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Large kinetic isotope effects in modern speleothems

Patrick J. Mickler^,1, Libby A. Stern^,1 and Jay L. Banner^,1

¹ Department of Geological Sciences, University of Texas, Austin, Texas 78712, USA

The application of stable isotopes in speleothem records requires an understanding of the extent to which speleothem calcite isotopic compositions reflect the compositions of the cave waters from which they precipitate. To test for equilibrium precipitation, modern speleothem calcite was grown on glass plates, so that the carbon and oxygen isotope composition of the calcite and the water from which it precipitated could be directly compared. The plates were placed on the tops of three actively growing stalagmites that occupy a 1 m² area in Harrison's Cave, Barbados, West Indies. Only some of the plate ¹³C values and none of the plate ¹⁸O values correspond to equilibrium values, indicating significant kinetic isotope effects during speleothem calcite growth. We investigate herein mechanisms that may account for the kinetic isotope effects.

On each plate, speleothem calcite was deposited with distinct ¹⁸O and ¹³C compositions that increase progressively away from the growth axis, with up to 6.6 ¹³C and 1.7 ¹⁸O enrichments. The positive ¹³C versus ¹⁸O trends are likely a result of ¹⁸O and ¹³C Rayleigh-distillation enrichment in the HCO₃^– reservoir owing to progressive CO₂ degassing and CaCO₃ precipitation. The magnitude of the ¹³C versus ¹⁸O slope is likely controlled by the extent to which CO₂ hydration-hydroxylation reactions buffer the oxygen isotope composition of the HCO₃^– reservoir during calcite precipitation. Complete oxygen isotopic buffering of the HCO₃^– reservoir by CO₂ hydration-hydroxylation reactions will produce a vertical ¹³C versus ¹⁸O slope in calcite sampled along a growth layer. As oxygen isotope buffering of the HCO₃^– reservoir decreases to no buffering, the ¹³C versus ¹⁸O slope in calcite sampled along a growth layer will decrease from vertical to 0.52 at the cave temperature. In this study, modern speleothem calcite sampled along the growth layer produced a ¹³C versus ¹⁸O slope of 3.9, indicating incomplete oxygen isotope buffering of the HCO₃^– reservoir during calcite precipitation.

Both modern and Holocene speleothem calcite from Barbados, sampled temporally along the growth axis, shows similar positive ¹³C versus ¹⁸O slopes. These results, along with the spatial variations in glass plate calcite carbon and oxygen isotope compositions, suggest that the isotopic composition of the Holocene speleothems is in part controlled by non-equilibrium isotope effects. In addition, there is a correlation between stalactite length and oxygen and carbon isotope ratios of calcite precipitated on the corresponding stalagmite and glass plate, which may be due to ¹³C and ¹⁸ O enrichment of the HCO₃^– reservoir during CO₂ degassing–calcite precipitation along the overhanging stalactite.

We compiled 165 published speleothem stable isotope records with a global distribution and found that most of these records show a positive covariation between ¹³C and ¹⁸O values. Speleothem stable isotope records may be influenced by kinetic isotope effects such that temperature-controlled equilibrium fractionation models alone cannot directly explain the significance of the variations in these records. Advancing the interpretation of these records requires the calibration of cave environmental conditions with the non-equilibrium isotope effects that cause ¹³C and ¹⁸O covariations in speleothems.

Key Words: kinetic isotope effects • speleothems • oxygen isotopes • carbon isotopes • non-equilibrium • Barbados

This article has been cited by other articles:

				J. L. Banner, A. Guilfoyle, E. W. James, L. A. Stern, and M. Musgrove Seasonal Variations in Modern Speleothem Calcite Growth in Central Texas, U.S.A. Journal of Sedimentary Research, August 1, 2007; 77(8): 615 - 622. [Abstract] [Full Text] [PDF]