Microslag as a stratigraphic tracer to quantify floodplain processes during the last centuries

This research has two objectives: (I) to improve the methodology of using small-sized iron slag (microslag) as a stratigraphic marker to reconstruct the centennial-scale floodplain evolution and (II) to study the geomorphic evolution of the Ardennian rivers using the evaluation of the concentration of microslag in alluvia. In addition, ¹⁴C dating, topographic surveys and sedimentological analysis were carried out. Using these data, floodplain processes have been quantified in several Belgian rivers (Ardennes massif): fine floodplain sedimentation, lateral mobility rates and channel incision phases. A major hillslope erosion episode occurred during the Bronze Age. Archaeological data and historical sources indicate Roman and High Middle Age occupations in the area but no evidence of erosion or increased floodplain sedimentation have been observed. Several peat layers have been dated in the Chavanne floodplain to approximately 1000 BP and probably indicate very low anthropogenic activity and a high level of groundwater. Before 1000 CE, the fluvial style of the Ardennian rivers was probably different, characterized by a low-level forested floodplain, which explains the presence of frequent peat layers within the alluvial sequences and abandoned channels. From the eleventh century onwards, a generalized increase in sedimentation rates occurred, coinciding with a higher concentration of charcoal in the alluvium and also the formation of colluvial deposits. Analyses of slag concentrations allowed us to reconstruct the evolution of the floodplain topography. In Ardennian rivers, the mean floodplain aggradation over the last several centuries is between 3 and 20 cm/century, depending on the study site. Such differences in floodplain sedimentation rates can be explained by several factors such as the distance to past cultivation areas, the degree of woodland clearance, flow velocities during floods and the incision of the riverbed during last centuries, which reduces the frequency of inundation and, therefore, sedimentation on floodplains.