Evidence of episodic permafrost conditions during the W eichselian Middle Pleniglacial in the Hengelo Basin (The Netherlands)

Abstract

An ice-wedge cast level, dated between ca 41,000 and 36,600 years BP. has been found in Tubantian (Weichselian) deposits near Hengelo (Twente district, Eastern Netherlands). So far, well developed ice-wedge casts of Middle Pleniglacial age have not been recorded for the Netherlands. It is argued that mean annual temperatures were near the upper limit to permit ice-wedge development. In that case substrate and hydrological conditions are critical. Sedimentary structures overlying the ice-wedge cast level point to thaw lake development. Palaeoecological evidence shows that the sequence started in open (periodically running) water. Under a declining water budget a phase with terrestrial peat sedentation was initiated which, after a short interval, culminated in an increasingly dry environment as indicated by xerophilous taxa. The ice-wedge cast level is associated with taxa indicative of bare, disturbed grounds and with the break up of a continuous vegetation cover. Subsequently a phase with (temporary) running water, predominantly standing, shallow water and a return to running water are recognized. The top of the sequence shows a vegetation which can be compared with a chionophilous arctic dwarf shrub tundra. Vegetation development could be explained in terms of changing moisture conditions, while temperature changes are of minor importance.