Affiliation:
1. Ecologie Dynamique des Systèmes Anthropisés (EDYSAN) UMR 7058 CNRS – Université de Picardie Jules Verne 1 rue des Louvels 80037 Amiens France
2. Ecologie des Hydrosystèmes Naturels et Anthropisés UMR 5023 CNRS – Université Lyon 1 6 rue Raphaël Dubois F‐69622 Villeurbanne Cedex France
3. Laboratoire de Géographie Physique (LGP) UMR 8591 CNRS – Université Paris 1 Panthéon‐Sorbonne – Université Paris Est Créteil 2 rue Henri Dunant 94320 Thiais France
4. Laboratoire des Sciences du Climat et de l'Environnement, UMR CNRS CEA 1572 Avenue de la Terrasse 91198 Gif‐sur‐Yvette France
5. Institute for Botany and Landscape Ecology University of Greifswald Domstraße 11 17489 Greifswald Germany
Abstract
As in most chalk river valleys in NW Europe, the sedimentary fill of the Somme valley is mainly composed of fluviogenic alkaline peat. The site of Morcourt exhibits a thick and well‐preserved fluvial sequence (10 m, including 6 m of peat). This sequence provides the framework for reconstructing fluvial and palaeoenvironmental dynamics from the end of the Upper Weichselian Pleniglacial (~20 000 cal. a BP) to the High Middle Ages (~700 cal. a BP). The palaeoenvironmental reconstruction is based on a high‐resolution stratigraphic study of 60 transect cores, 36 radiocarbon dates and sedimentological, geochemical, pollen and plant macrofossil analyses. There are three main phases in the development of the valley floor: (i) after the incision of the abandoned Pleniglacial braided river channels, a first generation of localized peat developed during the Bølling and the Allerød interstadial (<1 m thick); (ii) at the beginning of the Holocene, a peat formation phase began in the deepest parts of the valley and then spread over the valley floor by the end of the Middle Holocene (~4700 cal. a BP), with the limited runoff shifting to a small lateral channel; and (iii) at the transition to the Late Holocene, environmental changes, driven by the intensification of human activities and perhaps accentuated by climatic modifications, caused the incision of the peat system as a result of the formation of a channel. This channel drained the valley, and then mixed a detrital load into the peat. By the Low Middle Ages, the system had been altered to such an extent that the peat was completely covered by organic silty alluvium. The water table was lower, which definitively inhibited peat formation. The Morcourt sedimentary record (thickness and continuity) and the resumption of turfigenesis during the Late Holocene are remarkable in NW Europe, making this site a model of continuous morpho‐fluvial evolution since the Lateglacial.