Calibration of the dual clumped isotope thermometer for carbonates

International audienceTheD47(paleo)thermometer has opened a new avenue to determine carbonate formation temperatures independent of theoxygen isotopic composition of the fluid from which the carbonate crystallized. A major limitation of this thermometer isrelated to kinetic effects if homogeneous isotopic equilibrium is not attained during carbonate precipitation. Dual clumpedisotope thermometry – the high-precision analysis ofD48along withD47in CO2evolved from phosphoric acid digestion ofcarbonates – makes it possible to resolve temperature from the kinetic information recorded in an individual carbonate phase.Therefore, it provides a new opportunity to identify (bio)mineralization pathways and to determine carbonate formation tem-peratures devoid of a kinetic bias, based solely on isotopic analysis of a single carbonate phase.Identification of the nature and extent of kinetic effects as well as the reconstruction of accurate formation temperaturesrequires knowledge of the position of equilibrium inD47vsD48space. Here, we presentD47andD48data of carbonates thatwere previously considered as having crystallized closest to equilibrium in a temperature range of 8 to 1100°C. Across thisrange, the temperature dependences ofD47andD48are best expressed by the following fourth order polynomials of 1/T:D47(CDES 90) (‰) = 1.038 (5.897 1/T3.521 1031/T2+ 2.391 1071/T33.541 1091/T4) + 0.1856D48(CDES 90) (‰) = 1.028 (6.002 1/T1.299 1041/T2+ 8.996 1061/T37.423 1081/T4) + 0.1245with CDES 90 representing the Carbon Dioxide Equilibrium Scale at a reaction temperature of 90°C. In its entire tem-perature range, ourD47(CDES 90) - T - relationship agrees within 2 ppm with two previousD47(I-CDES) - T - relationshipsreported by Jautzy et al. (2020) and Anderson et al. (2021). Accuracy of our proposedD47(CDES 90)D48(CDES 90)equilibrium relationship is independently confirmed by additional dual clumped isotope data of experimental and geothermalcarbonates which precipitated from potentially equilibrated dissolved inorganic carbon pools at a temperature range of25–100°C. Furthermore, we reprocessed original dual clumped isotope data of natural carbonates (Bajnai et al., 2020)and compared their composition to the position of equilibrium inD47vsD48space. These results corroborate preliminaryevidence that the hydration/hydroxylation reactions became rate-limiting during the calcification of a speleothem-like sample,a warm water coral, a cold water coral and a brachiopod, finally evoking significant departures of carbonate-D47and -D48from dual clumped isotope equilibrium