Abstract
The optically stimulated luminescence (OSL) signals of quartz and K-feldspar are known to bleach poorly within some glacial settings, and can present a major challenge to dating applications. However, because the OSL signal is extremely sensitive to sunlight exposure history, the residual luminescence signals of modern glacial sediments also encode information about transport and depositional processes. Through examination of the residual luminescence properties (equivalent dose (De) and overdispersion values) of a suite of modern glacial sediments from different depositional settings (sandar, proglacial delta and main meltwater channel), this study provides insights not only into which sediments are likely to be fully bleached within glacial settings, but also into how OSL can be used to trace different depositional processes across sedimentary landforms. Improved understanding of the processes of sediment bleaching will enable better sample selection and may improve the accuracy and precision of OSL dating of glacial sediments.The luminescence signals of both coarse-grained quartz and K-feldspar with similar sediment sources are found to be sensitive to both depositional process and specific depositional setting. Whereas modern braid-bar-head deposits from the Nigardsdalen ice-proximal proglacial delta typically have ages of ≤3 ka, similar depositional features from the Fåbergstølsgrandane sandur have residual ages of ≥26 ka. Exploration of changing residual luminescence signals across individual sandur and proglacial delta braid-bar features shows that braid-bar-head deposits can retain large residual De values, while the partner braid-bar-tail deposits are almost completely bleached. The quartz OSL signal and K-feldspar IRSL50 and post-IR IRSL250 signals are shown to bleach at the same rate across the same bar feature and the IRSL50 K-feldspar signal is also shown to be completely bleached for bar-tail deposits in Nigardsdalen. Therefore the IRSL50 K-feldspar signal is suitable for dating some glacial deposits, circumventing the challenges associated with dim quartz signals.
Highlights
Stimulated luminescence (OSL) dating is one of the favoured methods for dating glacial sediments, partial resetting of luminescence signals can present a major challenge within glacial environments (e.g. Gemmell, 1988; Mejdahl and Funder, 1994)
This study explores the causes of variability in residual luminescence signals from depositional features through measuring the luminescence properties of multiple sediments from individual landforms
Samples GRAN54, GRAN58 and GRAN59 are sampled from braid-bar-head deposits and have the greatest residual ages (Kfeldspar ages are !26.6 ka, quartz ages are !3.4 ka), GRAN55 is sampled from a braid-bar-mid and GRAN56 and GRAN57 are sampled from braid-bar-tails and have the lowest residual ages for both the K-feldspar and quartz fractions
Summary
Stimulated luminescence (OSL) dating is one of the favoured methods for dating glacial sediments, partial resetting (bleaching) of luminescence signals can present a major challenge within glacial environments (e.g. Gemmell, 1988; Mejdahl and Funder, 1994). There has been considerable research on quantifying the unbleached residuals in glacial settings Fuchs and Owen, 2008), little research has explored the processes of sediment bleaching. King et al (2013) recorded unbleached residual signals of 0e3 ka for glaciofluvial bar deposits 2 km from the icemargin, but were unable to identify a single cause for the residual variability, attributing it to a combination of specific depositional setting, sediment source and transport distance. If the causes of unbleached residual luminescence signals can be well constrained it may be possible to exploit changing residual signals in the exploration of depositional pathway tracing.
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