1. Introduction
Since its first applications (e.g. Schauble et al., 2003; Wang et al.,
2004; Ghosh et al., 2006; Eiler, 2007), carbonate clumped isotope
analysis has developed into a valuable tool for paleothermometry in the
geosciences. Clumped isotope analysis is based on the thermodynamic
principle that molecules with multiple heavy isotopes (so-called
“multiply-substituted isotopologues”) have lower vibrational energies
than molecules containing lighter isotopes (Urey, 1947). Consequently,
the increase in system entropy at higher temperatures causes a decrease
in the occurrence of multiply-substituted isotopologues, and
“clumping” of heavy isotopes within the same molecule is favored in
low-energy systems (Eiler, 2007). In carbonates, this principle causes
heavy carbonate ions (e.g.13C18O16O2;
mass 63 or12C18O216O;
mass 64) to become more abundant with decreasing calcification
temperatures (Ghosh et al., 2006). The distribution of these
isotopologues is proportional in the CO2 gas after
reaction of carbonates with acid (e.g.13C18O16O; mass 47
and 12C18O2, mass 48
respectively) and is measured with reference to the distribution of
isotopologues in a fully scrambled heated CO2 gas with
the same isotopic composition:
\begin{equation}
{}_{47}\left[\%0\right]=\left(\frac{R^{47}}{R^{47}*}-1\right)\ \ \ (1)\nonumber \\
\end{equation}In which R47 is the ratio of CO2molecules with mass 47 (predominantly13C18O16O)
relative to CO2 with the most common mass 44
(12C16O2) in the
sample, and R47* is the same ratio in stochastic
equilibrium (Daëron et al., 2016). This ∆47 value is a
measure for the degree of “clumping” in the sample which depends on
its calcification temperature.
The main advantage of carbonate clumped isotope analysis over previous
paleothermometers is its basis on thermodynamic principles and its
independence from the chemistry of the precipitation fluid (Eiler,
2007). The latter represents an improvement over the often-used oxygen
isotope paleothermometer (δ18O), which requires
knowledge of the oxygen isotope composition of the precipitation fluid
(δ18Ow; e.g. Epstein et al., 1953; Kim
& O’Neil, 1997). The clumped isotope method has many applications, most
notably to reconstruct absolute temperature variability throughout
Earth’s history (Rodríguez-Sanz et al., 2017; Henkes et al., 2018;
Vickers et al., 2020a; de Winter et al., 2021a; Meckler et al., 2022;
Agterhuis et al., 2022).
Inter-lab standardization of carbonate ∆47 measurements
resolved former offsets between labs using different CO2preparation methods and reconciled the clumped isotope temperature
calibration of calcites with the results of thermodynamic ab
initio models (Bernasconi et al., 2018; 2021; Petersen et al., 2019;
Jautzy et al., 2020). A unified linear calibration was established
through re-standardized ∆47 values of carbonates
precipitated at a wide range of known temperatures (0.5-1100°C; Anderson
et al., 2021). This eliminates concerns over the confounding effects of
differences in the origin of carbonates (e.g. biogenic vs. inorganic;
Henkes et al., 2013), varying mineralization rates (Daëron et al.,
2019), different acid digestion temperatures and different carbonate
mineralogies (e.g. dolomite vs. calcite; Müller et al., 2019) on the
clumped isotope thermometer.
The unified calibration dataset includes only one aragonitic carbonate,
insufficient to test for different clumped isotope temperature
dependencies between aragonites and calcites (Anderson et al., 2021).
Results of ab initio models suggest that such a difference
between the two polymorphs may exist (Schauble et al., 2006; Guo et al.,
2009) and experimental studies disagree on a difference in acid
fractionation factor (AFF) between calcite and aragonite (Guo et al.,
2009; Müller et al., 2019; Petersen et al., 2019). These uncertainties
are confounded by the fact that most carbonates used in current
calibrations are precipitated under natural circumstances with
indirectly estimated or else poorly controlled temperature regimes (e.g.
Kele et al., 2015; Peral et al., 2018). This potential
∆47 offset between aragonite and calcite risks
introducing an unknown bias using the unified temperature calibration on
aragonite data (e.g. Caldarescu et al., 2021); a severe limitation given
the common occurrence of aragonite in biogenic calcifiers (e.g.
bivalves; Kennedy et al., 1969, gastropods; Taylor and Reid, 1990, and
foraminifera; Hansen, 1979) as well as inorganic natural carbonates
(e.g. speleothems; Frisia et al., 2000, and travertines; Kele et al.,
2015).
This study presents new clumped isotope results from precisely
temperature controlled, lab-grown aragonitic Arctica islandicabivalve shells. The bivalve Arctica islandica is a highly
utilized climate archive, and a promising substrate for clumped
isotope-based paleothermometry (e.g. Witbaard et al., 1997; Burchardt
and Simonarson, 2003; Schöne et al., 2005; Schöne and Fiebig, 2009;
Butler et al., 2013). Our dataset resolves potential vital effects on
clumped isotopes in aragonitic mollusks by comparing specimens grown
under the same controlled conditions. Combined with data from aragonite
samples from previous studies (Kluge et al., 2015; Kele et al., 2015;
Müller et al., 2017; Breitenbach et al., 2018; Bernasconi et al., 2018;
Piasecki et al., 2019; Caldarescu et al., 2021) standardized to the new
I-CDES reference frame (Bernasconi et al., 2021) the study aims to offer
a detailed investigation of the clumped isotope temperature dependence
in aragonites.