2.3 Clumped isotope analysis
The clumped isotope composition of 278 aliquots of shell aragonite were analyzed over two 6-month periods (March – August 2020; May – November 2021) on two Thermo isotope ratio mass spectrometers (one MAT253 and one MAT253 plus) coupled to Kiel IV carbonate preparation devices (seeAppendix IX ). Clumped isotope results were processed relative to the Intercarb-Carbon Dioxide Equilibrium Scale (I-CDES) through an empirical transfer function (ETF) based on measurements of ETH standards (ETH-1, -2 and -3) and their accepted I-CDES values (Bernasconi et al., 2021). Isotopic values were calculated using the latest IUPAC values (Daëron et al., 2016). No AFF was applied since I-CDES standardization is assumed to correct for acid fractionation (Bernasconi et al., 2021). Long-term accuracy and reproducibility of ∆47 results were assessed based on repeated measurements of the IAEA-C2 check standard (∆47_IAEA on MAT253 plus: 0.6382 ± 0.026‰; ∆47_IAEA on MAT253: 0.6445 ± 0.046‰; 1σ). Results were indistinguishable from the accepted value for IAEA-C2 (0.6409 ± 0.003‰; 95% CL; Bernasconi et al., 2021). Full results of all sample aliquots and standards used to standardize the results are provided inS2 .