ICE DYNAMICS AND DEGLACIATION OF SW NORWAY CONSTRAINED BY LIDAR MAPPING AND COSMOGENIC NUCLIDE EXPOSURE DATING OF GLACIAL LANDFORMS

Southwestern Norway is a key area for understanding interactions between the Fennoscandian and British ice sheets, breakup of the Norwegian channel ice stream and chronology of late glacial to Holocene deglaciation. The classical study of Bjørn Andersen in the 1950s outlined several moraine zones including the Younger Dryas (Ra) deposit. Using new remote sensing data we have remapped the entire area in greater detail than before and together with comprehensive new geochronological constraints we present significant new insights into past ice-sheet behavior in SW Scandinavia.

We have acquired airborne LiDAR terrain data, together with digital 3D infrared aerial photographs, to map glacial landforms and deposits in detail. In addition we have used four field-seasons to field map and sample for exposure dating. We have targeted moraines, drumlins and eskers and in total we have mapped approximately 4000 km².

For geochronology, we collected 48 samples for 10Be surface exposure dating which we combine with new radiocarbon dates from lake records. Our results show that the ice surface lowered early in the late-glacial, deglaciating mountains of modest altitude (450 m.a.sl.) already at 18 ka BP. The deglaciation in this dissected coastal landscape was complex, with early retreat of fjord glaciers that calved back to fjord heads, with thin ice backstepping onshore. We find little evidence of regional ice-margin positions, an important feature of Andersen's previous reconstruction. Conversely, most of the area outside the Ra is barren, with only few, isolated moraine ridges that are confined to separate valleys. The ice sheet melted back about 50 km inland, before it readvanced to the Ra position. Here, a complex morphology and our 10Be data indicate several readvances reaching the same position. We present evidence for early thinning and break-up of the ice sheet in SW Norway, already at 19-18 ka BP, which is linked to ice draw-down during collapse of the Norwegian channel ice stream. The deglaciation during Bølling-Allerød interstadials was relatively rapid and without major halts. However, the Ra moraine zone is complex and indicates at least one major- and one smaller readvance.