Possible signs of discontinuous permafrost
DOI:
https://doi.org/10.26485/BP/1976/26/15Keywords:
cylindrical column of sand, permafrost structures, ice cores, permafrost degradationAbstract
Article in French.
ORIGINAL TITLE: Indices possibles de pergélisol discontinu
Near Dourdan, 50 km southwest of Paris, a cylindrical column of siliceous sand with calcareous cementation is found. Its origin is explained by the infiltration of limestone-rich water seeping from the overlying limestone through a karst vent or by the presence of a structure disrupting the Quaternary permafrost layer.
At Orveau, 50 km south of Paris, a wide, flat-bottomed valley, now dry, is filled from 1 to 6 m by a series of Quaternary periglacial calcareous gravels, with glacier-formed blocks topped by a 0.4 to 1.2 m thick layer of aeolian siliceous sand. The contact between them is marked by a brown discoloration of the sand, with clay at the base. In various places, this contact creates very distinct V-, U-, or finger-shaped pockets, reaching a depth of 2 m, where the limestone has been dissolved. These pockets are not attributed to tree roots. Their origin can be explained by the hypothesis of permafrost, where rainwater and summer meltwater seep down through holes and, thanks to temperatures above 0°C, keep them open. Near Geneva, in Quaternary sand, brown iron formations form pockets of a very similar shape, possibly also formed by permafrost structures.
Near Montreal, in late-glacial deltaic and marine sands, vertical, cylindrical structures formed under a shallow layer of water. In cross-sections, they appear as columns ranging from 2 to 35 cm in diameter and 1 to 4 m high. These structures represent the internal form of cylindrical chimneys carved by ascending water currents in sandy accumulations. These phenomena occurred on the periglacial coast of the Champlain Sea and resulted from the interaction between sea level fluctuations and permafrost dynamics. The permafrost that appeared immediately after the sediments were exposed determined the formation of underground ice cores. It was then destroyed by warming caused by marine invasion, which led to ice core melting, the formation of thermokarst basins, subsidence, and overloading, leading to increased groundwater pressure, and ultimately to the rupture of the remaining thinned permafrost. The appearance of sudden and violent upwelling flows was responsible for the opening of cylindrical chimneys. These events, associated with permafrost degradation, seem to suggest the functioning of a permafrost skimmer.
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