Ph-D position in Geophysics
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We are seeking a PhD student in Geophysics. The project concentrates on the geomagnetic field variations during the last geomagnetic reversal to document the mechanisms governing the field during these critical transitional periods. The project combines modelling and laboratory measurements. The first phase of the study relies on calculation of 10Be production during reversals in presence of multipolar fields which likely dominates the transition between the two polarity states. The second phase involves paleomagnetic measurements combined to high resolution 10Be measurements from a large number of sedimentary cores that recorded the reversal from different locations around the globe. The candidate will have excellent training in Geophysics and Geomagnetism, good skill at modelling and a strong motivation for laboratory measurements.
Sedimentary and volcanic records of the geomagnetic field intensity provide the unique source of information on the evolution of the geodynamo. The paleomagnetic data contain a broad spectrum of dipole moment fluctuations with polarity reversals and excursions that typically occur during periods of very low field intensity. Much has been learned for the past 30 years from records of paleointensity relying on natural remanent magnetization of sediments, but uncertainties persist concerning the amplitude and the timing of the variations.
The 10Be cosmogenic nuclide provides an alternative approach to study changes in geomagnetic intensity. During periods of low geomagnetic dipole strength and, therefore, lower shielding (cut-off rigidity), the flux of galactic cosmic rays to the atmosphere increases and generates additional collisions with atoms which increase the production of cosmonuclides. The relationship between geomagnetic dipole strength and cosmogenic nuclides production is well understood. However 10Be production is not so well constrained in presence of complex multipolar fields which very likely prevail during reversals. The amount of meridional mixing of 10Be in the upper atmosphere is another important factor. In the present dipolar field configuration, calculations of 10Be production rate as a function of latitude show very little production below 30° of latitude, most of it being concentrated close to the pole. However, there are evidences of 10Be production at low latitudes during large geomagnetic events, suggesting either the existence of inter-latitudinal mixing in the atmosphere or the proximity to a pole since the field geometry is not dipolar any more during the reversal. The relationship between the 10Be signal and relative paleointensity should indicate the distance to the closest pole and by iteration at multiple locations document the field geometry when the field reverses.
The first part of the thesis will concentrate on predictions of 10Be production at various locations by simulating reversals dominated by multipolar fields with and without latitudinal atmospheric mixing. The second aspect will focus on the acquisition of detailed records of relative paleointensity. In parallel, high resolution measurements of oxygen isotopes will be conducted to construct the time scale of each record. We will favour carbonated sediments to acquire detailed oxygen isotopic stratigraphies from the variations of the 16O /18O ratio measured in foraminifera.
The magnetic measurements will be conducted at IPGP while the isotopic data will be acquired at LSCE (Gif/Yvette). The 10Be measurements will be conducted on the mass accelerator (ASTER) of Cerege (Aix en Provence). The PhD student will have excellent training in Geophysics and geomagnetism, excellent ability in computers and a strong motivation for experimental studies and magnetc measurements.
Nr of positions available : 1
Early stage researcher or 0-4 yrs (Post graduate)
First Stage Researcher (R1)
|Degree||Master Degree or equivalent|
|Degree||Master Degree or equivalent|
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