Laboratory precipitation experiments provide the basis for the common calibration of both the oxygen isotope and the clumped isotope thermometers. These focus on CaCO 3 crystals that form deep in the bulk of the solution, often by the bubbling of N 2(g) through a saturated Ca(HCO 3 ) 2 solution, following the classic experiments on McCrea (1950). Here we examine oxygen and clumped isotopes in CaCO 3 that was precipitated at the surface of a solution that undergoes passive CO 2 degassing. This CaCO 3 is affected by enhanced disequilibrium that is associated with degassing, and has therefore higher δ 18 O and lower Δ 47 values than those of the accepted thermometer calibrations. These offsets from the common thermometers calibrations increase with decreasing temperature for Δ 47 but decrease with decreasing temperature for δ 18 O. The Δ 47 -δ 18 O 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 δ 18 O and counteracts some of the degassing related enrichment. This effect seems not to influence Δ 47 . The physical setup of CaCO 3 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.
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