Rainfall and cave water isotopic relationships in two South-France sites
Résumé
This article presents isotopic measurements (δ18O and δD) of precipitation and cave drip water from two sites in southern France in order to investigate the link between rainfall and seepage water, and to characterize regional rainfall isotopic variability. These data, which are among the longest series in France, come from two rainfall stations in south-west France (Le Mas 1996–2012, and Villars 1998–2012; typically under Atlantic influence), and from one station in the south-east (Orgnac 2000–2012; under both Mediterranean and Atlantic influence). Rainfall isotopic composition is compared to drip water collected under stalactites from the same sites: Villars Cave (four drip stations 1999–2012) in the south-west, and Chauvet Cave (two drip stations 2000–2012) in the south-east, near Orgnac. The study of these isotopic data sets allows the following conclusions to be drawn about the rainfall/drip water relationships and about rainfall variability: (1) the cave drip water isotopic composition does not show any significant changes since the beginning of measurements; in order to explain its isotopic signature it is necessary to integrate weighted rainfall δ18O of all months during several years, which demonstrates that, even at shallow depths (10–50 m), cave drip water is a mixture of rain water integrated over relatively long periods, which give an apparent time residence from several months to up to several years. These results have important consequences on the interpretation of proxies like speleothem fluid inclusions and tree-ring cellulose isotopic composition, which are used for paleoclimatic studies; (2) in the Villars Cave, where drip stations at two different depths were studied, lower δ18O values were observed in the lower galleries, which might be due to winter season overflows during infiltration and/or to older rain water with a different isotopic composition that reaches the lower galleries after years; (3) local precipitation is characterized by local meteoric water lines, LMWL, with δ18O/δD slopes close to 7 in both areas, and correlations between air temperature and precipitation δ18O are low at both monthly and annual scales, even with temperature weighted by the amount of precipitation; (4) the mesoscale climate model REMOiso, equipped with a water isotope module, allows the direct comparison of modeled and observed long term water isotope records. The model slightly overestimates rainfall δ18O at the respective sampling stations. However, it simulates very well not only the seasonal rainfall isotopic signal but also some intra-seasonal patterns such as a typical double-peak δ18O pattern in winter time.