Detrital magnetization of marine sediments. Signals in natural environment and laboratory experiments
Start: 01 October 2012
End: 26 April 2016
Supervisor(s):
Jean-Pierre Valet, Julie Carlut
Related team(s):
Paleomagnetism
Status: Sustained
Doctoral dissertation - Cyrielle Tanty
This thesis deals with the acquisition of detrital remanent magnetization by marine sediments by coupling the study of magnetic signals from natural sediments deposited in a turbulent environment (turbidites) and collected in situ with those of natural and artificial sediments redeposited in the laboratory. The study of four turbidites of different origins, thickness and composition showed that their sedimentary and magnetic granulometries decrease as they rise in the column and thus that sedimentary particles and coarse magnetic grains reach the bottom first. The exception is the smallest turbidite. For the three largest events, the distribution of magnetic grain size with depth satisfies a common scaling law. Another feature is the decrease in the alignment of magnetic particles between the upper and lower layers. Sediments deposited under calmer conditions (small turbidites and hemipelagic layers) are characterized by good magnetic alignment. These results suggest that hydrodynamic conditions and the amount of mobilized particles strongly influence the degree of magnetic grain alignment. The absence of bioturbation and the rapid accumulation of sediment prevent any reorientation of the magnetic grains. Re-deposition experiments have shown that detrital remanent magnetization is strongly dependent on the presence and concentration of sediment and to a lesser extent on lithology. Concentration inhibits the rotation of magnetic grains. The detrital magnetization acquired in a field of the same intensity as the Earth, decreases with the concentration of sediment and reaches values that are almost zero above 55%. Experiments in the presence of magnetofossils also suggest that the techniques used for relative paleointensity reconstructions in sediments are appropriate. The types of magnetic particles (pseudo-monodomain, multidomain, and monodomain) have the same acquisition profiles as a function of sediment concentration but the values of the magnetization ratios are different. The ratios are closer to those of a natural sediment in the presence of BMTs. The linearity of the magnetization with magnetic field intensity is confirmed up to 100 µT in the case where the magnetic carriers are BMTs and no saturation effect is observed. For carbonate concentrations of 20-40%, 10% of the magnetic grains seem to be able to reorient after deposition, which is comparable to natural sediments.
