New insights obtained from joint inversions for the radial profile of mantle viscosity

We discuss recent inferences of mantle viscosity obtained via formal joint inversion of data associated with glacial isostatic adjustment (henceforth 'GIA') and mantle convection. The former includes a set of decay times obtained from post-glacial relative sea level histories at sites in previously glaciated regions, while the latter includes spherical harmonic coefficients (up to degree and order 8) of the present-day free-air gravity anomaly field. The analyses described here indicate that many previous inferences of an isoviscous mantle based on GIA data sets may have been significantly biased by a misinterpretation of the resolving power of the so-called 'Haskell number' (10²¹ Pas). Furthermore, we find that both the GIA and convection data sets are well fit by a radial viscosity profile characterized by a large (~ 2 orders of magnitude) increase from the base of the lithosphere to the core-mantle-boundary. This demonstration indicates that there is no compelling reason to invoke a transient mantle rheology. Final word in regard to these issues awaits an improvement in the quality of the GIA data base.