Microslag as a stratigraphic tracer to quantify floodplain processes (Lienne catchment, Belgium)

Abstract

This paper focuses on two objectives: (i) the presentation of the methodology for using small-sized iron slag (called microslag) as a stratigraphic marker to reconstruct the centennial-scale floodplain evolution and (ii) the study of the geomorphic evolution of the Ardennian rivers situated in the Lienne catchment (southern Belgium) by means of microslag evaluation, 14C dating, topographic surveys and sedimentological analysis. Using these data, the floodplain history since the Late Pleistocene is reconstructed, fine floodplain sedimentation and lateral mobility rates are estimated, and channel incision phases are identified. The first Holocene 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 the Lienne catchment, the mean floodplain aggradation over the last several centuries is between 2.7 and 16 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 recent centuries, which reduces the frequency of inundation and, therefore, sedimentation on floodplains.