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 andMatuyama 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 
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