Active deglaciation results in faster solid earth deformation rates than predicted by models that are used on longer glacial-cycle timescales. We test the hypothesis that the discrepancy can be explained by transient mantle viscosities.

In mechanical experiments on mantle rocks, steady-state viscous flow is preceded by transient creep during and after changes in stress. Transient deformation following stress changes initially occurs at fast rates and decays while the rock viscosity gradually increases. New efforts in microphysical modelling, calibrated against experimental deformation, provide a novel flow law that captures both transient as well as steady-state viscous behaviour. The flow law describes dislocation creep, where interactions between dislocations lead to internal stresses that counteract loading. We complement the flow law by a model that describes the evolution of these internal stresses with progressive deformation, and thereby allows for variable viscosity. We use this flow law in numerical models to study stress history-dependent glacial isostatic adjustment (GIA). First, we investigate the relevance of this flow law for GIA, using a simple 1D model. For typical loads induced by ice-mass changes, we predict that the asthenospheric viscosity may temporarily reduce by 1 to 2 orders of magnitude compared to the long-term, steady-state viscosities.

Second, we study the contribution of transient dislocation creep to present-day GIA by using regional 3D finite element models. We focus on the Amundsen Sea Embayment in Antarctica, and test whether transient rheology can provide a better fit to GNSS time series than steady-state mantle rheology. Satellite altimetry and firn models provide a spatio-temporal view of ice load changes for the last three decades, and we test the sensitivity to ice load changes in the pre-observational era.

Recent studies demonstrate that feedback between vertical velocities of bedrock and ice-sheet evolution, and the speed of grounding line retreat, depends strongly on mantle viscosity. As transient rheology affects effective viscosity when ice loads change, transient rheology may be an important factor to consider in ice sheet-bedrock motion interactions.