Premiers ages U/Th sur la Grotte des Demoiselles (Hérault): Evolution karstique et relation paléoclimatique

The U/Th method has been applied to eight speleothem samples from the Demoiselles Cave (Herault, France), as a first attempt to decipher the history of this cave and compare it to that of another cave in the same region of southern France (Clamouse Cave). Mass spectrometry (U and Th contents) and a spectrometry [( 234 U/ 238 U) ratios and ( 230 Th) activities] have been used to date these samples. The U contents of the eight samples are very low, between 0.05 and 0.19 ppm. The samples have very low 232 Th contents, and thus appear essentially devoid of detrital contamination. The initial ( 234 U/ 238 U) activity ratios calculated for five samples are constant (within 2σ uncertainties) with a mean value of 1.14. Such a low ratio is consistent with the rapid infiltration of the seepage water through a thin, strongly fractured limestone cover in shallow oxidizing conditions. The ages determined here are consistent with a growth of speleothems during warm and humid periods, with at least four stages of deposition: the first two are older than 250-300 ky; the two most recent episodes correspond to the last interglacial period (isotopic sub-stages 5.1 and 5.5, ca 80 and 120 ky respectively), and to the Holocene. The same sub-stages of the last Interglacial correspond to two main episodes of deposition in the nearby Clamouse Cave. Major deposition certainly took place during the two early stages, whereas the two later stages seem to have only added a final touch to the cave ornamentation. In this respect, the Demoiselles Cave appears different from the Clamouse Cave, where the recent stages are the most developed. This can be attributed to the thicker rock cover above the roof of the Clamouse Cave.