KINETIC ENRICHMENT OF STABLE ISOTOPES IN CRYOGENIC CALCITES

The C-13 and O-18 contents of cryogenic calcites formed by expulsion during the freezing of bicarbonate groundwaters are examined. Samples from karst caves within the permafrost region of northern Yukon, Canada, have deltaC-13-values as high as + 17.0 parts per thousand, representing the most isotopically enriched freshwater carbonates yet reported. To account for such enrichments, calcium bicarbonate solutions were frozen and sublimated under controlled laboratory conditions. The rapid rate of reaction is shown to effectively preclude isotopic equilibration during bicarbonate dehydration, resulting in a kinetic partitioning of C-13 between CO2 and CaCO3. We find a value of 31.2 +/- 1.5 parts per thousand for 1000ln13alpha(KIE)(13alpha(KIE) = 1.032), which is considerably greater than the equilibrium fractionation factor (13epsilon(CaCO3-CO2)) of 10.3 parts per thousand at 0-degrees-C. This kinetic isotope effect (KIE) represents the ratio of the absolute reaction rate constants (13k(d)/12k(d)) for the two isotopic species during the dehydration of dissolved bicarbonate. Similar results for deltaO-18-values confirm that the reaction proceeds without isotope exchange. The KIE of O-18 is determined to be 1.006 for this reaction at 0-degrees-C. These data are compared with the KIE which occurs during the reverse reaction: CO2 hydroxylation by reaction with OH- in hyperalkaline waters.