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Kinetic fractionation of O-18 and clumped isotopes (CO)-C-13-O-18 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 epsilon(13,18)(CaCO3-HCO3) in a cave analogue experiment of calcite precipitation (Hansen et at 2019) that shows a dependence on experimental precipitation rates, E An expression, epsilon(kin)(CaCO3-HCO3) = epsilon-lambda.C-eq( )ab alpha(ab)(prec)/F, 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 c(eq) with respect to calcite. The constant, epsilon, is expressed by the eq kinetic fractionation epsilon=1-alpha(rare)(prec)/alpha(ab)(prec) where alpha denote the rate constants of precipitation for the rare and abundant isotopologues. The second constant, lambda, is determined by the differing equilibrium concentrations of HCO3- isotopologues with respect to calcite and the P-CO2 in the surrounding atmosphere. Fitting this expression to the experimental data one obtains the parameters epsilon and lambda for different temperatures. Regarding these results the temporal evolution of (18)delta(CaCO3) (t) and Delta(47) (t) is discussed for the experimental conditions and for cave environments. This has implications to the application of 1000ln alpha(18)(CaCO3)-(H20) as a paleo-thermometer. It shows the reason why so many differing calibrations have been reported. These results analogously can be applied also to clumped isotopes (CO)-C-13-O-18 and the calibration of the Delta(47)-thermometer with regard to spcleothcm calcite. In summary, a better understanding of the problems arising in the search for generally valid calibrations of O-18 and Delta(47) paleothermometers is presented.

acta carsologica

Kinetic fractionation of the isotope composition of 18O, 13C, and of clumped isotope 18O13C in calcite deposited to speleothems. Implications to the reliability of the 18O and Δ47 paleothermometers