13 C 18 O clumping in speleothems: Observations from natural caves and precipitation experiments M. Dae ¨ron a,c,⇑ , W. Guo b,c , J. Eiler c , D. Genty a , D. Blamart a , R. Boch d , R. Drysdale e , R. Maire f , K. Wainer a , G. Zanchetta g a Laboratoire des Sciences du Climat et de l’Environnement, CEA – CNRS – UVSQ, Gif-sur-Yvette, France b Geophysical Laboratory, Carnegie Institution of Washington, Washington, DC, USA c Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, CA, USA d Fakulta ¨t fu ¨ r Geo- und Atmospha ¨ renwissenschaften, University of Innsbruck, Austria e Melbourne School of Land and Environment, University of Melbourne, Australia f UMR 5185 Ame ´nagement, De ´veloppement, Environnement, Sante ´ et Socie ´te ´s, CNRS, Pessac, France g Dipartimento di Scienze della Terra, University of Pisa, Italy Received 14 December 2009; accepted in revised form 26 October 2010; available online 21 March 2011 Abstract The oxygen isotope composition of speleothems is an important proxy of continental paleoenvironments, because of its sensitivity to variations in cave temperature and drip water d 18 O. Interpreting speleothem d 18 O records in terms of absolute paleotemperatures and d 18 O values of paleo-precipitation requires quantitative separation of the effects of these two param- eters, and correcting for possible kinetic isotope fractionation associated with precipitation of calcite out of thermodynamic equilibrium. Carbonate clumped-isotope thermometry, based on measurements of D 47 (a geochemical variable reflecting the statistical overabundance of 13 C 18 O bonds in CO 2 evolved from phosphoric acid digestion of carbonate minerals), potentially provides a method for absolute speleothem paleotemperature reconstructions independent of drip water composition. Appli- cation of this new technique to karst records is currently limited by the scarcity of published clumped-isotope studies of mod- ern speleothems. The only modern stalagmite reported so far in the literature yielded a lower D 47 value than expected for equilibrium precipitation, possibly due to kinetic isotope fractionation. Here we report D 47 values measured in natural speleothems from various cave settings, in carbonate produced by cave pre- cipitation experiments, and in synthetic stalagmite analogs precipitated in controlled laboratory conditions designed to mimic natural cave processes. All samples yield lower D 47 and heavier d 18 O values than predicted by experimental calibrations of thermodynamic equilibrium in inorganic calcite. The amplitudes of these isotopic disequilibria vary between samples, but there is clear correlation between the amount of D 47 disequilibrium and that of d 18 O. Even pool carbonates believed to offer excellent conditions for equilibrium precipitation of calcite display out-of-equilibrium d 18 O and D 47 values, probably inherited from prior degassing within the cave system. In addition to these modern observations, clumped-isotope analyses of a flowstone from Villars cave (France) offer evi- dence that the amount of disequilibrium affecting D 47 in a single speleothem can experience large variations at time scales of 10 kyr. Application of clumped-isotope thermometry to speleothem records calls for an improved physical understanding of DIC fractionation processes in karst waters, and for the resolution of important issues regarding equilibrium calibration of D 47 in inorganic carbonates. Ó 2011 Elsevier Ltd. All rights reserved. 0016-7037/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.gca.2010.10.032 ⇑ Corresponding author at: Laboratoire des Sciences du Climat et de l’Environnement, CEA – CNRS – UVSQ, Gif-sur-Yvette, France. Tel.: +33 6 59 41 50 35. E-mail address: mathieu@daeron.fr (M. Dae ¨ron). www.elsevier.com/locate/gca Available online at www.sciencedirect.com Geochimica et Cosmochimica Acta 75 (2011) 3303–3317