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 delta O-18 and lower Delta(47) values than those of the accepted thermometer calibrations. These offsets from the common thermometers calibrations increase with decreasing temperature for Delta(47) but decrease with decreasing temperature for delta O-18. The Delta(47)-delta O-18 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 delta O-18 and counteracts some of the degassing related enrichment. This effect seems not to influence Delta(47). 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. (C) 2014 Elsevier Ltd. All rights reserved.