The eustatic reduction of shoreline diagrams: Implications for the inference of relaxation-rate spectra and the viscosity stratification below Fennoscandia

Abstract

One of the main objectives of quantifying glacial-isostatic adjustment is to infer the viscosity of the Earth. An efficient method of determining this parameter is based on the inversion of the observational relaxation-rate spectrum (RRS) constructed from a shoreline diagram representing the relative sea level heights at discrete past time epochs along a given profile. The advantage of using the observational RRS for the inversion is that it is largely insensitive to the Pleistocene ice sheet geometry and history. Implicit assumptions of the method are: the approximate radius and centre of the ice sheet are known, the shorelines are directed to the former ice sheet centre and refer predominantly to the period after deglaciation. So far, a drawback of the method has been that only a small number of shoreline diagrams have been published and only one diagram has been used to determine an RRS. To overcome this, we propose a simple method of constructing shoreline diagrams from sea level indicators (SLIs) and apply it to a set of SLIs for the Oslo Fjord area. Considering a total of three shoreline diagrams for Fennoscandia, we then demonstrate the robustness of the observational RRS inferred with respect to the diagram considered. We also show that the previous neglect of the eustatic reduction when inferring the observational RRS from a given shoreline diagram is only slightly reflected in the viscosity stratification determined. © 2005 RAS.