Revisiting a proof of concept in quartz-OSL bleaching processes using sands from a modern-day river (the Séveraisse, French Alps)

Conditions of sediment transport and deposition in highly dynamic fluvio-glacial environments enhance incomplete bleaching of luminescence signals during sunlight exposure. Whatever the geomorphic context or application, partial bleaching has been widely reported and remains a methodological limitation for application of Optically Stimulated Luminescence (OSL) dating methods, potentially resulting in sediment-burial age overestimation. This study focuses on the highly dynamic Séveraisse River (SW French Alps) where modern-day alluvial sands of a braided reach were sampled to assess the degree of quartz-OSL partial bleaching associated with superficial pre- and post-deposition geomorphic processes. Our original approach combines (i) a photogrammetry-based survey, (ii) sediment grain-size analysis, and (iii) measurements of both portable OSL luminescence signals and conventional quartz OSL equivalent doses in modern superficial (from 0.1 to 1 cm) and sub-surface (up to 30 cm) alluvial sands exposed to sunlight for at least 19 days. Our results show high but spatially variable residual luminescence signals at the surface, measured in all grain-size fractions with both the portable luminescence reader (≥5 x10⁶ cts/g) and conventional quartz-OSL doses (≥80 Gy), even within the uppermost millimetres of the exposed alluvial surface. Our data thus highlight poor luminescence bleaching in the Séveraisse's modern sands, during both pre-depositional transport and post-depositional exposure. In addition, our study reveals, for the first time, the significant sunlight attenuation over a few millimetres within modern alluvial sediments, perhaps conditioned by dark sand grains, and/or by superficial blanketing by silts (i.e. waning flow stage) that leads to a porosity decrease and very low sunlight penetration. We suggest the occurrence of a critical sediment layer (i.e. only a few mm thick) that could play a key role in bleaching processes for alluvial surfaces, with strong implications for our understanding of residual doses in braided systems' sandy deposits and the dynamics of such alluvial surfaces.