Effects of different constants and standards on the reproducibility of carbonate clumped isotope (Δ47) measurements: Insights from a long-term dataset

Rationale Carbonate clumped isotope (Delta(47)) thermometry examines the temperature-dependent excess abundance of the C-13-O-18 bond in the carbonate lattice. Inconsistent temperature calibrations and standard values have been reported among laboratories, which has led to the use of equilibrated gases and carbonate standards for standardization. Furthermore, different acid fractionation factors and isotopic parameter sets have been proposed for improving inter-laboratory data comparability. However, few long-term datasets have been generated to explore the effects of these factors on the long-term reproducibility of Delta(47) data within a laboratory. Methods Four standards (ISTB-1, NBS-19, GBWO4416, and GB04417) were analyzed as unknowns by isotope ratio mass spectrometry from 2015 to 2019. The values of Delta(47) were calibrated using the ETH standards. We investigated the Assonov, Brand, and Gonfiantini isotope parameter sets for carbon and oxygen isotopes, as well as two correction schemes of equilibrated gas and carbonate standardization, using the same sample measurements to determine which procedures enhanced reproducibility. ISTB-1 (calcite) and ZK312-346W (dolomite) were measured to determine the 90 degrees C acid fractionation factor. Results The corrected 90 degrees C acid fractionation factors are 0.076 +/- 0.008 parts per thousand for ISTB-1 and 0.077 +/- 0.009 parts per thousand for ZK312-346W. The choice of isotope parameter set had no significant influence on final Delta(47) values in this study. However, using the Assonov parameters to calculate Delta(47) values improved the reproducibility of the results. The use of carbonate standards improved reproducibility through time compared with the use of equilibrated gases for standardization. Conclusions At 90 degrees C, the acid fractionation factors of calcite and dolomite are statistically indistinguishable. We find an insignificant effect from changing the isotope parameter set, suggesting that the choice of isotope parameter set among laboratories is not a major factor affecting inter-laboratory reproducibility. We find that using carbonate standards improved the reproducibility of results, suggesting that the use of carbonate standards may help to achieve inter-laboratory comparability of results in future studies.