Glacial isostatic adjustment and the free air gravity anomaly as a constraint on deep mantle viscosity.

Abstract

A gravitationally self-consistent theory for relative sea-level variations forced by Pleistocene deglaciation events is employed to explore the extent to which RSL and free air gravity observations together constrain the viscosity of the mantle beneath the seismic discontinuity at 670 km depth. A trade-off is revealed between errors in the assumed deglaciation history and errors in the inferred value of the viscosity of the lower mantle. Taking full account of such uncertainty, plausible values of the viscosity beneath the transition region are bounded above by 1023 poise (cgs units). The preferred value is at least a factor of 2 lower than this and is strongly constrained by the observed free air gravity anomaly over Hudson Bay. The calculations described in detail here show for the first time that the relatively large gravity anomalies observed over sites of Würm- Wisconsin deglaciation do not require any pronounced increase of mantle viscosity with depth in order to explain them.