Combined single grain and cobble luminescence dating of poorly bleached glaciofluvial deposits from the Swiss Alpine foreland

Abstract

Constraining the age of glaciofluvial deposits is essential for reconstructing paleoglacial dynamics. While luminescence dating is among the most commonly used approaches, incomplete resetting of the signal before sediment deposition makes its application to such deposits challenging. Single grain (SG) and cobble luminescence dating have been used to account for heterogeneous bleaching with different success, and are tested here in combination on glaciofluvial deposits from one site in the northern Alpine foreland. Previous chronological investigations indicate that deposition of the proglacial outwash occurred ca. 29 ka ago. SG feldspar results on sand lens and gravel sandy matrix samples underestimate the previous age estimates due to the occurrence of grains with low equivalent dose (D_e) values. When low D_e values are discarded, the estimated ages are in agreement with the independent constraints. Luminescence-depth profiles obtained from four cobbles show (1) no or only shallow bleaching, and (2) high variability between cores and surfaces of the same clast. Both aspects highlight the necessity of measuring numerous clasts and several cores in order to detect well-bleached clasts and profiles. The limited signal resetting can be explained by the proximity of the deposit to the glacial front. The intra-cobble variability is instead potentially related to heterogeneity in the cobbles’ lithology, implying differences in dosimetry, signal stability and light attenuation within the clasts. Electron microprobe analyses of feldspar in the clasts and comparison with the independent age constraints suggest that (1) the luminescence signal of the rock slices derive from both Na- and K-rich feldspars and (2) an average internal potassium content may be a good first order estimate for dose rate calculations. Overall, the present study highlights both the challenges and the potential of applying SG and cobble luminescence dating to partially bleached glaciofluvial deposits.