Mantle Rheology, Viscomagnetic Coupling At the Core-Mantle Boundary and Differential Rotation of the Core Induced By Pleistocenic Deglaciation

Abstract

S U M M A R YThe influence of the viscomagnetic coupling at the core-mantle boundary (CMB) on the differential rotation of the core and non-tidal acceleration induced by the Pleistocenic deglaciation is investigated for stratified mantle models with steadystate or transient rheologies. For a realistic modelling of the viscomagnetic coupling, the time-dependent viscoelastic topography at the CMB induces a westward drift of the core with respect to the mantle that can be correlated to the zonal component of the secular drift of the geomagnetic field. Starting from a reference model based on the Maxwell rheology and a viscosity profile inferred from J2 data and non-tidal acceleration, with 1021 P a s and 5 x 1021 P a s for upper and lower mantle respectively, we study first the effects of the hardening of the upper mantle in the transition zone between 420 and 670 km depth. This hardening is responsible for a 30 per cent reduction in the westward drift with respect to a uniform upper mantle. The impact of a viscosity decrease on the top of the lower mantle is considered next. The softening of the lower mantle beneath the 670 km discontinuity counteracts the effects of the transition zone, enhancing the differential rotation of the core. The D" layer is also implemented in order to analyse the influence of the decoupling at the bottom of the mantle, responsible for faster relaxation of the topography. If lower mantle viscosity is increased beyond the threshold of 10"Pas we obtain a differential rotation of the core in the opposite direction or an eastward drift. We also analyse the impact of a Burgers rheology for the lower mantle to simulate transient effects. For this rheology, steady-state viscosities in the lower mantle higher than Pa s, in agreement with estimates from long-wavelength geoid anomalies and recent findings from true polar wander, allow a westward drift of the core.Non-tidal acceleration is generally less affected by rheological variations than the differential rotation. Changes in the non-tidal acceleration induced by the various rheological models are, on the other hand, comparable with the error bounds in the observed values.

Secular Polar Motion and the Estimation of Mean Lower Mantle Viscosity

“…Table 2 has the ratio of at least 99.8 percent of all the ice sheet's mass. Therefore, the estimates of the ν LM can probably be acceptable, and its value differs from that obtained by Peltier et al [10] , but approximately agree with the recent results [8,9,[11][12][13][14]24] .…”

“…However, according to the relationship between mantle rheology and the true SDEP, Richard et al [11] concluded that the lower mantle requires a viscosity of at least 10 22 Pa·s to lower the computed rate of TPW to values observed by palaeomagnetism. The similar results of ν LM were confirmed by Lefftz et al [12] , Vermeersen et al [13] , and Mitrovica et al [14] , respectively.…”

Global post-seismic deformation

“…Spada ef af. 1991b; Lefftz, Sabadini & Legros 1994). More details about the spheroidal relaxation spectrum are presented in Section 2.4.…”

Gravitational core‐mantle coupling and the acceleration of the Earth

“…The calculations also assume that the outer core rotates like a rigid body relative to the mantle, when in fact it is a convecting liquid. Further, convection in the outer core will exert pressure, electromagnetic, and viscous forces at the core‐mantle boundary; these are not including here [ Lefftz et al , 1994; Fang et al , 1996]. The sum total of these effects would have to be calculated to obtain the complete torque.…”

“…Coupling between the liquid outer core of the Earth and the mantle has long been a suspected reason for changes in the length of day [ Le Mouel et al , 1993; Stacey , 1992; Lambeck , 1980]. Research heretofore has been aimed at viscous, electromagnetic, and topographic coupling at the core‐mantle boundary [e.g., Lefftz et al , 1994], mostly on the decadal timescale, and at gravitational coupling not related to the problem outlined below [ Getino , 1995; Xu and Szeto , 1994].…”