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.