The impact of regional-scale upper mantle heterogeneity on glacial isostatic adjustment in West Antarctica

Abstract. West Antarctica is underlain by a laterally heterogenous upper mantle, with localized regions of mantle viscosity reaching several orders of magnitude below the global average. Accounting for 3-D viscosity variability in glacial isostatic adjustment (GIA) simulations has been shown to impact the predicted spatial rates and patterns of crustal deformation, geoid, and sea level changes in response to surface ice loading changes. Uncertainty in the viscoelastic structure of the solid Earth remains a major limitation in GIA modeling. To date, investigations of the impact of 3-D Earth structure on GIA have adopted solid Earth viscoelastic models based on global- and continental-scale seismic imaging, with variability at spatial length scales > 150 km. However, regional body-wave tomography shows mantle structure variability at smaller length scales (~50–100 km) in central West Antarctica. Here, we investigate the effects of incorporating smaller-scale lateral variability in upper mantle viscosity into 3-D GIA simulations. Lateral variability in upper mantle structure at the glacial drainage basin scale is found to impact GIA model predictions for modern and projected ice mass changes, especially in coastal regions that undergo rapid ice mass loss. Differences between simulations adopting upper mantle viscosity structure inferred from regional- versus coarser continental-scale seismic imaging are large enough to impact the interpretation of crustal motion observations and reach up to ~15 % of the total predicted sea level change during the instrumental record. Incorporating a strong transition from lower viscosities at the mouth of the Thwaites and Pine Island glaciers to higher viscosities in the interior of the glacier basins results in a ~10–20 % difference in predicted sea level change in the vicinity of the grounding line over the next ~300 years. These findings have a range of implications for the interpretation of geophysical observables and improving constraints on feedbacks between the West Antarctic Ice Sheet and the solid Earth.