Towards successful OSL sampling strategies in glacial environments: deciphering the influence of depositional processes on bleaching of modern glacial sediments from Jostedalen, Southern Norway

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.