KINETIC AND EQUILIBRIUM CONTROLS ON SPELEOTHEM ISOTOPIC VARIATIONS
The d13C values of the majority of the modern calcite samples fall within the range of values predicted from the combined use of (1) fractionation factors from the literature, (2) measured temperatures and (3) measured d13C values of dissolved-inorganic-carbon (DIC). Some samples range from ~2.0 higher to ~2.0 lower than predicted values. The modern calcite 13C depletions are likely caused by kinetically driven changes in the fractionation between HCO3- and CaCO3 from equilibrium conditions (D13C CaCO3 - HCO3- ~ 2.0) to non-equilibrium conditions (D13C CaCO3 - HCO3- ~ 0), possibly caused by rapid growth of speleothem calcite. Modern calcite 13C enrichments may be caused by degassing of 13C-depleted CO2. Through Rayleigh distillation, this degassing would increase the d13C value of the HCO3- reservoir, as well as the CaCO3 precipitated from it.
The d18O values of modern calcite are not consistent with equilibrium precipitation (based on the combined use of fractionation factors, measured temperatures and measured d18O values of drip waters), and show 0.2 to 2.3 isotopic enrichments. Similar to the C-isotope system, the d18O value of modern calcite is likely controlled by kinetically-driven changes in the fractionation between HCO3- and CaCO3 from inferred equilibrium conditions (D18O CaCO3 - HCO3- ~ -6.3) to non-equilibrium conditions (D18O CaCO3 - HCO3- ~ 0), possibly caused by rapid growth of speleothem calcite. In contrast to the C-isotope system, the d18O value of modern calcite may not be affected by Rayleigh distillation because the d18O value of HCO3- is theoretically calculated to lie half way between that of CO2 and CaCO3. Degassing of 18O-enriched CO2 is thereby offset by precipitation of correspondingly 18O-depleted CaCO3.