Resetting of sediments mobilised by the LGM ice-sheet in southern Norway

Former geological field investigations in the Rondane area, east-central southern Norway, have proposed that the maximum Fermoscandian ice-sheet coverage occurred during the Late Weichselian Glacial Maximum (LGM, ca. 20ka) and that subsequent glaciofluvial sediments were first deposited in the early Holocene (after 10 ka). However, recent field investigations with ages from three internally consistent quartz optically stimulated luminescence (OSL) age series show an apparent deglaciation of northern Rondane in the period 20.0-13.8 ka. We examine here the possibility that these ages are too old because the sediment was not completely zeroed prior to deposition. Our investigations of incomplete bleaching use modern analogues, small aliquots, and single grains of quartz. First, the symmetric shape of small aliquot equivalent dose distributions suggests that the sediment was probably well bleached at deposition. This is supported by 5 modern analogue equivalent doses (D-e) of 0.6 Gy, similar to 1.5% of the typical D-e from the deglaciation sediments. Finally, from single grain studies on three samples, we conclude that there is no evidence for poor bleaching in these samples; thus the weighted mean gives the best estimates of D-e, and these are completely consistent with both large and small aliquot estimates for these samples. These comparisons between large aliquots, modern analogues, small aliquots and single-grain analyses help to validate the OSL ages and confirm the complete resetting of these sediments prior to deposition. (c) 2006 Published by Elsevier Ltd.