Non-equilibrium 18O and ^{13}C Enrichments in Modern Speleothems

The application of speleothem stable isotope records requires an understanding of the extent to which speleothem isotopic compositions reflect the compositions of the cave waters from which they precipitate. In order to test for equilibrium precipitation, we sampled modern speleothem calcite by coring actively growing speleothems and by growing calcite on glass plates under active drips, thereby allowing the direct comparison of the C and O isotopic composition of the calcite and the water from which it precipitated. At individual sites, only the lowest δ ¹³C values and none of the plate δ ¹⁸O values correspond to equilibrium values. On glass plates, speleothem calcite δ ¹⁸O and δ ¹³C values increase linearly away from the growth axis, with up to 6.6 per mil ¹³C and 1.7 per mil ¹⁸O enrichments. The positive δ ¹³C vs. δ ¹⁸O trends can be accounted for by Rayleigh-distillation of the HCO₃^{-} reservoir due to progressive CO_{2} degassing and CaCO_{3} precipitation, resulting in progressive ^{13}C and ^{18}O enrichment. The \delta^{13}C vs. \delta^{18}O slope is likely controlled by the ability of CO_{2} hydration/hydroxylation reactions to buffer the O isotopic composition of the HCO_{3}$^- reservoir during calcite precipitation. Complete O isotopic buffering of the HCO₃^{-} reservoir by CO^{2} hydration/hydroxylation reactions will produce a vertical \delta^{13}C vs. \delta^{18}O slope in calcite sampled along a growth layer. As O isotopic buffering of the HCO_{3}^- reservoir decreases to no buffering, the modeled δ ¹³C vs. δ ¹⁸O slope in calcite sampled along a growth layer will decrease from vertical to 0.4 at 26.6º C. The glass-plate calcite sampled along the growth layer has a δ ¹³C vs. δ ¹⁸O slope of 3.9, indicating incomplete O isotopic buffering of the HCO₃^{-} reservoir during calcite precipitation. We compiled 158 published speleothem stable isotope records with a global distribution and found that the majority of these records show a positive covariation between \delta^{13}C and \delta^{18}O values. In addition, speleothem records that show positive correlation, no correlation and negative correlation between \delta^{13}C and \delta^{18}O values for the entire record may contain short intervals that show strong positive correlation between \delta^{13}C and \delta^{18}$O values. It is likely that the stable isotopic composition of many speleothems is influenced by the non-equilibrium processes. Direct application of equilibrium fractionation factors may be unwarranted when interpreting speleothem stable isotopic variability in most studies.