Mantle Viscosity, Glacial Isostatic Adjustment and the Eustatic Level of the Sea

Tide gauge recordings of the secular variation of relative sea level are known to be strongly influenced by the ongoing global process of glacial isostatic adjustment. The east coast of the North American continent is heavily instrumented with tide gauge installations, many of which have been carefully maintained for over 50 years. Since this region traverses the collapsing forebulge of the Laurentide ice sheet and since the process of collapse is extremely well constrained on the basis of radio-carbon dated relative sea level histories from a dense set of locations, the region is globally unique in enabling an accurate decontamination of the tide gauge data using the 14C records themselves. It is shown herein that the decontaminated data define a residual signal which varies only slightly along the coast and which consists of an average rate of sea level rise near 2 mm yr-1.The relative sea level histories from sites along this coast also provide an excellent basis for testing theoretical models of the global glacial isostatic adjustment process that must be employed to decontaminate the tide gauge records at sites for which 14C records are unavailable. It is demonstrated that a mantle viscosity profile determined by the formal inversion of 14C controlled relative sea level histories from sites within the margins of the northern hemisphere ice sheets that existed at last glacial maximum enables a gravitationally and topographically self-consistent global model of glacial isostatic adjustment to accurately reconcile east coast rsl data. No viscosity structure has previously been derived that was successful in this regard. The global model based on this structure is therefore expected to provide an excellent basis for the removal of glacial isostatic adjustment effects from tide gauge recordings. The viscosity structure itself is also extremely close to models of the radial variation that have previously been shown to fit the requirements of non-hydrostatic geoid anomalies. This has important geodynamic implications concerning mantle rheology as it would appear to establish that transient and therefore non-linear creep mechanisms are not involved, since short timescale and long timescale viscosities are the same.