Carbonate clumped-isotope thermometer

      The reconstruction of oceanic temperatures is critical to understand the processes driving climate variations. This may in theory be achieved using several existing methods, but they can present substantial limitations. Carbonate clumped-isotope thermometry, based on precise ∆47 measurements, is a promising method for paleo-thermometry, presenting two major advantages: (1) in many natural carbonates, ∆47 only depends on crystallization temperature, and (2) carbonate clumped-isotope thermometry may be applied to many different kinds of carbonates (corals, foraminifera, coccoliths, bivalves, otoliths, soils...). Importantly, this therefor opens up the possibility to directly reconstruct past seawater δ18O values, providing insights into past global ice volumes and oceanic circulation patterns.

   In a thermodynamically equilibrated carbonate mineral, the heavy isotopes of carbon (13C) and oxygen (18O) preferentially bond ("clump") together to form [13C18O16O16O]2- carbonate groups. This preferential clumping of heavy isotopes fundamentally results from the relationships between zero point energies of the carbonate ion isotopologues (e.g., [13C18O16O2]2- vs [13C16O3]2-, [12C18O16O2]2-, and [12C16O3]2-). This can be expressed through the equilibrium constant for the exchange reaction following (Ghosh et al., 2006; Schauble et al., 2006; Eiler, 2007, 2011)

     The corresponding equilibrium constant varies as a function of the temperature (Schauble et al., 2006), and may be estimated by measuring the abundance of mass-47 in CO2 (corresponding mainly to 13C18O16O) obtained by reaction with phosphoric acid (McCrea, 1950; Ghosh et al., 2006). For well-preserved carbonates precipitated in isotopic equilibrium, the mass-47 CO2 abundance is slightly higher than expected for a random ("stochastic") distribution of isotopes. This mass 47 anomaly, noted ∆47, can thus be used as a direct tracer of crystallization temperature.

    Carbonate clumped isotope thermometry is well developed. Over the past few years it has benefitted from many methodological advances such as the implementation of an "absolute" reference frame anchored to theoretical equilibrium ∆47 values in CO2 (Dennis et al., 2011); improved procedures to measure ion current background values (He et al., 2012); the distribution of international carbonate standards (Meckler et al., 2014; Bernasconi et al., 2018; Bernasconi et al., submittes); the use of updated 17O correction parameters (Daëron et al., 2016) and the development of uncertainties propagation methods (Daëron et al., submitted). Concurrent with these methodological improvements, numerous calibration studies have been based on inorganic and biogenic carbonates (e.g. Kele et al., 2015; Kelson et al., 2017; Breitenbach et al., 2018; Peral et al., 2018; Daeron et al., 2019; van Dijk et al., 2019; Piasecki et al., 2019; Meinicke et al., 2020; Jautzy et al., 2020...). Using this standardisation, calibrations of various carbonates are in very good agreement (Anderson et al., submitted).