The influence of a finite glaciation phase on predictions of post-glacial isostatic adjustment

Abstract

We predict relative sea level (RSL) variations, present day three-dimensional (3D) crustal deformation rates and baseline velocities, and a set of anomalies in the gravitational field of the planet, using a suite of models for the space-time geometry of the Pleistocene ice sheets. These models are distinguished on the basis of the details of the glaciation phase, and they include the cases of an infinite glaciation phase (i.e., an assumption of isostatic equilibrium at the time of the last glacial maximum — LGM), a steady uniform glaciation, and a glaciation in which the bulk of the accumulation is limited to the final stages of the growth phase. Comparison of the observational records of postglacial RSL change with predictions based on the assumption of isostatic equilibrium at LGM have commonly been used to infer both the space-time geometry of the final deglaciation event and the mantle viscosity. We conclude that both these applications may be influenced significantly by the incorporation of a finite glacial cycle. As an example, RSL predictions, which assume isostatic equilibrium at LGM, and those which incorporate a relatively rapid glaciation phase, can differ by a factor of 2 or more at some sites within previously glaciated regions when an Earth model characterized by a lower mantle viscosity of 10 22 Pa s is adopted. We have also found that differences in the adopted model for the glaciation event are capable of explaining entirely the discrepancies ( ∼ 0.2 mm/yr) between previously published predictions of present day tangential velocities in North America. These discrepancies are characterized by a dominant long-wavelength signal and, hence, they do not necessarily influence (strongly) predictions of baseline velocities. Variations in the glaciation model can also have an important effect on predictions of peak gravity anomalies over previously glaciated regions, and secular variations in the zonal harmonics of the Earth's geoid. In both cases the effect may be sufficient to alter significantly the signal from other geophysical processes which may need to be invoked to reconcile the observational constraints.