Combined application of single grain and cobble luminescence dating to Alpine glaciofluvial deposits

Dating glaciofluvial deposits is fundamental to reconstruct paleoglacial and paleoclimatic dynamics. However, partial bleaching and lack of suitable sediments make the application of luminescence dating to such deposits challenging. Single grain and cobble luminescence dating have been successfully used to tackle these problematics [e.g. 1, 2] and are combined in this study on glaciofluvial deposits from the northern Alpine foreland (Finsterhennen, Switzerland). Previous chronological investigations at the study site [3, 4, 5] suggest that the deposition of the glaciofluvial gravel lying directly underneath the till from the regional Last Glacial Maximum (LGM) occurred ca. 29 kyr ago, in response to the LGM glacial advance. By comparison with the existing age constraints, the present study aims (1) to test the combined application of single grain and cobble luminescence dating to Alpine glaciofluvial deposits and (2) to refine the two methodologies. Samples were collected from well sorted sand lenses and moderately sorted gravel at approximately the same depth within the pre-LGM glaciofluvial gravel. Single grain (SG) feldspar luminescence measurements were conducted on sand lens and gravel sandy matrix samples, while luminescence depth profiles were measured in individual crystalline cobbles. Preliminary results show good agreement between SG ages from the sand lens and previous chronological constraints. SG results from the sandy matrix of the gravel horizon are instead underestimated, potentially because of challenges in dosimetry estimation, due to the heterogeneous lithology and grain size of the gravel layer. Heterogeneous dosimetry appears less problematic for burial age estimation of cobbles, since the variation of dose rate with depth into the cobble is dominated by the dosimetry of the cobble itself. Preliminary cobble luminescence depth profiles show shallow bleaching fronts, with large variability between cores and surfaces of the same cobble. The limited luminescence signal resetting can be explained by the proximity of the glaciofluvial deposits to the advancing LGM ice front and by sediment transportation in turbid water. The intra cobble variability is instead potentially related to the cobbles’ heterogeneous lithology, implying differences in dosimetry and light attenuation within the clasts. References [1] Duller, 2006, Quaternary Geochronology 1. [2] Jenkins et al., 2018, Quaternary Science Review 192. [3] Schlüchter, 2004, Quaternary Sciences 2. [4] Preusser et al., 2007, Boreas 36. [5] Pfander et al., 2022, Swiss Journal of Geosciences 115.