40Ar/39Ar dating of a tephra layer in the Pliocene Senèze maar lacustrine sequence (French Massif Central): constraint on the age of the Réunion–Matuyama transition and implications on paleoenvironmental archives

The sedimentary sequence cored in the Senèze maar (Velay, France) is very favourable for magnetostratigraphy of the Late Pliocene and for studying the paleoenvironmental changes that occurred during that period. The magnetostratigraphic studies revealed that the sediments from this lacustrine sequence were deposited during the Matuyama reverse period, except for the upper part where a short normal polarity episode was recorded. A tephra layer interbedded in the sediments contemporaneous with this normal polarity event is dated at 2.10±0.01 Ma by the 40Ar/39Ar method. Using this chronological marker, it can be inferred that the normal polarity episode recorded in the Senèze sequence corresponds to the Réunion event and that the age of the normal–reverse transition closing this event is ca. 2.09 Ma. These chronological constraints also permit to (1) demonstrate that the ‘Villafranchian’ mammal fauna found in the Senèze maar is younger than 2.09 Ma and (2) correlate the climatic events recorded in the Senèze sequence to the marine δ18O records. This suggests that the Senèze pollen sequence (5–120 m depth) ranges from isotopic stage 85 to 76.