文摘
Viscosity of the D?layer of the Earth¡¯s mantle, the lowermost layer in the Earth¡¯s mantle, controls a number of geodynamic processes, but a robust estimate of its viscosity has been hampered by the lack of relevant observations. A commonly used analysis of geophysical signals in terms of heterogeneity in seismic wave velocities suffers from major uncertainties in the velocity-to-density conversion factor, and the glacial rebound observations have little sensitivity to the D?layer viscosity. We show that the decay of Chandler wobble and semi-diurnal to 18.6 years tidal deformation combined with the constraints from the postglacial isostatic adjustment observations suggest that the effective viscosity in the bottom ?00 km layer is 1019-1020 Pa s, and also the effective viscosity of the bottom part of the D?layer (?00 km thickness) is less than 1018 Pa s. Such a viscosity structure of the D?layer would be a natural consequence of a steep temperature gradient in the D?layer, and will facilitate small scale convection and melt segregation in the D?layer.
