Ar-40/Ar-39 dating on tephra of the Velay maars (France): implications forthe Late Pleistocene proxy-climatic record

Pollen contents and magnetic records of the Pleistocene sedimentary sequenc es cored in the Praclaux and Lac du Bouchet maars (Velay region, French Mas sif Central) demonstrated that they have high potential as detailed climati c and environmental archives. Tephra layers interbedded in these sequences have been studied in order to perform isotopic dating. First, petrographic studies and Ar-40/Ar-39 ages have shown that these tephra layers are the wi tness of the same eruption from the Sancy volcano centre, located 100 lan t o the north. According to fourteen Ar-40/Ar-39 ages, carried out on sanidin e and biotite grains, this eruption occurred 275 +/- 5 ka ago. This result allows to construct a time scale for a composite high resolution record of the climate covering the last 300,000 years and to establish an absolute ch ronology for the palaeoclimatic events recorded in these sequences. These t ephra layers are interbedded in organic layers, deposited during an interst adial stage called the Amargiers interstadial which, according to its age, can be considered as equivalent to isotopic stage 9a. Moreover, the geograp hical extension of the tephra and its well-constrained age give to this tep hra the interest of a chronostratigraphic marker for upper Quaternary seque nces in southeastern France. (C) 1999 Elsevier Science B,V. All rights rese rved.