Oldest Optically‐Stimulated‐Luminescence Ages From a Composite‐Wedge Pseudomorph as Evidence of MIS 9d–8b Permafrost Between Fennoscandian and Alpine Palaeo‐Ice Sheets

ABSTRACT

Reconstructions of past permafrost have most frequently utilized relict landforms and sedimentary structures genetically related to the growth and decay of permafrost. However, the majority dated so far formed during the Last Glacial cycle, while older ones are much less common and have mostly been dated using purely relative and/or numerically aided stratigraphic methods. Here, we describe the stratigraphic position, morphology, internal structures and optically‐stimulated‐luminescence (OSL) ages of a rare Middle Pleistocene frost wedge recovered in a sedimentary section in Dolní Chabry, Czech Republic (50.14171° N, 14.44964° E, 302 m above sea level). The wedge developed in two phases taking place during MIS 9d–8b between 307 ± 30 ka and 277 ± 22 ka, which renders it virtually the oldest infill‐dated frost wedge in Europe. The frost wedge indicates the occurrence of a cold, dry and windy climate and the existence of permafrost at least during the coldest phases of MIS 8 in the interior of the wide belt between the Fennoscandian and Alpine palaeo‐ice sheets where no direct numerical ages for permafrost proxies have yet been available. As such, the OSL dates are unique and valuable, which is further enhanced by their exceptionally old ages.