Glacial Isostatic Adjustment and Relative Sea Level changes: the role of lithospheric and upper mantle heterogeneities in a 3D spherical Earth

The response of the Earth to the melting of the Late Pleistocene ice sheets is commonly studied by spherically layered models, based on well-established analytical methods. In par- allel, a few models have been recently proposed to circumvent the limitations imposed by spherical symmetry, and to reproduce the actual structure of the lithosphere and of the upper mantle. Their main outcome is that laterally varying rheological structures may significantly affect various geophysical quantities related to glacial isostatic adjustment (GIA), and partic- ularly post-glacial relative sea-level (RSL) variations and 3-D crustal velocities in formerly ice-covered regions. In this paper, we contribute to the ongoing debate about the role of lithospheric and mantle heterogeneities by new 3-D spherical Newtonian finite elements mod- els and we directly compare their outcomes with publicly available global RSL data. This differs from previous investigations, in that have mainly focused on extensive sensitivity anal- yses or have considered a limited number of RSL observations from formerly glaciated regions and their periphery. In our study the lithospheric thickness mimics the global structure of the cratons based on geological evidence, and the upper mantle includes a low-viscosity zone beneath the oceanic lithosphere. We use two distinct global surface loads, based upon the ICE1 and ICE3G deglaciation chronologies, respectively. Our main finding is that using all of the available RSL observations in the last 6000 years it is not possible to discern between homo- geneous and heterogeneous GIA models. This result, which holds for both ICE1 and ICE3G, suggests that the cumulative effects of laterally varying structures on the synthetic RSL curves cancel out globally, yielding signals that do not significantly differ from those based on the 1-D models. We have also considered specific subsets of the global RSL database, sharing similar geographical settings and distances from the main centres of deglaciation. When we consider the data from the margins of the Baltic region, a laterally varying lithospheric thick- ness improves significantly the agreement with the observations. This is not observed in other relevant situations, including the Hudson bay region. In the regions where the disagreement be- tween predictions and observations is particularly evident, further investigations are needed to improve the geometry of the heterogeneous structures and of the surface ice-sheets distribution.