Anomalous luminescence of subglacial sediment at Haut Glacier d'Arolla, Switzerland: a consequence of resetting at the glacier bed?

Luminescence has the potential to elucidate sediment transport pathways in glaciated environments because sediment derived from extraglacial sources should be at least partially reset whilst sediment produced by the erosion of subglacial bedrock should have substantial luminescence commensurate with a geological irradiation history. We set out to test the validity of these assumptions at Haut Glacier d’Arolla, Switzerland by measuring the equivalent dose of sediment sampled from extraglacial and subglacial locations. The limited volume of subglacial material that could be acquired dictated multiple-stimulation analysis of polymineral aliquots using a single-aliquot regenerative-dose (SAR) technique, comprising sequential measurement of Infra-Red-Stimulated Luminescence (IRSL), post-IR blue Optically-Stimulated Luminescence (OSL), and Thermally-stimulated Luminescence (TL). Surprisingly, although extraglacial samples exhibited equivalent doses that were indeed commensurate with partial-resetting of near-saturated, geologically-acquired signals, subglacial samples exhibited doses that were substantially lower. Further analyses indicate no obvious differences in the luminescence behaviour of subglacial and extraglacial samples, whilst glaciological arguments eliminate the possibility of extraglacially-reset sediment being present at the glacier bed. The results are therefore supportive of resetting of subglacial sediment in situ. We conclude that the properties of glacier ice, and the likely residence time of sediment at the ice-bed interface, do not support in situ bleaching by natural light, and that the efficacies of alternative resetting processes – specifically, the mechanical processes that produce and comminute subglacial sediment – should be further investigated.