INTERPRETING THE LANDSCAPE HISTORY OF NORTHERN SCANDINAVIA USING GEOMORPHOLOGY AND COSMOGENIC RADIONUCLIDE MEASUREMENTS

Detailed mapping of landforms in northern Scandinavia has identified preglacial surface remnants juxtaposed to glacially scoured surfaces, including glacial troughs. Model reconstructions of the Fennoscandian ice sheet indicate that most of the preglacial landscapes were not nunataks. Hence, these landforms and their boundaries have been interpreted in terms of the configuration, the basal thermal regime, and duration of three modes of glaciation known to have existed in the region: Fennoscandian ice sheets, mountain ice sheets, and cirque glaciation. The average subglacial thermal regime of both ice sheet types appears to have been cold-based on the uplands and warm-based in the main valleys where outlet glaciers and ice streams formed.

Cosmogenic radionuclide measurements were made on bedrock and glacial erratics associated with the different landforms in order to test the hypothesis of landscape preservation under ice sheets, and to constrain the spatial and temporal patterns of glacial erosion. Deglaciation ages from glacial erratics situated at high elevations confirm that the preglacial landforms were not nunataks. Apparent Be-10 and Al-26 surface exposure ages of bedrock are often well in excess of the deglaciation age indicating low bedrock weathering and erosion rates in regions formerly occupied by the ice sheets. This is not only manifested on preglacial surfaces but also locally in major trunk valleys where glacial bedrock erosion in excess of roughly 2 metres is confined to the valley base and decreases significantly up the valley sides. Nuclide ratios at some locations indicate that preglacial landforms have survived multiple glaciations.

The broad implication of these cosmogenic nuclide measurements is that they confirm the geomorphological interpretations that boundaries between relict and glacially modified landscapes represent englacial thermal boundaries rather than former ice levels, and that contrary to popular opinion, glaciological processes, at least during the last glacial cycle, have not generally been efficient agents of bedrock erosion. The chronologies and erosion patterns derived from the cosmogenic radionuclide analyses are currently driving efforts that combine GIS with sophisticated numerical modeling of ice sheet dynamics and isostatic response.