4 Million Years of stable and unstable geomagnetic polarity states: a tribute to Jean-Pierre VALET

Jean-Pierre Valet started his CNRS research career in 1982 at the “Centre des Faibles Radioactivités” (Gif sur Yvette). His Doctorate studies, led by C. Laj, delt with the detailed study of the Earth’s magnetic field reversals recorded in the Mio-Pliocene marine sediments of Crète (Eastern Mediterranean sea). After his Doctorate thesis held in 1985, he spent one year in the paleomagnetism laboratory led by L. Tauxe at the Scripps Institution of Oceanography (California). From 1990 Jean-Pierre Valet joined the Geomagnetism and Paleomagnetism Team of the « Institut de Physique du Globe de Paris » as Director of research and joined coring and drillin campaigns on the R.V. Marion-Dufresne and ODP/IODP cruise of the Joide-Resolutin. Studying magnetization of marine sediment sequences, he and his team deeply improved the understanding of the dynamics of polarity changes and provided robust stacked records of the geomagnetic dipole moment variation over the last 2 Ma (e.g. Sint-2000). They revealed a long term decay of the dipole field intensity prior the reversals, followed by rapid return to strong dipole field intensity after the reversals, suggesting a progressive loss of the geodynamo energy followed by a strong and rapid gain of energy drawn from the reversal. In 2010, Jean-Pierre joined a project developed since 2000 at CEREGE (Nat. Lab. Cosmogenic nuclides) by N.Thouveny and Didier Bourlès (1955-2021), conceived to decipher geomagnetic dipole intensity lows and highs using both paleomagnetism and accelerator mass spectrometry for detection of the cosmogenic isotope Be-10. From 2015 to 2020, this project was led by Jean-Pierre with 2.5 M€ funding by the European Research council. The final results describe and critically analyze the 4 Ma proxy records of the geomagnetic dipole variations produced both by sediment magnetization paleointensity records and by cosmogenic Beryllium production records tuned by the magnetospheric screening of cosmic rays (e.g. Valet et al. 2024, Valet et al., submitted). Jean-Pierre Valet authored about 200 peer-reviewed articles and directed 12 PhD. CNRS Silver medal (2001), Fellow of the American Geophysical Union since 2003, Petrus Peregrinus Medal of the European Union of Geosciences (2010).