
sampling location (Renema et al., 2015) and were constructed via a
cross-correlation with the SIS Look-Up Table (Version 4: 08/04; McAr-
thur et al., 2001; McArthur and Howarth, 2005). The ages are 10.1 ±0.6
(2 SD) Ma when considering the pristine dataset only, while the altered
sections correspond to an apparent age of 9.7 ±0.7 Ma (2 SD) (Fig. S1).
4.3. Internal age model
4.3.1. Age and growth rate determination using sub-daily resolved trace
element data
The internal age model based on the evaluation of daily Mg/Ca cy-
cles using the Daydacna programme (see Arndt et al., 2023) yielded an
age estimate of 20,916 ±1220 days (2 SD). Assuming a Miocene year
had 365 days, this translates to 57.3 ±3.3 years (2 SD). Given the size of
the data set (628,612 measurements per element) and the need to esti-
mate the uncertainty via Monte Carlo simulations, the entire record was
divided into 14 subsets for data processing. Details of the parameters
used by Daydacna and the results of each of the individual segments can
be found in the supplementary online material (Table S4, Figs. S2-S15).
The internal age model, i.e. the distance-time relationship, is displayed
in Fig. 3.
4.3.2. Application of the age model to the δ
18
O and δ
13
C data
Stable oxygen and carbon isotopic compositions were measured from
powder milled from slabs, while elemental ratios were obtained from
corresponding thin sections. Although the ablation and milling tracks
were therefore positioned very close to each other within the shell (a few
millimetres apart on opposite slabs), the growth pattern within the shell
varies slightly between the two tracks. Furthermore, cracks in the
ontogenetically youngest parts prevented the collection of sufcient
unaltered powder for reliable oxygen and carbon isotope measurements
in these growth areas, while the cracks could be avoided using laser
ablation. This results in a slightly shorter isotope record compared to the
elemental ratio record. To reduce the potential offset and to increase
comparability between the datasets we use the dark bands within the
shell as anchor points in order to align the two datasets onto the same
distance scale (Fig. 2).
The banding pattern, linked to the stable isotope dataset, was
quantied by optical analysis of slab photographs. In the two ontoge-
netically oldest slabs (numbers 6 and 5) the lower sampling resolution of
0.3 mm allowed us to assign the attribute “dark band” and “light band”
to each δ
18
O and δ
13
C datapoint. The ‘quarry-like’ sampling at 0.1 mm
sampling resolution applied to the ontogenetically younger slabs
(numbers 4 to 1) made it difcult to optically distinguish between in-
dividual samples on the slab images. Therefore, we evaluated a nor-
malised brightness curve extracted from light microscope images and
assigned dark bands wherever brightness values were below 0.6 for 10
consecutive pixels. The attribute “dark band” and “light band” were then
assigned to the isotope values by distance along the milled track. We
used a xed threshold (0.6) to recognise dark bands. As such, we cannot
exclude the possibility that bands which were slightly darker compared
to the surrounding shell, but not dark enough to cross the threshold, may
have been missed.
We could not optically determine the nature of the banding pattern
at the sites of the sub-daily resolved El/Ca measurements, because in
thin sections the differences in brightness between dark and light
banding was not sufcient for a clear identication of the bands. Based
on the observation that Mg/Ca is elevated in dark bands (Fig. S16, Fig. 2;
I. Arndt et al.
Palaeogeography, Palaeoclimatology, Palaeoecology 661 (2025) 112711
5
see also Arndt et al., 2023), the banding pattern determined for the δ
18
O
and δ
13
C data set could be aligned to the Mg/Ca peaks, dened as
maxima with values above the median +2 SD. The variability of El/Ca
relative to the independently determined banding, seen in overview
measurements conducted at a lower sampling resolution on the sample
slabs, is shown in Fig. S16. The distance value associated with the cor-
responding Mg/Ca peak centre was matched to the centre of the dark
band in the stable isotope data set. Finally, we assigned a new distance
value to each isotope value by linearly interpolating between the dark
band calibration points, derived by matching the Mg/Ca peaks to the
image processing of the micromilled slabs. With the stable isotope
dataset now on the elemental ratio distance scale, the distance-time-
relationship determined for elemental ratios could be applied to the
stable carbon and oxygen isotope data as well.
4.4. Elemental ratios
The LA-ICPMS elemental ratio dataset includes 628,612 data points
per element, on average equivalent to 10,971 datapoints per year of
growth, or ~30 per day. B/Ca values range from 0.01 mmol/mol to 0.14
mmol/mol with an average of 0.05 mmol/mol. The minimum value for
Na/Ca is 2.9 mmol/mol, the maximum value 27.9 mmol/mol and the
average is 13.2 mmol/mol. Mg/Ca ranges from 0.10 to 1.42 mmol/mol
with an average of 0.30 mmol/mol. Average Sr/Ca is 2.1 mmol/mol,
with a minimum of 1.1 mmol/mol and narrow peaks which reach values
of up to 10.4 mmol/mol. Ba/Ca ranges from 0.5 to 7.2
μ
mol/mol with an
average of 2.4
μ
mol/mol (Table S5).
To test for multi-annual periodicities in the El/Ca data, multi-taper
spectral analysis was performed on 30.42-day (monthly) averaged
data using Acycle (Li et al., 2019), with an AR1 process as the null hy-
pothesis (see Fig. S17 for all spectra). One of the signicant periodic
signals is in the range of 3.6 to 4.4 years and is preserved in B/Ca, Na/Ca
and Ba/Ca. Another signicant peak occurs between 7.6 and 8.2 years in
Na/Ca and Ba/Ca. Furthermore, signals of around 10, 13 and 17 years
are seen in B/Ca, Mg/Ca, Sr/Ca and Ba/Ca.
At a seasonal scale, elevated Mg/Ca values coincide with B/Ca and
Sr/Ca maxima as well as Na/Ca minima. This pattern is clearly linked to
the appearance of dark banding in the shell (Fig. 2) and appears mostly
twice per year with varying intensity (Fig. 4 A,B). The pattern shows
irregularity with some increases or decreases in El/Ca, present in the
form of two or three nearby peaks. These ndings are supported by the
spectral analysis (Fig. S17), which indicates that an approximately half
year periodic signal is present in B/Ca, Na/Ca, Mg/Ca and Sr/Ca,
exceeding the 90 % condence level. Signicant annual cyclicity is
present in the signals of Na/Ca, Mg/Ca, Sr/Ca and Ba/Ca where some of
these elemental ratios also show other signicant periodicities around
the yearly frequency band (Fig. S17). In addition, sub-seasonal vari-
ability in the form of short-term events can be seen in the data. These
multi-day events are most prominently seen as Sr/Ca peaks and usually
appear in areas of elevated Mg/Ca and dark banding. B/Ca, Na/Ca and
Mg/Ca briey decrease (overall within areas of increased Mg/Ca and B/
Ca values) while Ba/Ca increases together with the sharp increase in Sr/
Ca (Fig. 4 C,D). In total, we identied this elemental pattern of short-
term extreme events with varying intensities 35 times in the dataset,
so on average less than once per year. Fourteen events were moderate
while the remaining twenty-one showed clear peaks. The events seemed
to be clustered and most appear either between 21 and 29 or between 39
and 51 years (Fig. 4).
4.5. Isotopic data
4.5.1. Dual clumped isotopes
The eight replicate measurements yielded a mean δ
13
C
[VPDB]
of 3.05
±0.40 ‰ (2 SD) and δ
18
O
Arag[VPDB]
of −1.73 ±0.07 ‰ (2 SD). The
clumped isotopic composition (Δ
47
) based on these replicates is 0.587 ±
0.007 ‰ (2 SE) while the Δ
48
value is 0.239 ±0.021 ‰ (2 SE). The data
point plots within uncertainty indistinguishably from the Δ
47
-Δ
48
equilibrium line of Fiebig et al. (2024) (Fig. 5), indicating growth in
isotopic equilibrium and validating the suitability of Δ
47
as a reliable
temperature proxy in Tridacna. The temperature calculated from the Δ
47
value ±2 SE lies between 25.6 and 30.3 ◦C with a mean of 27.9 ◦C
(Table S6). By inserting the clumped isotope derived temperature into
the empirical temperature-δ
18
O
sw
-δ
18
O
Arag
relationship for aragonitic
molluscs of Grossman and Ku (1986), we calculated a δ
18
O
sw [SMOW]
of
−0.43 ±0.50 ‰ for this late Miocene location.
4.5.2. Stable oxygen and carbon isotopes
δ
18
O
Arag
on the shell age model shows a broadly cyclic variability
between −2.86 ‰ and -1.27 ‰ with a mean of −2.16 ‰ (Fig. 6 A). The
cyclicity is typically approximately yearly (Fig. 6 B). δ
13
C values range
from 2.06 ‰ to 3.60 ‰ with a mean of 2.99 ‰ (Table S7). No distinct
yearly signal can be observed in δ
13
C, however multi-annual cycles are
Fig. 3. Age model for TOBI with the shaded blue area reecting the 2.5th to 97.5th percentile interval, indicating 20,916 ±1220 days (2 SD) or 57.3 ±3.3 years (2
SD) of growth. Overall, the growth rate is relatively constant throughout the shell apart from seasonal variability. (For interpretation of the references to colour in
this gure legend, the reader is referred to the web version of this article.)
I. Arndt et al.
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Fig. 4. Elemental ratios versus time. A: Overview over the entire record of ~250 mm, representing ~57 years. Cyclicities are seen in all elemental ratios. While B/Ca,
Mg/Ca and Sr/Ca positively co-vary, Na/Ca is anticorrelated with the other trace element ratios. Ba/Ca is characterised by a less regular pattern, with several peaks
in phase with B/Ca, Mg/Ca and Sr/Ca. B: Close-up of the section from 20 to 30 years, highlighting the consistent presence of two peaks or troughs per year. C: Close-
up of the section of 25 to 27 years to highlight the presence of short seasonal peaks, most clearly visible in Sr/Ca. D: Close-up of a ~two-months period, namely 9320
to 9380 days (25.53 to 25.70 years), with a 38-day-long seasonal peak spanning from about 9323 to 9360 days and one short-term event spanning over 3 days (from
9353 to 9356), represented by a positive peak in Sr/Ca and Ba/Ca and correspondingly negative Mg/Ca, Na/Ca, and B/Ca excursions. Daily cycles are visible
particularly well between 9370 and 9380 days.
I. Arndt et al.
Palaeogeography, Palaeoclimatology, Palaeoecology 661 (2025) 112711
7
present (Fig. 6 A). No clear link with the banding pattern is visible in the
case of either δ
18
O
Arag
or δ
13
C as both minima and maxima can occur in
dark bands.
The spectral analysis of the monthly binned δ
18
O
Arag
and δ
13
C data
shows that a yearly periodicity is present with 90 % condence over an
AR(1) null hypothesis in the δ
18
O
Arag
but not the δ
13
C dataset. Regarding
long-term variabilities, δ
18
O
Arag
shows signicant periodicities at 4.6
and 7.6 years, while δ
13
C shows signicant periodicities at 3.9 and 4.4
years as well as at 7 and 8.3 years (Fig. S17).
Using the temperature equation for aragonitic molluscs from Gross-
man and Ku (1986), and a constant δ
18
O
sw
of −0.43 ‰ (derived via dual
clumped isotope measurements), we arrive at an apparent seasonal
temperature range of 23.8 to 31.3 ◦C (see discussion).
4.5.3. Strontium isotopes
The seasonally-resolved strontium isotopic data of the shell were
measured across three years of growth, covering six light areas and six
dark bands. We found no difference in
87
Sr/
86
Sr between the dark bands
with 0.708904 ±0.000021 (2 SD) and light bands with 0.708903 ±
0.000014 (2 SD) (Table S2). Furthermore, all twelve measurements are
within error of each other and indistinguishable from the SIS value of
0.708883 ±0.000016 (2 SD) (Fig. S18).
5. Discussion
The multiproxy dataset presented in this study spans nearly six de-
cades at sub-daily to seasonal resolution. It provides insights not only
into multi-annual but also seasonal and shorter-term aspects of late
Miocene tropical climate in the Indo-Pacic region. This is, to the best of
our knowledge, the rst sub-daily resolved, multi-decadal record iden-
tifying sub-seasonal patterns in a tropical palaeoenvironment.
For specimens like our late Miocene Tridacna TOBI, for which daily
banding could not be resolved optically throughout the entire shell,
daily geochemical cycles are more reliable indicators of daily growth
(Arndt et al., 2023). Prior research has revealed that daily cycles occur in
Sr/Ca and Mg/Ca of Tridacna shells and are possibly linked to the light-
dependent incorporation of these elements in the shell, which could be
induced by daily variability in photosymbiotic activity (Sano et al.,
2012; Warter and Müller, 2017; Warter et al., 2018) and/or light-
enhanced calcication (Rossbach et al., 2019; Sano et al., 2012). In a
previous Tridacna culturing study, Warter et al. (2018) demonstrated
that the number of bands and El/Ca cycles was equal to the culturing
period in days. We found that the Mg/Ca cycles in our sample, ranging
from a few to tens of
μ
m, align well with the microscopical banding
pattern where it is visible (Arndt et al., 2023). Because of the similarity
of our observations to that from the cultured Tridacna in Warter et al.
(2018) we are condent that daily El/Ca cycles are preserved in our
fossil sample. Our Python based script Daydacna (Arndt et al., 2023;
Arndt and Coenen, 2023) can create an internal age model for long
datasets and is likely to be useful even in cases of good increment visi-
bility, as cycle quantication is semi-automated and there is no need for
counting (tens of) thousands of daily increments. Converting data from
Fig. 5. Dual clumped isotope (Δ
47
, Δ
48
) space. The late Miocene Tridacna
sample TOBI lies within fully error propagated 2 SE of the proposed Δ
47
/Δ
48
temperature equilibrium relationship of Fiebig et al. (2024).
Fig. 6. A: Stable carbon and oxygen isotope variability as a function of relative growth time across the shell. The shaded areas reect the uncertainty (2 SD). The rst
year could not be sampled (see the section on preservation). The grey bars indicate position and width of the visible dark bands transferred to the time scale. B:
Closeup of the section of 20 to 30 years with vertical lines indicating the middle of each year. δ
18
O
Arag [VPDB]
values show asymmetric yearly cycles while δ
13
C
[VPDB]
cycles are multi-annual. C: Closeup of the section of 45 to 55 years. At the lower sampling resolution (300
μ
m steps, seasonal resolution) cycles are less clear.
I. Arndt et al.
Palaeogeography, Palaeoclimatology, Palaeoecology 661 (2025) 112711
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the distance to the time scale facilitates both timing and duration of
multi-annual, seasonal or short-term events to be elucidated in the
elemental and isotopic record and can help to understand the connection
between shell growth, shell structure and proxy data.
5.1. Equilibrium calcication, temperature and δ
18
O
sw
derived from dual
clumped isotopes
Through dual clumped isotope analysis, we are able to verify the
assumption that Tridacna calcify in isotopic equilibrium, which has only
previously been inferred based on observations of modern Tridacna
shells, in which measured δ
18
O
sw
and temperature values match the
respective values calculated via δ
18
O
Arag
(e.g. Aharon and Chappell,
1986; Arias-Ruiz et al., 2017; Duprey et al., 2015; Elliot et al., 2009).
Furthermore, this nding demonstrates that the isotopic signature was
not signicantly impacted by open-system diagenesis e.g. through
heating and uid‑carbonate interaction with isotopically distinct
diagenetic uids, as this could result in an offset from the equilibrium
curve (Staudigel et al., 2023a, 2023b). The resulting temperature, in-
dependent of assumptions of the oxygen isotopic composition of
seawater, of 27.9 ±2.4 ◦C (2 SE) is similar to modern temperatures in
the region (e.g. Purwandari et al., 2019; Teichberg et al., 2018). The
calculated mean δ
18
O
sw
of −0.43 ±0.50 ‰, reconstructed from the
clumped isotope derived temperature, is in agreement with previous
reconstructions of late Miocene δ
18
O
sw
(Lear et al., 2000; Rohling et al.,
2022).
5.2. Multi-annual cycles
Overall, the multi-annual variabilities in elemental and isotopic ra-
tios are not very pronounced. However, the two clearly resolvable pe-
riodicities, especially in the δ
18
O
Arag
, δ
13
C, B/Ca, Na/Ca and Ba/Ca
time-series, include signicant signals between 3.6 and 4.6 as well as 7
to 8.2 years. In the modern ocean, the El Ni˜
no climate phenomenon
typically occurs every two to seven years (McPhaden et al., 2020) and is
linked to weaker easterly trade winds through a weakened Walker cir-
culation, reduced precipitation in the West Pacic and a weakened and
cooler ITF (Feld et al., 2000; Timmermann et al., 2018; Yamanaka
et al., 2018; Zhang et al., 2016). Similarly, in the late Miocene, ENSO
like ocean-atmosphere dynamics appear to have affected precipitation
patterns and the ITF strength, inducing changes in the water currents
owing into the reef and thus impacting local temperature, seawater
isotopic composition and nutrient availability, all recorded in the shell
geochemistry. Owing to their longevity, modern and Holocene speci-
mens of Tridacna have already been shown to be a suitable archive for
recording ENSO variabilities in the δ
18
O composition of the shell (Shao
et al., 2020; Welsh et al., 2011). Our ndings afrm the previously
suggested hypothesis that ENSO was present in the late Miocene,
following the formation of an incipient Indo-Pacic proto-warm pool
with a La Ni˜
na-like mean state as early as 11.6 Ma (Nathan and Leckie,
2009; Batenburg et al., 2011; Sosdian and Lear, 2020; Fox et al., 2021;
Gallagher et al., 2024).
5.3. Seasonal cycles
Seasonal patterns are visible in the δ
18
O
Arag
proles which display
yearly cyclicity, as well as in some El/Ca datasets, which show a pattern
of two peaks per year, some of which are double peaks. Amongst the El/
Ca data, the seasonal peaks are most clearly seen as positive Mg/Ca and
Sr/Ca as well as negative Na/Ca peaks (Fig. 4). Increased Mg/Ca in
Tridacna has previously been linked to elevated temperatures (Arias-
Ruiz et al., 2017; Ayling et al., 2015; Warter et al., 2015), reduced light
availability and physiological stress, especially on short (daily) time
scales (Warter et al., 2018), as well as reduced growth rates (Arndt et al.,
2023). Similarly, studies on other bivalves indicate elevated Mg/Ca in
areas of reduced growth (Schleinkofer et al., 2021) and increased
organic content (Schleinkofer et al., 2021; Sch¨
one et al., 2010). Obser-
vations regarding Sr/Ca in Tridacna are ambiguous, some studies indi-
cate that Sr/Ca is correlated to SST (Mei et al., 2018; Yan et al., 2013;
Yan, 2020) while others have shown no signicant correlation between
Sr/Ca and SST (Arias-Ruiz et al., 2017; Batenburg et al., 2011; Elliot
et al., 2009; Warter et al., 2018). Like Mg/Ca, Sr/Ca is known to be
affected by diurnal changes in solar irradiance (Hori et al., 2015; Sano
et al., 2012) and physiological performance and growth (Carr´
e et al.,
2006; Gillikin et al., 2005; Warter et al., 2018). Na plays a role for
calcication in Tridacna as Na
+
/K
+
-ATPase maintains the gradient of
these molecules between the calcication site and cytosol of the adja-
cent cells, while Na
+
/Ca
2+
exchangers maintain charge balance by
removing Na
+
from the calcifying uid to provide Ca
2+
for calcication
(Boo et al., 2019; Boo et al., 2017). Na/Ca could be affected by kinetics
and the concentration of the ions in the extrapallial uid, dependent on
ion input through ion exchange and ion deposition into the shell during
calcication. Low Na/Ca occur during low growth rate phases charac-
terised by elevated Mg/Ca, which implies that the Ca concentration in
the extrapallial uid could be enriched relatively to Na, inducing low
Na/Ca in the shell. However, more research on Na/Ca in Tridacna shells
grown in controlled settings is needed to fully understand the causes for
Na/Ca variability and its implications. Similarly, B/Ca is not established
as an unambiguous environmental or physiological performance proxy
in Tridacna. However, recent studies on Arctica islandica show signicant
negative correlation between shell B/Ca and temperature (Sch¨
one et al.,
2023), which may have a mechanistic basis via the impact that tem-
perature has on physiological processes that control the calcication site
borate/carbon dynamics.
The strong peaks in Mg/Ca and Sr/Ca coupled to low growth rates in
our sample could be caused by the seasonal passing of the ITCZ over the
near-equator reef twice a year. While the ITCZ had its northernmost
seasonal position near the equator at 15 Ma, it shifted northwards during
the late Miocene and reached a seasonal northernmost position of ~10◦
N at 8 Ma (Groeneveld et al., 2017). The passage of the ITCZ would be
coupled to cloudy and rainy conditions, which would reduce both solar
irradiation onto the sea surface and induce reduced growth rates in the
photosymbiont-bearing tridacnids. In the present, the complex, regional
precipitation patterns in the Indo-Pacic are driven by rainy seasons,
linked to the ITCZ situated above the location and air convection being
at its strongest (Yuan et al., 2023). Recent monitoring from Baik and
Triangle reefs in Darvel Bay, situated on the eastern shore of Borneo
(Macassar Strait) at 5◦N (Mills et al., 2023), indicates that precipitation
peaks around November–January and May–July in the years 2018 to
2020, while light availability was high and cloud cover low in Febru-
ary–April and August–October, which corresponds to relatively elevated
temperatures. The authors suggested that seasonal turbidity can nega-
tively inuence Tridacna growth (Mills et al., 2023). In our data, Ba/Ca
are most variable during these periods of elevated Mg/Ca and Sr/Ca. Ba/
Ca has been suggested as a potential proxy for riverine inux (Elliot
et al., 2009) and primary productivity in Tridacna (Arias-Ruiz et al.,
2017; Elliot et al., 2009; Hori et al., 2015) and other molluscs (Fr¨
ohlich
et al., 2022; Gillikin et al., 2008; Marali et al., 2017), and short-term Ba/
Ca peaks have been introduced as indicators for storm events in Tridacna
(Komagoe et al., 2018), and tsunamis in other mussels (Sano et al.,
2021). During the rainy season, increased precipitation induces more
run-off which increases nutrient availability for primary productivity,
while the cloudy conditions and thus reduced solar irradiation inhibit
primary productivity. Regional short-term changes in this balance could
cause increased primary productivity variability, reected in our sam-
ple’s Ba/Ca. We do not expect that seasonal differences in riverine inux
strongly affected the reef, as we would expect more pronounced Ba/Ca
peaks than observed, potentially including seasonal changes in
87
Sr
/
86
Sr in such a scenario. Even though El/Ca proxies in Tridacna, and
marine bivalves in general, can often not be linked to a single environ-
mental factor and are partially physiologically determined (Sch¨
one
et al., 2023; Warter et al., 2018), the overall observations of
I. Arndt et al.
Palaeogeography, Palaeoclimatology, Palaeoecology 661 (2025) 112711
9
environmental settings causing El/Ca excursions as described above
combined with the regional setting in which the clam grew, make it
probable that the seasonal elemental variability observed in our sample
is a recorder of the cloudier rainy season linked to the ITCZ passing.
The yearly δ
18
O
Arag
variation could be induced by temperature and/
or δ
18
O
sw
changes. The mean δ
18
O
sw
value of −0.43 ‰, derived from the
clumped isotope temperature and δ
18
O
Arag
, is higher than the previously
assumed δ
18
O
sw
value of −0.88 ‰ for the Miocene tropics (Batenburg
et al., 2011) but reects the reconstructed late Miocene whole ocean
δ
18
O
sw
value of around −0.5 ‰ (Lear et al., 2000; Rohling et al., 2022).
If the δ
18
O
sw
value was seasonally constant, the temperature range
necessary to explain the observed intra-shell δ
18
O heterogeneity would
be over 7 ◦C. The typical seasonal cyclic δ
18
O
Arag
range is <1 ‰, which
would indicate typical intra-annual temperature ranges of 26 to 30 ◦C.
This range is larger than recent seasonal SST variability of about 2 ◦C in
the Makassar strait (e.g. Purwandari et al., 2019; Teichberg et al., 2018).
It is likely that part of the δ
18
O
Arag
variability is induced by δ
18
O
sw
changes due to regional evaporation, precipitation or changes in the
isotopic composition of the water owing into the reef. Changes in both
temperature and δ
18
O
sw
can be induced through changes of water
entering the reef either through runoff from land, upwelling and/or
changes in surface currents. The sample grew in the Makassar strait,
which is an essential part of the ITF, through which water ows
southwards from the Pacic into the Indian Ocean. The passing of the
ITCZ and the connected change in wind forcing inuences the ow,
temperature and salinity of the ITF (Kuhnt et al., 2004). Such changes
could have affected the ow of water currents regionally, thus poten-
tially impacting water temperature and isotopic signatures in the late
Miocene reef environment TOBI grew in, leading to the irregular annual
δ
18
O
Arag
cycles recorded in the shell.
5.4. Short-term events
Short-term events, spanning a few days only, are most prominently
seen as Sr/Ca peaks (Fig. 4), with concomitant, yet less-pronounced
peaks in Ba/Ca, while Mg/Ca, Na/Ca and B/Ca show negative excur-
sions (Fig. 4 C,D). These events, with durations of a few days only, occur
within the overall elevated Mg/Ca dark band areas grown during the
rainy season. We hypothesise that they indicate extreme weather events,
most likely characterised by very heavy precipitation, as Tridacna have
been shown to record such events in their elemental composition, e.g. in
Ba/Ca (Komagoe et al., 2018). The coincident increase in Sr/Ca and Ba/
Ca could indicate increased runoff from land, linked to increased
turbidity in the reef and reduced light availability, while decreased Mg/
Ca and B/Ca might be an indicator of reduced water temperatures.
Finally, we note that Mg/Ca heterogeneity in bivalve shells has also been
linked to crystal structure (Sch¨
one et al., 2013). It is therefore also
possible that the recorded Mg/Ca drop in our sample is crystal structure
related (i.e., controlled by kinetic processes), because these short-term
events visibly impact the structure of the clam shell (Fig. S19). We
cannot completely rule out the possibility of very short growth hiati
during the extreme weather events, although giant clams are considered
to have continuous growth throughout the year (Schwartzmann et al.,
2011) and have recently been shown to continue growing during short
extreme events such as heat waves (Fousiya et al., 2024). Furthermore,
the presence of yearly and half-yearly cycles in the spectral analysis
(Fig. S17) indicate that these events are very likely not connected to
longer growth hiati (spanning weeks or more).
6. Conclusion and Outlook
This contribution afrms the importance of giant clams for recon-
structing pre-Pleistocene palaeoclimates by presenting the rst, to our
knowledge, multi-decadal (57 ±3 years), multi-proxy record from a
giant clam at daily time resolution. Using this record from a late Miocene
tropical reef, we provide insights into climate patterns on multi-annual,
seasonal and daily time scales across several decades, in addition to
reconstructing the occurrence of short-term extreme weather events in a
late Miocene tropical reef. Multi-annual periodicities of around four
years suggest atmosphere-ocean interactions akin to multi-annual cyclic
climate patterns like today’s ENSO system. On a (sub-)annual scale, we
observed two key periodicities. The rst cycle is visible as yearly vari-
ability in δ
18
O
Arag
indicating changes in temperature and/or δ
18
O
sw
,
likely linked to different water currents entering the reef. The second
intra-annual pattern consists of peaks in elemental ratios occurring
approximately twice a year, which we interpret as being driven by a
cloudy and rainy seasonal signal coupled to the passing of the ITCZ over
the equatorial reef twice a year. In addition, we found 35 short-term
extreme weather events across ~57 years, most clearly seen in Sr/Ca
peaks, likely indicating short-term heavy precipitation events during the
rainy season, linked to increased runoff and turbidity in the reef. Finally,
our dual clumped isotope results demonstrate that our Tridacna sample
calcied, within uncertainty, in isotopic equilibrium, enabling us to
reconstruct average SST at 10 Ma of ~28 ◦C and a mean δ
18
O
sw
of
around −0.4 ‰. Future studies may include seasonal clumped isotope
analysis to better deconvolve the seasonal temperature and δ
18
O
sw
changes (see e.g. de Winter et al., 2021; Kniest et al., 2024).
While the multi-decadal nature of our data set shows that these
patterns hold true over long time spans and do not simply reect a few
unusual years, our sample represents one snapshot in time at one lo-
cality. More datasets such as this are required to assess local effects and
gain a deeper understanding of past seasonality and extreme weather
patterns on a larger scale. We suggest that future studies could follow
this approach of placing highly resolved geochemical data onto an in-
ternal age model, as it has great potential to help understanding past
seasonality and palaeo-weather. Ultimately, we envision that a network
of such datasets from the same time interval could be used to map multi-
annual and seasonal climate phenomena like ENSO activity and past
ITCZ positions, while a comparison over time can provide information
on the shifts in seasonality and frequency of extreme weather events in a
changing climate. Finally, such (sub-)seasonal climate time series are
crucial not only for a deeper understanding of past climate systems but
also to test the skill of climate models (Carr´
e and Cheddadi, 2017;
Cauquoin et al., 2019; Schmidt et al., 2014; Tierney et al., 2020).
CRediT authorship contribution statement
Iris Arndt: Writing – review & editing, Writing – original draft,
Visualization, Validation, Software, Methodology, Investigation, Fund-
ing acquisition, Formal analysis, Data curation, Conceptualization.
Miguel Bernecker: Writing – review & editing, Visualization, Valida-
tion, Software, Methodology, Investigation, Formal analysis, Data
curation. Tobias Erhardt: Investigation, Writing – review & editing.
David Evans: Writing – review & editing, Validation, Supervision,
Investigation. Jens Fiebig: Writing – review & editing, Validation, Re-
sources, Methodology, Funding acquisition, Conceptualization. Max-
imilian Fursman: Writing – review & editing, Validation. Jorit Kniest:
Writing – review & editing, Validation. Willem Renema: Writing –
review & editing, Validation, Resources, Conceptualization. Vanessa
Schlidt: Writing – review & editing, Validation, Conceptualization.
Philip Staudigel: Writing – review & editing, Validation, Investigation.
Silke Voigt: Data curation, Methodology, Visualization, Writing – re-
view & editing, Validation. Wolfgang Müller: Writing – review &
editing, Validation, Supervision, Resources, Project administration,
Methodology, Investigation, Funding acquisition, Data curation,
Conceptualization.
Declaration of competing interest
The authors declare the following nancial interests/personal re-
lationships which may be considered as potential competing interests:
Iris Arndt reports nancial support was provided by German
I. Arndt et al.
Palaeogeography, Palaeoclimatology, Palaeoecology 661 (2025) 112711
10
Research Foundation. Wolfgang Muller reports nancial support was
provided by German Research Foundation. If there are other authors,
they declare that they have no known competing nancial interests or
personal relationships that could have appeared to inuence the work
reported in this paper.
Data availability
All data sets presented in this study are available as tables S1 to S7 at
Zenodo with licence CC BY 4.0 (https://doi.
org/10.5281/zenodo.14609845).
Acknowledgements
We would like to thank the Stiftung Polytechnische Gesellschaft as
well as the Deutsche Forschungsgemeinschaft grant (DFG MU 3739/6-1)
for providing nancial support for the corresponding author (IA). We
are grateful for the expert sample preparation conducted by Maria Bladt,
S¨
oren Tholen and Niels Prawitz, as well as technical support of Linda
Marko, Alexander Schmidt and Richard Albert during LA-ICPMS and
solution MC-ICPMS measurements, data reduction and microscope im-
aging. We further thank Christian Bredefeld for help with micromilling
and Sven Hoffmann for technical assistance and stable isotope mea-
surements. Manuel Schumann and Phil Dolz are thanked for preparing
equilibrated gas standards. Discussions with Douglas Coenen claried
aspects of time-series analysis and Python coding. The VeWA con-
sortium (Past Warm Periods as Natural Analogues of our high-CO
2
Climate Future) by the LOEWE programme of the Hessen Ministry of
Higher Education, Research and the Arts, Germany, co-funded aspects of
this research (DE, WM). FIERCE is nancially supported by the Wilhelm
and Else Heraeus Foundation and by the Deutsche For-
schungsgemeinschaft (DFG: INST 161/921-1 FUGG, INST 161/923-1
FUGG and INST 161/1073-1 FUGG), which is gratefully acknowl-
edged. This is FIERCE contribution No. 186. We thank the editors and
two anonymous reviewers for their valuable comments on an earlier
version of this manuscript, which helped to improve both content and
presentation.
Appendix A. Supplementary data
Supplementary data to this article can be found online at https://doi.
org/10.1016/j.palaeo.2024.112711.
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