An extensive MIS 12 Alpine glaciation in the Jura Mountains: insights from cave sediments and burial dating

The Middle Pleistocene glaciations of the Alps and the Jura Mountains are poorly understood relative to the Last Glacial Maximum (LGM). However, they played a critical role in shaping the landscape of the Northern Alpine Foreland. In the Jura Mountains, karstic features and their allochthonous sediments represent an underutilized resource for understanding the dynamic history of the Alpine Ice Field and Jura Ice Cap. This study combines field investigation of glacial sediments with measurements of cosmogenic 10 Be and 26 Al concentrations in Alpine cobble and gravel samples from the Grottes aux Fées. Two isochron-burial dating methods and P-PINI age modeling were used to calculate burial ages. Good agreement was found between these techniques, with ages being 420 ± 50 ka, 440 ± 30 ka, and 400 ± 60 ka, respectively. This suggests an Alpine glacier associated with the MIS 12 glacial cycle advanced over the Grottes aux Fées, in contrast to the LGM where no evidence supports an advance of the Alpine Ice Field to this location. These observations are consistent with an MIS 12 advance that extended at least 6 km beyond the LGM Alpine Ice Field position and may have contributed to the development of the External Moraine Complex. Analysis of cobble petrography indicates this MIS 12 glacier catchment was similar to the LGM Rhone glacier, with significant ice contribution from the southern Valaisian Alps. A Middle Pleistocene Alpine Ice Field advance beyond LGM extents may have been facilitated by enhanced climate forcing and/or a less incised landscape. • Glacial sediments with Alpine lithologies are present in caves in the Jura Mountains beyond the LGM Alpine Ice Field extent. • Alpine cobbles from the Grottes aux Fées have lithologies consistent with glacial transport from the southern Valaisian Alps. • 10 Be and 26 Al concentrations from quartzite clasts produced consistent ages using Isochron-burial dating and P-PINI modeling. • Alpine sediments were deposited in the Grottes aux Fées ca. 400 ka, likely associated with the MIS 12 glaciation.