First dating of an early Chibanian (Middle Pleistocene) glacial overdeepening in the Alpine Foreland using the ⁴He/U-Th method

The ⁴He/U-Th dating method can be used to estimate the residence time of pore waters in low-permeable rocks and consolidated sediments, serving as a proxy for sediment deposition time. This residence time is inferred from the accumulation time of radiogenic ⁴He measured in the pore water being produced by the local decay of U and Th in the sediment matrix. We applied the ⁴He/U-Th method to date the pore waters of unconsolidated sediments from a glacial overdeepening in the Swiss Plateau (northern Alpine Foreland), where prior studies suggested sediments older than Marine Isotope Stage (MIS) 6 (191−130 ka). We show that compact and fine-grained (glaci)lacustrine sediments provide low-permeability conditions that allow ⁴He to accumulate in the pore water and be preserved in the pore space. The ⁴He/U-Th dating indicates that the sediments between 40 m and 140 m are 606 ± 122 ka. The dated infill was deposited in a glacial overdeepening eroded by a foreland glaciation larger than that of the Last Glacial Maximum (LGM). The results reveal extensive foreland glaciations and intense glacial overdeepening erosion during the early part of the Chibanian (i.e., Middle Pleistocene). This work highlights the potential of the ⁴He/U-Th method for dating sediments in similarly favorable hydrogeological settings.