Low viscosity of the bottom of the Earth's mantle inferred from the analysis of Chandler wobble and tidal deformation

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 18.6years tidal deformation combined with the constraints from the postglacial isostatic adjustment observations suggest that the effective viscosity in the bottom ∼300km layer is 10 ¹⁹-10 ²⁰Pas, and also the effective viscosity of the bottom part of the D' layer (∼100km thickness) is less than 10 ¹⁸Pas. 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.