THE INFLUENCE OF A FINITE GLACIATION PHASE ON PREDICTIONS OF POSTGLACIAL ISOSTATIC-ADJUSTMENT

We predict relative sea level (RSL) variations, present day three-dime nsional (3D) crustal deformation rates and baseline velocities, and a set of anomalies in the gravitational field of the planet, using a sui te of models for the space-time geometry of the Pleistocene ice sheets . These models are distinguished on the basis of the details of the gl aciation 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 glaci ation in which the bulk of the accumulation is limited to the final st ages of the growth phase. Comparison of the observational records of p ostglacial RSL change with predictions based on the assumption of isos tatic equilibrium at LGM have commonly been used to infer both the spa ce-time geometry of the final deglaciation event and the mantle viscos ity. We conclude that both these applications may be influenced signif icantly 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 region s 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 adopte d model for the glaciation event are capable of explaining entirely th e discrepancies (similar to 0.2 mm/yr) between previously published pr edictions 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 o f baseline velocities. Variations in the glaciation model can also hav e an important effect on predictions of peak gravity anomalies over pr eviously glaciated regions, and secular variations in the zonal harmon ics of the Earth's geoid. In both cases the effect may be sufficient t o alter significantly the signal from other geophysical processes whic h may need to be invoked to reconcile the observational constraints.