Glacial isostatic adjustment in Fennoscandia for a laterally heterogeneous earth

Abstract

Glaciation and deglaciation in Fennoscandia during the last glacial cycles has significantly perturbed the Earth’s equilibrium figure. Changes in the Earth’s solid and geoidal surfaces due to external and internal mass redistributions are recorded in sequences of ancient coastlines, now either submerged or uplifted, and are still visible in observations of present-day motions of the surface and glacially induced anomalies in the Earth’s gravitational field. These observations become increasingly sophisticated with the availability of GPS measurements and new satellite gravity missions. Observational evidence of the mass changes is widely used to constrain the radial viscosity structure of the Earth’s mantle. However, lateral changes in earth model properties are usually not taken into account, as most global models of glacial isostatic adjustment assume radial symmetry for the earth model. This simplifying assumption contrasts with seismological evidence of significant lateral variations in the Earth’s crust and upper mantle throughout the Fennoscandian region. We compare predictions of glacial isostatic adjustment based on an ice model over the Fennoscandian region for the last glacial cycle for both radially symmetric and fully 3-D earth models. Our results clearly reveal the importance of lateral variations in lithospheric thickness and asthenospheric viscosity for glacially induced model predictions. Relative sea-level predictions can differ by up to 10–20m, uplift rate predictions by 1–3mmyr−1 and free-air gravity anomaly predictions by 2–4mGal when a realistic 3-D earth structure as proposed by seismic modelling is taken into account.