PhD Subject for a 3-year contract starting in September 2015: Geomagnetic dipole moment variations since 3 Ma reconstructed by coupling paleomagnetism and cosmogenic nuclide geochemistry.


The hypothesis of a modulation of the geomagnetic field intensity by the effects of the axial precession and obliquity of the Earth’s on the outer core fluid mechanics is nowadays supported by numerical and analogic modeling. However, clues provided by sedimentary paleomagnetism at the Million year scale remain unclear and ambiguous because sediment magnetization processes are often suspected to be influenced by unremovable orbitally constrained paleoenvironmental conditions. Therefore, we choose to reconstruct time series of atmospheric cosmogenic Beryllium 10 production rate which depends on the geomagnetic field intensity over the last 3 Ma (Brunhes and Matuyama epochs) and check their spectral content. High sedimentation rate (5 à 10 cm/ka) sedimentary sequences were selected at low and middle latitudes in order to cover the Brunhes and  Matuyama epochs which contain numerous dipole field collapses that triggered excursions and reversals. The Berylllium adsorbed on the mineral fraction (authigenic Be), extracted using leaching procedures, will be measured by accelerator mass spectrometry (AMS) (Be-10) and by flameless absorption spectrophotometry (Be-9). The Be-10/Be-9 ratio will allow reconstructing Be-10 production variations. Cosmogenic Be-10 production time series, as well as paleomagnetic and paleoclimatic time series, will be analyzed in order to check their phase relationships and measured their frequency spectra ( spectral analyses and wavelet).


The required competences are combining geochemistry / geophysics and data analyses.


Research Directors :

Nicolas THOUVENY and Didier BOURLES

Professors at Aix-Marseille University

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__________________________________________________________ Régis BRAUCHER Chargé de recherche CNRS CEREGE UMR 7330 LN2C (Laboratoire National des Nucléides Cosmogéniques) Plateau d'Arbois BP 80 13545 Aix en Provence FRANCE Tel: 33 (0)4 42 97 15 09 Fax: 33 (0)4 42 97 15 59