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

The sedimentary sequence cored in the Seneze maar (Velay, France) is very f avourable for magnetostratigraphy of the Late Pliocene and for studying the paleoenvironmental changes that occurred during that period. The magnetost ratigraphic studies revealed that the sediments from this lacustrine sequen ce were deposited during the Matuyama reverse period, except for the upper part where a short normal polarity episode was recorded. A tephra layer int erbedded in the sediments contemporaneous with this normal polarity event i s dated at 2.10 +/- 0.01 Ma by the Ar-40/Ar-39 method. Using this chronolog ical marker, it can be inferred that the normal polarity episode recorded i n the Seneze sequence corresponds to the Reunion event and that the age of the normal-reverse transition closing this event is ca. 2.09 Ma. These chro nological constraints also permit to (1) demonstrate that the 'Villafranchi an' mammal fauna found in the Seneze maar is younger than 2.09 Ma and (2) c orrelate the climatic events recorded in the Seneze sequence to the marine delta O-18 records. This suggests that the Seneze pollen sequence (5-120 m depth) ranges from isotopic stage 85 to 76. (C) 2000 Elsevier Science B.V. All rights reserved.