Abstract
Abstract. During the last glacial maximum, a large ice sheet covered Scandinavia, which depressed the earth's surface by several 100 m. In northern central Europe, mass redistribution in the upper mantle led to the development of a peripheral bulge. It has been subsiding since the begin of deglaciation due to the viscoelastic behaviour of the mantle. We analyse relative sea-level (RSL) data of southern Sweden, Denmark, Germany, Poland and Lithuania to determine the lithospheric thickness and radial mantle viscosity structure for distinct regional RSL subsets. We load a 1-D Maxwell-viscoelastic earth model with a global ice-load history model of the last glaciation. We test two commonly used ice histories, RSES from the Australian National University and ICE-5G from the University of Toronto. Our results indicate that the lithospheric thickness varies, depending on the ice model used, between 60 and 160 km. The lowest values are found in the Oslo Graben area and the western German Baltic Sea coast. In between, thickness increases by at least 30 km tracing the Ringkøbing-Fyn High. In Poland and Lithuania, lithospheric thickness reaches up to 160 km. However, the latter values are not well constrained as the confidence regions are large. Upper-mantle viscosity is found to bracket [2–7] × 1020 Pa s when using ICE-5G. Employing RSES much higher values of 2 × 1021 Pa s are obtained for the southern Baltic Sea. Further investigations should evaluate whether this ice-model version and/or the RSL data need revision. We confirm that the lower-mantle viscosity in Fennoscandia can only be poorly resolved. The lithospheric structure inferred from RSES partly supports structural features of regional and global lithosphere models based on thermal or seismological data. While there is agreement in eastern Europe and southwest Sweden, the structure in an area from south of Norway to northern Germany shows large discrepancies for two of the tested lithosphere models. The lithospheric thickness as determined with ICE-5G does not agree with the lithosphere models. Hence, more investigations have to be undertaken to sufficiently determine structures such as the Ringkøbing-Fyn High as seen with seismics with the help of glacial isostatic adjustment modelling.
Highlights
During the last colder climatic phase with average surface temperatures being about 10 ◦C lower than today (Petit et al, 1999), northern Europe – like other parts in the world – was covered by an extensive ice sheet
We start presentation of the results with a discussion of the best-fitting three-layer earth models (Table 1) for each ice model and regional relative sea-level (RSL) data set, which includes a brief presentation of results of the different groupings of sea-level data
We have to note that the spatial resolution of this model is two degrees and smaller features may not be clearly identified. This is the first time that the regional earth structure in the southern Baltic Sea has been investigated with the help of regionally categorized RSL data and glacial isostatic adjustment (GIA) modelling
Summary
During the last colder climatic phase with average surface temperatures being about 10 ◦C lower than today (Petit et al, 1999), northern Europe – like other parts in the world – was covered by an extensive ice sheet. The mass of this so-called Fennoscandian ice sheet deformed the earth’s crust into the mantle, leading to surface depressions of several hundreds of metres underneath the ice. Beyond the ice-covered area, a peripheral bulge developed around the ice sheet due to the bending of the elastic lithosphere outside the ice-covered. These changes are, due to the viscoelastic and time-delayed behaviour of the mantle, still observable today
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