Glacial isostatic adjustment and Earth rotation: Refined constraints on the viscosity of the deepest mantle

We explore the extent to which two properties of Earth's present-day rotational state may be applied to constrain the viscosity of the lower regions of the lower mantle. The analysis builds upon recent advances in understanding the radial resolving power of the relative sea level data of postglacial rebound which have demonstrated anew that no significant increase of viscosity across the 660-km seismic discontinuity appears to be allowed by these data. Such observations do not constrain the viscosity of the mantle below about 1400 km depth, however, and here we estimate the average viscosity in this deepest region by invoking observations of the present-day rate and direction of polar wander and the present-day magnitude of the nontidal acceleration of the rate of planetary rotation. Since the rotational response to the glaciation cycle depends only upon the degree 2 spherical harmonic components of the induced deformation, this response is expected to provide the most specific sampling possible of deep mantle properties. Our analysis is based upon the use of glacial excitation functions calculated from complete gravitationally self-consistent solutions of the sea level equation for the most recent refinement of the history of ice sheet thickness variations across the last glacial-interglacial transition, namely, the ICE-4G model. This analysis shows that when Earth rotation observations are combined with relative sea level constraints, the viscosity of the deepest mantle is required to be similar to 1 order of magnitude higher than the viscosity of the upper part of the lower mantle. Such significant elevation of viscosity in the lowermost region of the lower mantle is also in accord with recent inferences based upon analysis of the nonhydrostatic geoid. The analyses presented herein therefore suffice to effect a reconciliation between the requirements of these distinct geophysical observations.