Plain language summary
Clumped isotope analysis is a highly accurate method for reconstructing temperatures in Earth’s past climate from calcium carbonate fossils such as shells and corals. Unfortunately, calibration studies so far were predominantly based on samples of calcite, a common calcium carbonate mineral. It is therefore unknown whether these clumped isotope calibrations, yield accurate temperature reconstructions when applied on aragonite, a different carbonate mineral of which corals and many shells are made. Therefore, we grew mollusks that build their shell out of aragonite in a lab by constant water temperatures to test the clumped isotope method on aragonitic shells. We find no significant difference in the temperature sensitivity of the method between our aragonites and the previous calibrations and show that the temperature calibration can be improved by combining data from different minerals. However, we find subtle differences in the temperature dependence of clumped isotopes between hot (>100°C) carbonates and cold (<30°C) carbonates, which cause previous calibrations to underestimate temperatures of colder carbonates. We conclude that using carbonate samples grown at temperatures close to the temperatures of the samples used in climate reconstructions can eliminate a bias of 2.7 ± 2.0°C (95% confidence level), resulting in more accurate reconstructions of past temperatures.