Uplift and sea level constraints on 3-D upper mantle viscosity in Northern Europe

Northern Europe experiences vertical land motion and sea level changes as a consequence of past changes in ice sheet cover in Fennoscandia and the British Isles. The process, called glacial isostatic adjustment (GIA), is controlled by the subsurface structure. Numerical models of GIA can be compared to observations of uplift or past sea level changes to constrain the subsurface structure, and such models can also be used to correct present-day sea level observations to reveal sea level changes due to climate change. GIA models for northern Europe usually adopt a homogeneous upper mantle viscosity even though seismic studies indicate contrasting elastic lithosphere thickness and upper mantle structure between Northwestern Europe and Eastern Europe. This raises the question whether the effect of lateral variations in structure (3-D viscosity) can be detected in observations of GIA and whether including such variations can improve GIA model predictions. In this study, we compare model output from a finite element GIA model with 3-D viscosity to observations of paleo sea level and current vertical land motion. We use two different methods to derive 3-D viscosities, based on seismic velocity anomalies and upper mantle temperature estimates. We use three different reconstructions of the Eurasian ice sheet, one based on an inversion using a 1-D viscosity model, and two others based on glacial geology and modelling. When we use these two reconstructions, we find that the data are fit better using 3-D viscosity models. Models with two separate 1-D viscosities for Fennoscandia and for the British Isles cannot replicate a 3-D model because a 3-D model redistributes GIA-induced stresses differently from a combination of models with 1-D viscosities. The fit to data across Fennoscandia is improved when, as indicated by seismic models, the upper mantle viscosity is higher than for the rest of Northern Europe. The best fit is obtained with a model with dry olivine rheology, in agreement with other evidence from Fennoscandia.