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

Abstract

Kinetic fractionation of 18O and clumped isotopes 13C18O 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 of 18δCaCO3 (t) and Δ47(t) is discussed for the experimental conditions and for cave environments. This has implications to the application of 1000lnα18 CaCO3-H2O 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 13C18O 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 of 18O and Δ47 paleothermometers is presented. Key words: Calcium carbonate, 13C and 18O isotopes, clumped isotopes, kinetic fractionation between HCO3- and calcite, Δ47 paleothermometer. Kinetična frakcionacija izotopov 18O, 13C in izotopskega skupka 18O13C v sigah in zanesljivost paleotermometrov 18O in Δ47 Kinetična frakcionacija 18O in skupka 13C18O v kalcitu, ki se kot siga odlaga v jamskih okoljih, dela težave pri interpretaciji paleoklime na osnovi teh proksijev. Zato potrebujemo boljše razumevanje procesov, od katerih je odvisen izotopski zapis v sigah. V tem delu s hevrističnim pristopom interpretiramo nedavno pridobljene podatke frakcionacij . Podatki, pridobljeni ob izločanju kalcita v pogojih, podobnih jamskim, kažejo, da na frakcionacijo bistveno vpliva hitrost izločanja kalcita (Hansen et al. 2019). V pogojih, ko je izločanje bistveno hitrejše od raztapljanja, velja zveza . Pri izpeljavi te enačbe upoštevamo, da so kinetične konstante izločanja in ravnotežna konstanta kalcita različne za različne izotopologe. Konstanto ε lahko izrazimo s kinetično frakcionacijo , kjer je α konstanta izločanja za redke oziroma večinske izotopologe. Drugo konstanto λ dobimo iz razlik ravnotežnih koncentracij izotopologov HCO3- glede na kalcit in na atmosferski pCO2 . S prilagajanjem izraza eksperimentalnim podatkom dobimo ε in λ pri različnih temperaturah. To omogoča obravnavo časovne odvisnosti 18δCaCO3 (t) in Δ47(t) pri eksperimentalnih pogojih in v jamskem okolju. Rezultati so pomembni za razumevanje uporabnosti paleotermometra 1000lnα18CaCO3-H2O in hkrati pokažejo na vzrok različnih kalibracij, kot jih zasledimo v literaturi. Rezultate lahko uporabimo tudi za izotopske skupke 13C18O in kalibracijo Δ47- za kalcitno sigo. Članek predstavi nov pogled na iskanje splošno veljavne kalibracije paleotermometrov 18O in Δ47. Ključne besede: kalcit, izotopa 13C in 18O, izotopski skupki, kinetična frakcionacij a med HCO3- in kalcitom, paleotermometer Δ47.