Deep-Earth Geophysics

Morphogenesis and dissipation at the Core-Mantle Boundary:
At the core-mantle boundary (CMB), there is a striking contrast in physical and chemical properties between the mantle and the core. The mantle is rocky, composed mostly of crystalline silicate with a density of  5600 kg/m3 near the CMB, while the core consists of a dense (density of 104 kg/m3) molten iron alloy with low viscosity. The CMB is therefore expected to be the seat of physical and chemical interactions between core and mantle (see Narteau et al., 2001),. The hot liquid core can corrode the overlying mantle and dissolve it. The core fluid is consequently enriched in light elements, and may become saturated enough to crystallise. In any case, it is likely that an ocean of light elements is floating on the top of the core. In our work, we try to analyse such a layer with classical tools of sediment transport and bedform dynamics. Thus, the CMB may acquire a roughness and a topography on length scales larger than the grain size of the mantle material, superimposed to the topography  at the scale of hundreds to thousands km, due to mantle convection. If it happens to be the case, we will estimate the dissipation resulting from the interaction of the fluid core motion with this topography (see  Le MouŽl et al., 2006).

Reversal of the Earth magnetic fields:

The behaviour of the geomagnetic field over geological times, with its long periods of stable polarity (chrons) and its quick reversals, is one of the most  fascinating phenomena in Earth's Sciences, whose mechanism has not yet been fully elucidated. From a multiscale alpha/omega dynamo model (see Narteau and Le MouŽl, 2005), we concentrate on the phenomenology of reversals and the duration of chrons. In the model, cyclones at different length scales contribute to the alpha-effect.During polarity chrons, a progressive population inversion invariably yields to a change of sign of the alpha-effect which initiates reversals. The dipole weakens and turbulent motions are able to cascade through a wide range of length scales to produce large fluctuations of the alpha-effect. These fluctuations (1) reinforce magnetic fields with opposite polarity, or (2) trigger large instabilities in direction and intensity of the dipole. This behaviour is reflected in the real Earth by the existence of chrons and reversals. The simulations also predict different degrees of complexity in the reversal process which depends on the spontaneous change in sign of the alpha-effect. The existence of similar oscillations in the detailled reversal records suggest that geomagnetic reversal would result from equivalent underlying mechanisms (see Narteau et al., 2007).