Quaternary glacial events in the Pyrenees from U-series dating of speleothems in the Niaux-Lombrives-Sabart caves, Ariège, France

Located at approximately 150 m above the present base level, the caves of Niaux, Lombrives and Sabart are an old drainage system, which worked between the Vicdessos and Ariège Valleys. In these caves, three successive sedimentary units were studied in detail. Each unit consists of two parts: the lower deposit is detrital, of fluvial origin, and testifies to a hydrological working of the conduit system; the upper deposit of each unit is mostly speleothems, corresponding to a draining of the conduits as the palcokarst evolved above base level. This is similar to the present environment of the caves. Only the speleothems from the upper part of each unit were dated by the 230Th/234 U method. The dates, from 27 samples, are in good agreement with Europe and North America data. The data from the middle (250 to 200 ka BP) and upper (90 to 20 ka BP) fluvial sediments correspond to major glacial events, which re-activated the karst system. Two further events resulted in a lack of carbonated sedimentation, the first between 350 and 290 ka BP, the second between 175 and 130 ka BP. Comparing the data from other places, these two events can be related to glacial periods of minor importance in the study area, because of the lack of erosion and of detrital sediments. The oldest detrital sediments are covered by speleothems older than 350 ka BP and, partly, older than 720 ka BP (from paleomagnctic data to be confirmed); from sedimentary data, they may not be related to a glacial event. They are perhaps contemporary with the area's initial cave formation.