CGRG Bibliography of Canadian Geomorphology
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Author : Clark, I.D.; and Lauriol, B.
Date : 1992.
Title : Kinetic enrichment of stable isotopes in cryogenic calcites
Publication : Chemical Geology
Issue : 102(1-4):
Page(s) : 217-228.
Abstract
The 13C and 18O 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 13C-values as high as + 17.0, representing the most isotopically enroched 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 13C between CO2 and CaCO3. We find a value of 31.2 ± 1.5 for 1000ln13KIE(13KIE = 1.032), which is considerably greater than the equilibrium fractionation factor (13CaCO3/1bCO2) of 10.3 at 0°C. This kinetic isotope effect (KIE) represents the ratio of the absolute reaction rate constants (13kd/12kd) for the two isotopic species during the dehydration of dissolved bicarbonate. Similar results for 18O-values confirm that the reaction proceeds without isotope exchange. The KIE of 18O is determined to be 1.006 for this reaction at 0°C. These data are compared with the KIE which occurs during the reverse reaction: CO2 hydroxylation by reaction with OH- in hyperalkaline waters.
Bibliography of Canadian Geomorphology