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 .