Equilibrated gas and carbonate standard-derived paired clumped isotope (Δ47 and Δ48) values on the absolute reference frame

Rationale: Carbonate clumped isotope geochemistry has primarily focused on mass spectrometric determination of mass-47 CO2 for geothermometry, but theoretical calculations indicate paired analysis of the mass-47 (13C-18O-16O) and mass-48 (12C-18O-18O) isotopologues (denoted with Δ47 and Δ48 notation) can be used to study non-equilibrium isotope fractionations and refine temperature estimates. We are one of two labs currently utilizing paired Δ47 and Δ48 measurements to study equilibrium and kinetic isotope effects in carbonates. Additional work is needed on different instruments to define standard Δ48 values against equilibrated gases and evaluate Δ48 values using carbonate transfer functions. Methods: We determined Δ47 and Δ48 of standards using isotope ratio mass spectrometry during the time interval of 2015-2021 on a Thermo Fischer MAT 253 mass spectrometer with a common acid bath digestion system and two Nu Instruments Perspective mass spectrometers with common acid bath and individual reaction vessel digestion systems. A total of 5,581 Δ47 and 4,212 Δ48 measurements of carbonates, and 183 Δ47 and 195 Δ48 measurements of gas standards are used from robust correction intervals over multiple years. We report statistical methods for data screening and quality assurance. Results: Equilibrated gas-based standard values support the accuracy of the recently proposed Δ47 I-CDES reference frame and provide robust constraints on Δ48 values for carbonate standards. We provide further constraints on the equilibrium Δ47 vs Δ48 relationship and report values of Devils Hole cave calcite. We demonstrate regression-based acid digestion fractionation factors, Δ*63-47 and Δ*64-48, agree with experimental data. Conclusions: We show that accurate determination of paired Δ47 and Δ48 is possible using both equilibrated gas and carbonate-based standardization methods. We report new equilibrium Δ47 vs Δ48 regressions that can be used to identify kinetic effects and quantify biases that may affect temperature reconstructions from unknown samples and constrain regression-based acid digestion fractionation factors.