The origin of closed depressions in Northeastern France: A new assessment

Over 10,000 closed depressions (CDs) are found in the silty plains of Northeastern France. These small wetlands support the growth of rare plant species. Although their origins, which could be anthropogenic or geologic due to salt/gypsum lens dissolution, have been debated for 150 years, they have not yet been the focus of an integrated study. In 39 geological borings along a 15-km² strip, no salt/gypsum lenses and more than 260 CDs were recorded using LiDAR. All of the investigated CDs have a bathtub form with a flat bottom. Complete excavations clearly showed a cut contact between the sediment and the horizontal marl substratum at the bottom, and a cut at the edges of the upper marl layers. Radiocarbon dating of sediment bottoms showed that sedimentation began between the second Iron Age and the Roman period. The frequencies of pollen and Sporormiella-type depict an open landscape with grassland, pasture and cropland. These convergent findings challenge the hypothesis that CDs formed naturally and suggest that they area instead anthropogenic. Because no soil deposits were found around the CDs, digging may have been intended to marl the surrounding acidic silty soils. The high density and small size of CDs will allow the detailed reconstruction of landscape and biodiversity modifications in the region for the two last millennia.