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.