TRACING THE POST-YOUNGER DRYAS RETREAT OF THE NORTHERN FENNOSCANDIAN ICE SHEET USING COSMOGENIC RADIONUCLIDE EXPOSURE AGES

The deglaciation chronologies of the northern and northeastern margins of the Fennoscandian Ice Sheet are relatively poorly constrained. This is because one of the principal tools used to trace and date the deglaciation pattern for this ice sheet, the occurrence of deglaciation varves, only applies in areas that were post-glacially submerged by the Baltic Sea. Moreover, a paucity of radiocarbon dates has prevented detailed reconstructions of the timing of the post-Younger Dryas shrinkage of the ice sheet to its final deglaciation configuration in the northern Swedish mountains. We are tracing the deglaciation of the Fennoscandian Ice Sheet from its YD terminal moraines in northern Norway and eastern Finland towards the area of final deglaciation in the northern Swedish mountains, using cosmogenic nuclide apparent exposure ages of depositional and erosional features related to the former ice sheet margin. Because the ice sheet was predominantly warm-based close to its YD-margin, the dominant morphology is one of eskers and aligned lineation systems, such as crag-and-tails. Abundant meltwater eroded bedrock locally to considerable depth and deposited fans or deltas perched above current local base levels. Subglacial conditions during final deglaciation in the central areas were generally cold-based, inhibiting the formation of eskers and lineation systems, although there are widespread (lateral) meltwater channel erosional imprints and occasional plucking scars where, locally, pressure-melting conditions were reached. Each geomorphological setting was examined for its value in providing deglaciation ages, testing the initial assumptions that, (i) abundant erosion on crags of crag-and-tails, across transverse erosional scarps, and in meltwater channels has exposed bedrock surfaces without a prior exposure history and (ii) depositional features contain embedded boulders without a prior exposure history (on the surfaces of eskers and deltas, and erratics). Preliminary results indicate that meltwater channels, transverse erosional scarps, and erratics yield deglaciation ages that are consistent with ages provided by other methods, but that crag-and-tails yield apparent exposure ages that are too old, presumably because of a prior exposure history that was not fully removed by glacial erosion.