Rapid alluvial sedimentation aided expansion of Moissac during the High Middle Ages along the Tarn River in southern France

Abstract

We examine medieval urban expansion of Moissac, a town located at the outlet of the Tarn River basin draining 15,400 km2 in southern France, in order to understand the association of Moissac's expansion to river position and sedimentation patterns in the river valley. A chronostratigraphic framework was derived from three auger holes and an archaeological excavation using radiocarbon dating and standard field and laboratory characterization of sediments. Five radiocarbon dates were obtained from one auger hole, along with particle size, magnetic susceptibility, organic matter, and quantitative soil color analyses to augment field descriptions. Results indicate Moissac spawned from the Abbey of Saint-Pierre on the apex of a small alluvial fan surrounded by backswamps ca. 850 CE, and urban growth radiated down the fan toward the river bottomlands throughout the following several centuries. At ca. 1000–1200 CE, human-induced soil erosion in the watershed caused very rapid aggradation of surrounding alluvial backswamps (at long-term average sedimentation rates of 20–30 mm/y). Such rapid bottomland accretion probably was driven by rampant agricultural soil erosion linked with wet climatic periods and enhanced flooding. This bottomland accretion enabled urban expansion and agricultural activities to cover the former backswamp and wetlands by the late Middle Ages (ca. 1300–1500 CE). Although we normally associate human-induced soil erosion with negative consequences of ecosystem services, the rapid bottomland accretion and resulting habitable and arable land can be viewed as a positive ecosystem service outcome resulting from human-induced soil erosion.