Abstract
ABSTRACTThe application of optically stimulated luminescence (OSL) dating within glacial settings can be limited where sediments have not had their OSL signal fully reset by sunlight exposure. Heterogeneous bleaching can result in age overestimations, and although it is recognized that certain depositional settings are more likely to have experienced sufficient sunlight exposure to bleach the OSL signal, no comprehensive study has empirically investigated the processes of sediment bleaching, or the variability in bleaching between deposits of the same type and within the same glacial catchment. A suite of modern glacial and glaciofluvial sediments from Fåbergstølsdalen, southern Norway, have been analysed to explore the controls that sedimentary processes and depositional setting have on bleaching of the OSL signal of quartz. There is considerable variability in the residual OSL ages of similar modern deposits, which reflects high sensitivity of the OSL signal to sediment source, sedimentary process, transport distance and depositional setting. Overdispersion values are greatest for the sediments which have been most heterogeneously bleached and these sediments have the lowest residual ages. Sampling strategies that incorporate sufficient consideration of the depositional framework of sample settings can minimize the effects of unbleached residuals on OSL age determinations.
Highlights
A major challenge to optically stimulated luminescence (OSL) dating within glacial environments is identifying sediments that have been fully exposed to sunlight, and so have had their luminescence signal fully reset
The increased bleaching of the paraglacial deposits relative to the subglacial sediments is reflected in their grain size analysis characteristics: the paraglacial sediments remain poorly sorted (Table S3), they are better sorted than the subglacial deposits, exemplified by mesokurtic, rather than leptokurtic, grain size distributions
Changing overdispersion values between the different deposits indicates that transport and depositional processes modify overdispersion because of sediment bleaching, and deposits with the greatest overdispersion values have experienced the greatest bleaching
Summary
A major challenge to optically stimulated luminescence (OSL) dating within glacial environments is identifying sediments that have been fully exposed to sunlight, and so have had their luminescence signal fully reset (bleached). Thrasher et al, 2009a), understanding how depositional processes influence bleaching opportunities for sediments may enable more informed sample selection within glacial settings. Alexanderson (2007) measured the OSL of six bed-load samples of ripple-laminated, sandy, longitudinal- and side-bar deposits from five different, ice-distal Greenlandic rivers. She recorded unbleached residual doses of 0.1–1 Gy for rivers draining perennial snowfields of !2 km distance from her sampling sites, which equate to residual ages of between 50
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