Inferences of mantle viscosity based on ice age data sets: Radial structure

Abstract

We perform joint nonlinear inversions of glacial isostatic adjustment (GIA) data, including the following: postglacial decay times in Canada and Scandinavia, the Fennoscandian relaxation spectrum (FRS), late-Holocene differential sea level (DSL) highstands (based on recent compilations of Australian sea level histories), and the rate of change of the degree 2 zonal harmonic of the geopotential, J(2). Resolving power analyses demonstrate the following: (1) the FRS constrains mean upper mantle viscosity to be similar to 3 x 10(20) Pa s, (2) postglacial decay time data require the average viscosity in the top similar to 1500 km of the mantle to be 10(21) Pa s, and (3) the J(2) datum constrains mean lower mantle viscosity to be similar to 5 x 10(21) Pa s. To reconcile (2) and (3), viscosity must increase to 10(22) - 10(23) Pa s in the deep mantle. Our analysis highlights the importance of accurately correcting the J(2) observation for modern glacier melting in order to robustly infer deep mantle viscosity. We also perform a large series of forward calculations to investigate the compatibility of the GIA data sets with a viscosity jump within the lower mantle, as suggested by geodynamic and seismic studies, and conclude that the GIA data may accommodate a sharp jump of 1-2 orders of magnitude in viscosity across a boundary placed in a depth range of 1000-1700 km but does not require such a feature. Finally, we find that no 1-D viscosity profile appears capable of simultaneously reconciling the DSL highstand data and suggest that this discord is likely due to laterally heterogeneous mantle viscosity, an issue we explore in a companion study.