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