Pleistocene Glaciation and the Viscosity of the Lower Mantle

The non-tidal acceleration of the Earth, revealed by astronomical observations and records of eclipses in antiquity, is attributed to the change in the Earth's moment of inertia resulting from isostatic response to the most recent deglaciation and rise in sea level. The isostatic response time for a spherical harmonic deformation of degree two is calculated on this basis to be either ∼ 2000 y or ∼ 100 000 y. A correlation of the geopotential with the potential that would have existed following deglaciation indicates that any large scale anomalies resulting from deglaciation have already decayed. This rules out the 100 000 relaxation time; thus the relaxation time of the Earth is ∼ 2000 y for degree two. Calculations of the relaxation time spectrum of a layered, gravitating spherical viscous earth model indicates that a model with a uniform mantle viscosity of ∼ 1022 poise, except for fine structure in the upper few hundred kilometres, can satisfy a relaxation time of 3000 y for degree two as well as the relaxation time of ∼ 4000 y for degree twenty which results from studies of post-glacial uplift in Fennoscandia. A zone of high viscosity in the lower 800 km of the mantle has a significant effect on the degree two relaxation time. This rules out any substantial increase in viscosity in the lower mantle. The calculated viscosity permits rapid polar wandering and convection in the lower mantle.