RADIAL PROFILE OF MANTLE VISCOSITY - RESULTS FROM THE JOINT INVERSIONOF CONVECTION AND POSTGLACIAL REBOUND OBSERVABLES

We present new inferences of the radial profile of mantle viscosity th at simultaneously fit long-wavelength free-air gravity harmonics assoc iated with mantle convection and a large set of decay times estimated from the postglacial uplift of sites within previously glaciated regio ns (Hudson Bay, Arctic Canada, and Fennoscandia). The relative sea lev el variation at these latter sites is constrained by age-height pairs obtained by geological survey, rather than the subjective trends which are commonly used in glacial isostatic adjustment (GIA) studies. Our viscosity inferences are generated using two approaches. First, we ado pt a relative viscosity profile which is known to provide a good fit t o the free-air gravity harmonics and determine an absolute scaling whi ch yields a best fit to the GIA decay time constraints. Second, we per form an iterative, nonlinear, joint inversion of the two data sets. In both cases our inferred profiles are characterized by a significant i ncrease of viscosity (similar to 2 orders of magnitude), with depth, t o values of similar to 10(22) Pa s in the bottom half of the lower man tle. The new viscosity profiles are shown to satisfy constraints based on the postglacial uplift of both Fennoscandia (the classic Haskell [ 1935] number) and Hudson Bay which have commonly been invoked to argue for an isoviscous mantle. Furthermore, the models are used to predict a set of long-wavelength signatures of the GIA process. These include predictions of CIA-induced variations in (1) the length-of-day over t he late Holocene period; (2) the Earth's precession constant and obliq uity over the last 2.6 Myr; and (3) the present-day zonal harmonics of the geopotential, J(l)(l less than or equal to 7). The predictions (1 ) and (3) bound the late Holocene (and ongoing) mass flux between the large polar ice sheets (Greenland and Antarctic) and the global oceans to small values(less than or equal to 0.4 mm/yr equivalent eustatic s ea level rise).