The Bernard Brunhes Site Revisited

The famous Brunhes article establishing the reality of geomagnetic reversals was pub- lished in 1906. Brunhes' first researches in the field, however, date back to 1901-1902. It seems, therefore, appropriate to celebrate this 100th anniversary jointly with the 25th anniversary of the GSA publication of the classic Gubbio magnetostratigraphic study by our friend and colleague William Lowrie. We will report on the results obtained from the baked clays and the lava flow, which were resampled in 2001 at the original site at Pont Farin (Pontfarein in 1906), some 20 km west of Saint Flour. This formation belongs to the lower Miocene lava plateau locally called " Planèze ". The direction of the characteristic remanent magnetization is very close to the direction determined by Brunhes, which is the NRM direction, as no demagnetization techniques were avail- able at the time. WeSll discuss on how Bernard Brunhes was aware of the possibility that secondary magnetizations may have overprinted the original magnetization, and how he could rule out the possibility of significant overprint. We'll also discuss on how he realized that concordant directions from the clays and the lava would strengthen his argument for paleomagnetic stability, leading him to perform the first baked contact test of paleomagnetism. Paleointensity determinations, out of reach for Bernard Brun- hes, performed with the Thellier-Thellier technique indicate that the intensity of the field was about 30-35 microT around 20 millions years ago in Central France.