Middle to Late Holocene landscape changes and geoarchaeological implications in the marshes of the Dives estuary (NW France)

摘要

Holocene palaeogeographic changes in the lower Dives valley, a small estuarine system in Normandy, France, have been investigated using a combination of facies interpretation from borehole cores, ~(14)C dating and pollen analysis. The palaeoenvironmental record is broadly similar to that of other coastal and estuarine systems along the English Channel but exhibits specific features in comparison with large estuarine and coastal systems, especially as regards the narrow lower valley and its margins. The identified environments included uniform tidal (6800-5500, 2200-2000 cal. BP), freshwater fen (5500-3300 cal. BP), and drained floodplain (300-0 cal. BP). In contrast, landscape variability was high during the periods 3300-2300 and 1900-300 cal. BP in relation to gradients in environmental conditions from downstream to the upper reaches of the valley. A significant palaeogeographic change between ca. 3000 and 2200 cal. BP indicates that episodes of strong coastal hydrodynamic forcing, notably storm events, may have had dramatic consequences, with significant implications for human activities. The pollen diagrams show evidence of the opening up of woodland in order to develop pastures and agriculture on the edges of the freshwater marsh during the Bronze Age (4500-2800 cal. BP), although the settlement patterns remain unknown because of the sparse archaeological discoveries related to this period. During the Iron Age, increased land use in the valley bottom had a significant impact on the environment. From the Iron Age to the Late Roman period (2800-1500 cal. BP), the dramatic change caused by marine flooding and the large size of the tidal environment favoured the development of salt extraction sites from the coast to the inner estuary. From the Early Middle Ages, restricted tidal influences and the progressive silting of the valley bottom by overbank deposits enabled a lasting change, with the development of pastureland and the miscellaneous exploitation of the various environments. Finally, after the 17th century, the systematic reclamation and drainage of the wetland produced marked changes, inducing, in particular, a significant loss of landscape and ecological diversity. In this regard, the pattern of land use in the Middle Ages probably represents an example of sustainable landscape exploitation.