Pingo remnants in the northern Netherlands and north-western Germany
Bruijn, R. de
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In this study, a comparison was made between pingo remnant depressions in the northern Netherlands and north-western Germany. For this purpose, the dimensions, substrate and infill of 17 depressions have been studied. These depressions are located in four separate study areas: Friesland, Drenthe (the Netherlands), Ost-Friesland and the region between Cloppenburg and Visbek (Germany). In most cases, lithological cross sections have been constructed. In the laboratory, Loss on Ignition measurements were performed on the deepest infill of three cores, each derived from a different study area. Two cores were selected for the construction of an age model of the deepest infill. This model has been made based on correlation of important transitions in pollen assemblage in the records to the well-known vegetation development in the Netherlands, reflected in a high-resolution pollen diagram of the Uteringsveen pingo remnant in Drenthe. For several other depressions, the age of deepest infill was determined based on a pollen quickscan. All pingo remnant depressions are situated in areas where sands are covered by glacial till which belongs to the Drenthe Formation. This is in turn covered by coversands of the Boxtel Formation. Morphologically, four different types of depressions have been distinguished; (1) flat-based pingo remnant depressions, (2) cone-shaped pingo remnant depressions, (3) possible underdeveloped pingo remnant depressions, (4) depressions of which the origin is unclear and (5) depressions that are not pingo remnants. A clear geographical trend is evident, as all flat-based pingo remnants occur in the northernmost study areas. This is thought to be caused by regional differences in the substrate. Infill of the depressions is roughly similar for most sites, showing a hydroseral succession of organic material. Two depressions contain an infill of mainly sands. Vegetation development and aeolian activity during the time of earliest infill have been derived from pollen assemblages and LOI measurements. Results are consistent with pre-existing literature. Pollen analysis on samples of the deepest organic infill indicates that pingo collapse initiated in the Pleniglacial and Bølling. Pingo remnants with a Pleniglacial infill are thought to have formed by collapse because of mechanical failure of the former pingo, whereas remnants with an infill of Bølling age may have formed by climate-induced collapse during the transition to a warmer climate. Possible indicators for climate-induced collapse are proposed: more of the (weathered) pingo skin will have descended back into the depression, which is likely to be relatively small and should have an infill dating from a period of transition to a warmer climate. Former pingos in the study area are thought to have been of the hydraulic type, where pressurised water is forced upward, penetrating the glacial till in the substrate through weaker spots. This is implied by the presence of the impermeable substrate, the cone-shape of several of the pingo remnants and by indicators of both former and modern seepage conditions.