COBISS: 1.01 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 .47- PALEOTHERMOMETERS KINETIČNA FRAKCIONACIJA IZOTOPOV 18O, 13C IN IZOTOPSKEGA SKUPKA 18O13C V SIGAH IN ZANESLJIVOST PALEOTERMOMETROV 18O IN .47 Wolfgang DREYBRODT1, 2 Abstract UDC 552.545:54.027 Wolfgang Dreybrodt: 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 .47 paleothermometers 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, ., is expressed by the kinetic frac­tionation where . denote the rate constants of precipitation for the rare and abundant isotopologues. The second constant, ., 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 . and . for different temperatures. Regarding these results the temporal evolution of 18.CaCO3 (t) and .47(t) is discussed for the experimental conditions and for cave environments. This has implications to the application of 1000ln.18CaCO3-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 .47-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 .47 paleo-thermometers is presented. Key words: Calcium carbonate, 13C and 18O isotopes, clumped isotopes, kinetic fractionation between HCO3- and calcite, .47 paleothermometer. Izvleček UDK 552.545:54.027 Wolfgang Dreybrodt: Kinetična frakcionacija izotopov 18O, 13C in izotopskega skupka 18O13C v sigah in zanesljivost paleo­termometrov 18O in .47 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 . lahko izrazimo s kinetično frakcionacijo , kjer je . konstanta izločanja za redke oziro­ma večinske izotopologe. Drugo konstanto . dobimo iz razlik ravnotežnih koncentracij izotopologov HCO3- glede na kalcit in na atmosferski pCO2 . S prilagajanjem izraza eksperimental­nim podatkom dobimo . in . pri različnih temperaturah. To omogoča obravnavo časovne odvisnosti 18.CaCO3 (t) in .47(t) pri eksperimentalnih pogojih in v jamskem okolju. Rezultati so pomembni za razumevanje uporabnosti paleotermometra 1000ln.18CaCO3-H2O in hkrati pokažejo na vzrok različnih kalib­racij, kot jih zasledimo v literaturi. Rezultate lahko uporabimo tudi za izotopske skupke 13C18O in kalibracijo .47- za kalcitno sigo. Članek predstavi nov pogled na iskanje splošno veljavne kalibracije paleotermometrov 18O in .47. Ključne besede: kalcit, izotopa 13C in 18O, izotopski skupki, kinetična frakcionacij a med HCO3- in kalcitom, paleotermom­eter .47. 1 Faculty of Physics and Electrical Engineering, University of Bremen, Germany 2 Karst Research Institute ZRC SAZU, Titov trg 2, 6230 Postojna, Slovenia e-mail: dreybrodt@t-online.de Received/Prejeto: 01.09.2019 DOI: https://doi.org/10.3986/ac.v48i3.7710 ACTA CARSOLOGICA 48/3, 329-340, POSTOJNA 2019 WOLFGANG DREYBRODT INTRODUCTION The isotope compositions of 13C and 18O in calcite pre­cipitated inorganically to speleothems nowadays are widely used as proxies of paleo-climate (Fairchild & Baker 2012). To this end the temperature dependence of the calcite-water fractionation factor (1000 ln18 .calcite-water) from theoretical, laboratory and cave studies has been suggested as calibration to the growth temperature of speleothems. In the ideal case when precipitation of the calcite proceeds at very low growth rates in isotope equilibrium the 18O isotope composition reflects the tem­perature of growth. The calibration of this line has been constructed from two measurements of isotope fraction­ation factors 18.calcite–water of calcites from Devils Hole and Laghetto Basso (Corchia Cave, Italy) that grow extremely slow with growth rate of 10-11 mmol cm-2 s-1 in isotope equilibrium at 33.7°C (Coplen 2007) and 7.9°C (Daeron et al. 2019). However, most calcite deposited in caves to speleothems or farmed to glass plates at known tempera­ture and also synthetic calcite obtained from the labora­tory exhibit fractionation 1000 ln 18.calcite-water that deviates from the Coplen-Daeron line thus rendering its use as paleo-thermometer questionable. The reason is kinetic frctionation that depends on the difference of rate con­stants and equilibrium concentrations in the precipita­tion rate laws of calcium and bicarbonate for the different isotopologues with respect to calcite (Dreybrodt 2008, 2016; Dreybrodt & Scholz 2011). Little is perceived in the community about the pro­cesses that determine the evolution of the isotope com­position when water deposits calcite to a stalagmite. One usually assumes that classical Rayleigh distillation (Mook 2000) is active during precipitation of calcite. Using this concept a fractionation factor is found. If it deviates from the known equilibrium value, as is mostly the case, ki­netic fractionation is inferred. Recently Hansen et al. (2019) reported the evolution of the isotope composition of DIC and calcite deposited from a thin film of water super saturated with respect to calcite in a cave analogue experiment. They obtained val­ues that deviated unsystematically from the equilibrium values and inferred kinetic fractionation without further explanation. These results have recently been analyzed and interpreted by a kinetic model (Dreybrodt 2019). In a second step they have explored the evolution of the fractionations and . They sug­gested a dependence of this fractionation on the corre­sponding precipitation rates of calcite. Here we present a heuristic model that explains their observations and sheds light into the meaning of kinetic fractionation. Us­ing these results the reasons for the problems in using the 18O and also the .47 paleothermometers are discussed. 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 .47 PALEOTHERMOMETERS WOLFGANG DREYBRODT 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 .47 PALEOTHERMOMETERS WOLFGANG DREYBRODT 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 .47 PALEOTHERMOMETERS WOLFGANG DREYBRODT 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 .47 PALEOTHERMOMETERS WOLFGANG DREYBRODT 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 .47 PALEOTHERMOMETERS WOLFGANG DREYBRODT