Kinetic isotope effect in CO₂ degassing: Insight from clumped and oxygen isotopes in laboratory precipitation experiments

Laboratory precipitation experiments provide the basis for the common calibration of both the oxygen isotope and the clumped isotope thermometers. These focus on CaCO₃ crystals that form deep in the bulk of the solution, often by the bubbling of N_2(g) through a saturated Ca(HCO₃)₂ solution, following the classic experiments on McCrea (1950). Here we examine oxygen and clumped isotopes in CaCO₃ that was precipitated at the surface of a solution that undergoes passive CO₂ degassing. This CaCO₃ is affected by enhanced disequilibrium that is associated with degassing, and has therefore higher δ¹⁸O and lower δ₄₇ values than those of the accepted thermometer calibrations. These offsets from the common thermometers calibrations increase with decreasing temperature for δ₄₇ but decrease with decreasing temperature for δ¹⁸O. The δ₄₇-δ¹⁸O co-variance is therefore strongly temperature dependent, in contrast to theoretical predictions. This suggests an influence of additional fractionation in fast growing minerals, between DIC and calcite, that affects δ¹⁸O and counteracts some of the degassing related enrichment. This effect seems not to influence δ₄₇. The physical setup of CaCO₃ minerals growing at the solution surface is analogous to the degassing and precipitation processes that occur in stalagmites, such that these laboratory experiments may help shed light on the isotopic disequilibrium that is observed in speleothems.