The importance of lateral Earth structure for North American glacial isostatic adjustment

Abstract

The isostatic response of the Earth to past mass exchange between ice sheets and oceans, so-called glacial isostatic adjustment (GIA), is an important geodynamic process in North America. Modelling GIA observables provides one of the few direct methods for estimating Earth viscosity structure. We investigate here the requirement for anomalously high mantle viscosities to fit the relative sea level (RSL) data along the Atlantic and Gulf coasts of North America found in recent GIA studies. We demonstrate that this requirement is primarily related to the modelled geometry and evolution of the peripheral bulge of the Laurentide ice sheet. We show that a 3D Earth model with a global average viscosity in the upper mantle of 0.3×1021 Pas and 3×1022 Pas in the lower mantle, which is consistent with both the Richmond Gulf (Hudson Bay) relaxation time and several recent global analyses, is able to produce a much improved fit to the Gulf and Atlantic coast RSL data relative to the 1D Earth model results when no lateral structure is applied. Therefore, we conclude that realistic implementations of lateral structure can explain the markedly different viscosity inferences obtained using 1D GIA models and RSL data from different regions of North America. A necessary caveat is that there are significant differences among the computed RSL curves corresponding to the three different realisations of lateral structure considered here, demonstrating significant uncertainty associated with this model input.