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

Abstract

Rationale Carbonate clumped isotope (Δ47) thermometry examines the temperature‐dependent excess abundance of the 13C–18O 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 Δ47 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 Δ47 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°C acid fractionation factor. Results The corrected 90°C acid fractionation factors are 0.076 ± 0.008‰ for ISTB‐1 and 0.077 ± 0.009‰ for ZK312‐346W. The choice of isotope parameter set had no significant influence on final Δ47 values in this study. However, using the Assonov parameters to calculate Δ47 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°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.

Carbonate clumped isotope analysis (Δ47) of 21 carbonate standards determined via gas‐source isotope‐ratio mass spectrometry on four instrumental configurations using carbonate‐based standardization and multiyear data sets

“…We note that the reported interlaboratory variance for isotope ratios determined for the four standards that were calculated using carbonate‐based standardization (ETH‐4, IAEA‐1, IAEA‐2, and MERCK) in Bernasconi et al 21 ranges from 0.05‰ to 0.10‰, and for the four standards that were calculated using gas standardization (ETH‐1 to ETH‐4), the variance ranges from 0.05‰ to 0.09‰. A comparison of our determined values for ISTB‐1 with published values from Chang et al 42 indicates an offset of 0.04‰, which is within the range of variance between laboratories reported by Bernasconi et al 21 …”

“…These include ETH‐1, ETH‐2, ETH‐3, ETH‐4, IAEA‐C1, IAEA‐C2, and MERCK, as described in Bernasconi et al, 17 and NBS‐19, SRM 88B (dolomite), Mallinckrodt Calcium Carbonate, 102‐GC‐AZ01, Carmel Chalk, Carrara Marble, CMTile (another homogenized Carrara Marble), Spel 2‐8‐E, TV01, TV03, and VeinStrom. Carbonate standards obtained from the California Institute of Technology (CIT) and China University of Geosciences (CUG) include Coral Std, ISTB‐1, TB‐1, and TB‐2 42 . These carbonates span a range of δ 47 that is more than 30‰ and ~0.5‰ in Δ 47 .…”

The rise and demise of the Paleogene Central Tibetan Valley

“…The methodology in the ITPCAS is similar to that of the University of Chicago, except that the powdered carbonate samples were reacted with 1 to 2 ml of anhydrous phosphoric acid (ρ = ~1.93 g/cm 3 ) at 90°C for 15 min and that the measurement of Δ 47 is conducted on a 235 Plus IRMS. We further construct a standard transfer function to standardize our samples and the effect of acid digestion at 90°C ( 64 ).…”

Ice Sheet Expansion in the Cretaceous Greenhouse World

“…The raw Δ 47 values were corrected for non-linearity and baseline background effects of the mass spectrometer by empirical calibration 50 , and then standardised from a working gas reference frame to an absolute reference frame (ARF), which allows for interlaboratory comparison 51,52 . We further constructed a standard transfer function (STF) 53,54 to standardise our samples and the effects from different acid digestion temperatures 55 .…”