Palaeoclimate Research in Villars Cave (Dordogne, SW-France)
Titel:
Palaeoclimate Research in Villars Cave (Dordogne, SW-France)
Auteur:
Genty Dominique
Verschenen in:
International journal of speleology
Paginering:
Jaargang 37 (2008) nr. 3 pagina's 173-191
Jaar:
2008
Inhoud:
Villars Cave is a typical shallow cave from South-West France (45.44°N; 0.78°E; 175 m asl) that has provided several speleothempalaeoclimatic records such as the millennial scale variability of the Last Glacial period and the Last Deglaciation. Monitoring theVillars cave environment over a 13-year period has helped in the understanding of the stable isotopic speleothem content and inthe hydrology. For example, it was demonstrated that most of the calcite CaCO3 carbon comes from the soil CO2, which explainsthe sensitivity of the δ13C to any vegetation and climatic changes. Drip rate monitoring, carried out under four stalactites from thelower and upper galleries, has shown a well marked seasonality of the seepage water with high flow rates during winter and spring.A time delay of about two months is observed between the water excess (estimated from outside meteorological stations) and thedrip rate in the cave. A great heterogeneity in the flow rate amplitude variations and in the annual quantity of water between twonearby stalactites is observed, confirming the complexity of the micro-fissure network system in the unsaturated zone. At a dailyscale, the air pressure and drip rates are anti-correlated probably because of pressure stress on the fissure network. Cave air CO2concentration follows soil CO2 production and is correlated with its δ13C content. Since the beginning of the monitoring, the cave airtemperature, in both lower and upper galleries, displays a warming trend of ~+0.4°C±0.1/10yrs. This might be the consequence ofthe outside temperature increase that reaches the Villars Cave galleries through thermal wave conduction. Chemistry monitoringover a few years has shown that the seepage water of the lower gallery stations is significantly more concentrated in trace and minorelements (i.e. Sr, Mg, Ba, U) than the upper stations, probably due to the 10-20 m depth difference between these galleries, whichimplies a different seepage pathway and different water/rock interaction durations. There is also, in the elemental concentration(i.e. [Ca]), a seasonal signal which causes variation in the speleothem growth rates. Modern calcite deposit experiments conductedfor several years have permitted the calculation of vertical growth rates, which are extremely high in Villars (i.e. 1.0 to 1.75 mm/yr). Pollen filter experiments in the cave have demonstrated that most of the pollen grain found in the cave comes from the air andnot from the water. The specificity of the Villars Cave records is that the climatic variations were well recorded in the calcite δ13Cwhereas the δ18O is usually used in such studies. Overall, these results are helpful for the interpretation of speleothem records forpalaeoclimatic reconstructions, but more work is needed, especially numerical modelling of the temperature, chemistry and hydrology.
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