A new mechanism of phosphoric acid digestion reaction and theoretical re-calibration on the carbonate 13C-18O clumped isotope thermometry

The carbonate 13C-18O clumped isotope thermometry has been widely used for surface temperature systems. However, the existing D47-T relationships calibrated by different groups are incompatible, generating substantial confusions in the community. Here we propose a new molecular-level mechanism of the kinetic isotope effect (KIE) produced in the phosphoric acid digestion reaction of carbonates. We will show why the previous mechanism won't work and why this new one is reasonable, especially based on experimental evidences. This new mechanism, which suggests that there are three parallel pathways undergoing for the phosphoric acid digestion process, can explain well the discrepancies of the D47-T relationships obtained by different groups at slightly different experimental conditions (Fig.1). Based on the new mechanism and sophisticated quantum chemistry calculations, we show that the clumped isotope enrichments (i.e., Δ*47) can be significantly different from those provided by Guo et al. (2009) if considering the change of individual contributions of the three parallel pathways. Consequently, if we use these new Δ*47 values to plot the D47-T figure under the absolute reference frame (ARF), all the calibration lines conducted at 90°C (e.g., Dennis et al. (2011)) will be relocated. They can be much closer to those calibration lines conducted at 25°C than before in the low temperature range (Fig.1). In another word, there may not be much difference between these two groups of calibration lines. However, we obtain a slope very close to those conducted at 90°C but with distinct difference to that of Ghosh et al. (2006). Finally, we provide molecular-level explanations on the changes of calibration lines if the experiments are conducted under slightly different conditions. We show why the temperature of phosphoric acid digestion, the reaction time and the amount of sample can affect the position of calibration line but not the slope of it. This new mechanism not only can resolve existing disagreements on the carbonate D47-T calibrations but also can explain well the d18O enrichment differences (i.e., 1000lna*) in the phosphoric acid digestion conducted by different groups.