Distribution and chronology of Pleistocene permafrost features in France

Résumé En

Numerous periglacial features (polygons, nets, soil stripes, ice-wedge pseudomorphs and sand-wedge casts, involutions) have been listed in France through the examination of bibliographical sources, aerial photographs and field surveys in the context of the French Pleistocene permafrost database developed by the PACEA (Bordeaux) and LGP (Meudon) laboratories. These data show that a large part of the country was affected by permafrost during the Pleistocene and only the southernmost part of France seems to have been beyond its maximum extension. Large ice- wedge pseudomorphs are restricted to northern France and the Paris basin, whereas other parts of France only exhibit sand (or composite) wedges. This strongly suggests that the latter regions did not undergo permafrost phases long enough to allow growth of thick ground ice bodies. Further documentation of the database and the use of GIS tools to investigate the relationships between the distribution and dimension of the features, the nature of the host material, the latitude and other parameters, are still going on.By contrast to what has been shown for northern Europe, where most OSL ages obtained from the aeolian infill of wedge structures fall within the interval 21-15 ka, the first OSL data from southwest France indicate that the main phases of thermal contraction cracking may have occurred earlier, between ~21 and 46 ka, with a peak in the age probability densities at 26-27 ka. This shows that southwest France only recorded the coldest episodes of the last glaciation at the end of MIS3 and the beginning of MIS2. Epigenetic sand wedge polygons have probably recorded multiple events of thermal contraction. Comparison of the records from northern Europe and southwest France suggests a bias towards the most recent events in both regions. To tackle this issue, dating of sand wedges will involve collection of multiple small OSL samples in wedges or the use of single-grain OSL method. New OSL and 14C data from the main network of ice-wedge pseudomorphs in loess deposits of the Paris Basin point to an age close to 30 ka.