Series of Geomagnetic dipole lows (GDL) associated with polarity reversals and excursions have been reconstructed for the Brunhes and Matuyama epochs by coupling and cross-checking paleomagnetic records (relative paleointensity, RPI) and cosmogenic Beryllium records (authigenic 10Be/9Be ratio) along ocean clayey-carbonate sequences deposited at rates of 2 to 20 cm/kyr. The 10Be production is enhanced (quasi-doubled) at the time of polarity reversals (i.e. the B/M transition at ca 772 ka) and excursions (i.e. (Laschamp event at 41 ka). This strategy allows detecting eventual stratigraphic offsets (few cm to several dm) between the two expressions of a given GDL. In such cases, 10Be peaks being eventually located above RPI minima, this demonstrates the reality and importance of considering non-negligible locking depth of the (post-) depositional remanent magnetization and thus delays that reduce the precision/reliability of paleomagnetic markers as ultra-high resolution markers for intercorrelation and dating sedimentary archives. Meanwhile, the calibration of 10Be/9Be records using empirical and theoretical relationships between cosmogenic nuclide production and the geomagnetic dipole moment values, allows producing 10Be derived dipole moment records, independent from RPI sedimentary paleomagnetic records. These provide useful time series for spectral analyses and computing rates of changes of the past geomagnetic moment across reversals and excursions, and thus offer a reliable pacing frame to the 103 to 106 years scale regime of the Earth’s dynamo, opening perspectives on the secular to millenial decay of the modern geomagnetic field.
This study was funded by the French ANR (Agence Nationale de la Recherche) and by the ERC (European Research Council) FP7/2007–2013 advanced grant 339899
Co-authors: Q. SIMON1, D. BOURLES1, F. BASSINOT2, J.- P. VALET3
1- CEREGE AMU,CNRS,IRD,INRA, Coll. de Fr., Aix en Provence, France.
2- Lab. Sci. Climat Envir. LSCE, CEA – CNRS - UVSQ, Saclay, France.
3- Int. Phys. Globe, IPGP, Université Paris Sorbonne, France.
Nicolas THOUVENY is Geologist (paleo/rock magnetist) at CEREGE Aix en Provence, France. Doctorate in 1983 he entered CNRS in 1985 in the Quaternary Geology Lab. In Marseille; he passed his habilitation in 1990. He left CNRS in 1997 for a Professor position at UNIVERSITY of AIX-MARSEILLE. He was Director of CEREGE from 2010 to 2017, and is since January 2018 the Director of the Institute for Research and Observation in Space, Earth, Ocean and Biosphere Sciences, called OSU-Institut Pythéas (a Marseillan mathematician, astronomer, geodesist and explorator of the antiquity (330 av. JC) who measured the obliquity of the ecliptic, the latitude of Marseille and described the sunny polar nights.