Kinetic fractionation of the isotope composition of18O,13C, and of clumped isotope18O13C in calcite deposited to speleothems. Implications to the reliability of the18O and Δ47-paleothermometers

Abstract

Kinetic fractionation of18O and clumped isotopes13C18O in calcite precipitated to speleothems in cave environments renders the paleo-climatic interpretation of these proxies difficult. Therefore a better understanding of the processes generating the isotope imprint is needed. A heuristic approach is taken to interpret recent data of the fractionations in a cave analogue experiment of calcite precipitation (Hansen et al. 2019) that shows a dependence on experimental precipitation rates, F. An expression,, is derived that is based on uni-directional irreversible precipitation and is valid for large F when the forward rate of precipitation dominates the backward rate of dissolution. In that derivation it is assumed that the kinetic constants of precipitation rates are different for the different isotopologues and that this is also true for their equilibrium concentrations ceq with respect to calcite. The constant, ε, is expressed by the kinetic fractionation where α denote the rate constants of precipitation for the rare and abundant isotopologues. The second constant, λ, is determined by the differing equilibrium-concentrations of HCO3 isotopologues with respect to calcite and the pCO2 in the surrounding atmosphere. Fitting this expression to the experimental data one obtains the parameters ε and λ for different temperatures. Regarding these results the temporal evolution of18δCaCO3 (t) and Δ47 (t) is discussed for the experimental conditions and for cave environments. This has implications to the application of 1000lnα18CaCO3-H2Oas a paleo-thermometer. It shows the reason why so many differing calibrations have been reported. These results analogously can be applied also to clumped isotopes13C18O and the calibration of the Δ47-thermometer with regard to speleothem calcite. In summary, a better understanding of the problems arising in the search for generally valid calibrations of18O and Δ47 paleothermometers is presented.