The origin of closed depressions in Northeastern France: A new assessment David Etienne a,b, ⁎, Pascale Ruffaldi b , Stéphanie Goepp c , Frédéric Ritz d , Murielle Georges-Leroy b,e , Benoit Pollier a , Etienne Dambrine a a Biogeochemistry of Forest Ecosystem, INRA: National Institute for Agronomic Research, Forêt d'Amance, 54280 Champenoux, France b Chrono-environment Laboratory, University of Franche-Comte, 16 Route de Gray, 25001 Besançon Cedex, France c INRAP: National Institute for Archaeological Research, 12 Rue de Méric, 57050 Metz, France d ONF: Forest Office, Maison forestière de Guermange, 57810 Guermange, France e DRAC: Regional Direction for Culture, 6 Place de Chambre, 57045 Metz Cedex 01, France abstract article info Article history: Received 28 June 2010 Received in revised form 27 October 2010 Accepted 27 October 2010 Available online 2 November 2010 Keywords: Closed depressions Forest hollows Roman period Lorraine Land use Biodiversity 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. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Closed depressions (CDs) are defined as small, isolated wetlands filled by rain water where the sediment infill is encircled by hillslopes (Norton, 1986; Tiner, 2003). These closed depressions are quite common in many silty plains of Northeastern France; Linckenheld counted 10,000 to 30,000 CDs in Lorraine in 1927. The origins of these structures are still debated. The most commonly accepted hypothesis in Lorraine (Barth et al., 2001) is that CDs formed through soil collapse caused by the dissolution of salt or gypsum lenses deep below the structures (Delafosse, 1948; Coûteaux, 1969). Alternatively, CDs could be attributed to other natural phenomena like periglacial features/ processes (Slotboom, 1963; Pissart, 2003), karstic sinkholes (Van Den Eeckhaut et al., 2007; Galve et al., 2009), morainic kettle holes (Booth et al., 2004; Frielinghaus and Vahrson, 1998; Lamentowicz et al., 2008) and human activities around archaeological settlements (Wichmann, 1903; Grenier, 1906; Welter, 1908; Linckenheld, 1927), water ponds or small quarries for clay (Härbele, 1938), iron nodules (Wingert, 1996) or lime (Gillijns et al., 2005; Vanwalleghem et al., 2007). Most CDs in Lorraine are currently located in forests, but many can still be observed in the same geological setting in the grasslands in Luxembourg (Thoen and Herault, 2006). CDs are commonly located on the summits and upper slopes of hills and are rarely observed on steep slopes or in valley bottoms (Wichmann, 1903). Slotboom (1963) and Barth et al. (2001) studied the shapes and the vegetation types of hundreds of CDs and found that CDs are generally oval or circular in shape but may have a funnel form. Their diameters range between 10 and 50 m, and they are not encircled by any kind of accumulated soil. These temporary or permanent ponds are generally colonized by Alnus glutinosa and Salix sp. with Lemna minor in the free water area and Deschampsia cespitosa and Juncus effusus on the shore. Some are filled with peat and colonized by acidic peat land species (Betula pubescens, Menyanthes trifoliata and Sphagnum sp.), which are quite uncommon in this pedological context (Thoen and Herault, 2006; Herault and Thoen, 2008). Surprisingly, very few studies have been devoted to the formation of CDs or have used their infilling as an archive of environmental change (Gillijns et al., 2005; Vanwalleghem et al., 2007, 2008). Many recent investigations in modern forests have shown that both soil properties and plant diversity are strongly influenced by agricultural activity that occurred as much as two thousand years ago (Peterken and Game, 1984; Dupouey et al., 2002; Dambrine et al., 2007). This has urged research into archives to describe the historical Geomorphology 126 (2011) 121–131 ⁎ Corresponding author. Biogeochemistry of Forest Ecosystem, INRA: National Institute for Agronomic Research, Forêt d'Amance, 54280 Champenoux, France. Tel.: +33 3 81 66 62 83. E-mail address: david.etienne@nancy.inra.fr (D. Etienne). 0169-555X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.geomorph.2010.10.036 Contents lists available at ScienceDirect Geomorphology journal homepage: www.elsevier.com/locate/geomorph