Available online at www.ijs.speleo.it International Journal of Speleology Offcial Journal of Union Internationale de Spéléologie International Journal of Speleology 36 (1) 51-56 Bologna (Italy) January 2007 INTRODUCTION Carbon dioxide dynamics and transfer from soil to the deepest parts of an aquifer underlie the main karstifcation processes. Cavity ventilation plays an important role in speleothem formation and chemical equilibrium of water in the unsaturated zone of the aquifer. Meteorological parameters (barometric pressure and temperature), cave geometry and fracture networks control exchanges between subterranean and outside atmosphere (De Freitas et al., 1982; De Freitas & Littlejohn, 1987; Smithson, 1991; Christoforou, 1996; Buecher, 1999; Fernandez-Cortes, 2005). Gases currently studied in caves are CO 2 and 222 Rn. Their concentrations and variations can be explained by different parameters. Carbon dioxide concentrations in cavities vary from 0.03 % to more than 6 % and may have different sources : Batiot-Guilhe C., Seidel J.L., Jourde H., Hébrard O., Bailly-Comte V. 2007. Seasonal variations of CO 2 and 222 Rn in a Mediterranean sinkhole - spring (Causse d’Aumelas, SE France). International Journal of Speleology, 36 (1), 51-56. Bologna (Italy). ISSN 0392-6672. Carbon dioxide and 222 Rn monitoring of the atmosphere of a Mediterranean sink hole - spring (SE France) during two hydrological cycles (from September 2004 to September 2006) showed seasonal variations with very high concentrations during summer (greater than 6% and 20 000 Bq/m 3 , respectively). Gas dynamics in caves often show seasonal variations. Meteorological parameters (barometric pressure and temperature mainly), cave geometry and fracture networks control exchanges between the cavity and outside atmosphere. Carbon dioxide and 222 Rn may have different sources (atmosphere, soil, bedrock, deep gas diffusion, in situ oxidation of organic matter and, in some caves, the key role of swift underground streams). For a CO 2 origin, 13 C measurements on water and gas samples taken into the cavity suggest a superfcial origin. Radon-222 appears to be locally produced and transported by biogenic CO 2 . Further investigations will be carried out in order to study the relationship of gas-level variations with barometric pressure variations and piezometric level fuctuations within the aquifer. Keywords: : CO 2 , 222 Rn, Karst, Cave, 13 C, France, Languedoc-Roussillon Abstract: Received 7 July 2006; Revised 27 September 2006; Accepted 24 November 2006 - atmosphere which is characterized by low values (0.03 %; Renault, 1968), - soil, where biological activity produces one hundred times more CO 2 on average than in the atmosphere, and then is transported by water seeping through the soils and network of rock voids (Atkinson, 1977; White, 1988). In Mediterranean limestone soils, pCO 2 varies from 0.5 to 2% (Batiot, 2002), - oxidation by bacteria of organic matter in carbonated rocks or cavity deposits, - deep gas diffusion or transport (Wood & Petraitis, 1984; Wood, 1985). - in some caves, draughts cause, for instance, by a swift underground stream (Ek & Gewelt, 1985). High 222 Rn contents have been reported in caves and are primarily related to the uranium content of the bedrock or to 226 Ra, decay progeny of 238 U, readily transported from the source and precipitated as RaCO 3 on cave walls and deposits present in the cavity. Radon-222 is produced by the 238 U decay series and has a short half-life (3.82 days). Several studies carried out on 220 caves worldwide have shown concentrations ranging from 0.1 to 20 kBq/m 3 (Hakl et al., 1997a, b; Dueñas et al., 1999; Cigna, 2005). 1 Laboratoire HydroSciences Montpellier, Université Montpellier II, Maison des Sciences de l’Eau, Place Eugène Bataillon, CC MSE, F-34095 MONTPELLIER Cedex 5, France seidel@msem.univ-montp2.fr Paper presented during the International Conference Aqua in Med’06, Malaga (Spain) 24-28 April 2006. Seasonal variations of CO 2 and 222 Rn in a mediterranean sinkhole - spring (Causse d’Aumelas, SE France) Christelle Batiot-Guilhe 1 , Jean-Luc Seidel 1 , Hervé Jourde 1 , Olivier Hébrard 1 , Vincent Bailly-Comte 1