Numerical tests on the seismic visibility of metastable minerals in subduction zones

Pollen contents and magnetic records of the Pleistocene sedimentary sequences cored in the Praclaux and Lac du Bouchet maars (Velay region, French Massif Central) demonstrated that they have high potential as detailed climatic and environmental archives. Tephra layers interbedded in these sequences have been studied in order to perform isotopic dating. First, petrographic studies and 40Ar/39Ar ages have shown that these tephra layers are the witness of the same eruption from the Sancy volcano centre, located 100 km to the north. According to fourteen 40Ar/39Ar ages, carried out on sanidine 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 chronology for the palaeoclimatic events recorded in these sequences. These tephra layers are interbedded in organic layers, deposited during an interstadial stage called the Amargiers interstadial which, according to its age, can be considered as equivalent to isotopic stage 9a. Moreover, the geographical extension of the tephra and its well-constrained age give to this tephra the interest of a chronostratigraphic marker for upper Quaternary sequences in southeastern France.