Theoretical Calibration on the 13C-18O Clumped Isotope Thermometer

Abstract:

The kinetic isotope effect (KIE) arisen from phosphoric acid digestion of carbonates has been noticed since the beginning of stable isotope geochemistry. However, the molecular level details of this reaction have not been fully understood yet. Equilibrium ¹³C-¹⁸O clumped isotope distribution in carbonates has been suggested as a new thermometer for surface temperature systems; nevertheless, existing Δ₄₇-T relationships calibrated by several different groups are incompatible, generating substantial confusions and debates about those variations. Here we propose a new molecular-level mechanism with three parallel pathways for the phosphoric acid digestion of carbonates. We show that the KIE of such reaction can be different if the relative contributions of the three parallel pathways are changed. This new mechanism abandons completely a previously proposed molecular mechanism (i.e., Guo et al., 2009) and can explain (at least partly) why the Δ₄₇-T relationships provided by different groups are different. Together with a re-calculated equilibrium clumped isotope fractionation factors of carbonate minerals using a new volume-variable-cluster-model method with higher theoretical-level treatments and higher-order anharmonic corrections, we present a theoretical (equilibrium + KIE) Δ₄₇-T relationship for carbonates. Our theoretical calibration line is with large variations due to considering possible changes of relative contributions of the three parallel pathways and different carbonate mineral used. For minimizing the variation, we suggest using the same amount of sample, the same mineral and the same temperature of phosphoric acid digestion for this experiment.