The sensitivity of glacial isostatic adjustment predictions to a low-viscosity layer at the base of the upper mantle

Recent inferences of mantle viscosity based on convection-related observables (long-wavelength nonhydrostatic geoid and free-air gravity harmonics) indicate the presence of a relatively thin low-viscosity region at the base of the upper mantle. We explore the sensitivity of observables associated with glacial isostatic adjustment (GIA) to the viscosity and thickness of this region. These observables include post-glacial relative sea-level (RSL) variations within previously glaciated areas and in the "far field" of the Late Pleistocene ice sheets, as well as anomalies associated with the rotational state of the planet. The latter include present-day secular variations in the geopotential harmonic J₂ (or, equivalently, variations in the rotation rate) and polar wander speed. We find, in contrast to previous suggestions, that the GIA observables (with the exception of polar wander speed) are sensitive to fine-scale low-viscosity structure at the base of the upper mantle. For example, j₂ predictions can be altered by as much as 2 × 10^-11 yr^-1 by the inclusion of a low-viscosity zone. This signal is sufficient to significantly affect both inferences of mantle viscosity and constraints on the present-day melting of large polar ice sheets (Antarctic, Greenland) that are derived using this observable. Indeed, if a low-viscosity region exists, then models that do not include it can overestimate recent polar melting events by as much as 0.5 mm/yr of equivalent eustatic sea-level rise.