Historical clay extraction from paleo-channel deposits of the late-glacial Bergstraßenneckar in the Upper Rhine Graben, southwestern Germany

Abstract

Linear anomalies of vegetation vitality observed in satellite images motivated in-depth investigations of historical anthropogenic modification and exploitation of the paleo-floodplain of the late-glacial Bergstraßenneckar (BSN) in the Upper Rhine Graben near Mannheim (southwestern Germany). Stratigraphic investigations based on up to 1.7 m deep pits, sediment sampling, and laboratory analyses (grain size distribution; C, N, S; loss on ignition; X-ray fluorescence; morphoscopy of sand grains), as well as electrical resistivity tomography, reveal the presence of long parallel trenches cutting into the organic-rich and fine-grained natural strata which result from silting-up of the abandoned BSN channel during the Holocene. The linear features are interpreted as anthropogenic trenches and were later filled with sand. We identify an aeolian origin of the sand, which points to the use of sand, e.g., from the nearby Bettenberg dune of Last Glacial Maximum (LGM) to late-glacial age. The samples for optically stimulated luminescence dating (OSL) from the fill of the trenches show a wide range of equivalent doses and insufficient bleaching as sand was filled in lumps during shoveling. This results in ages ranging from the LGM to 300 years, depending on the aliquot and age model. This wide range indicates incomplete bleaching and is in agreement with the manual filling process in historical times. Further corroboration is provided by data from the Hesse State Archive at Darmstadt through a license for clay mining and brick burning at the study site dated to 1865 CE, explicitly requiring immediate fill. Local-scale clay pits for mud-brick production have been known about in western Europe since Roman times. However, access to the resources in the BSN channels in 1865 CE was only possible after a significant fall in groundwater tables following the regulation campaign of the Rhine system starting in the first half of the 19th century, which, in a wider context, illustrates the extent to which large-scale anthropogenic changes in the fluvioscape have cascading effects down to the local scale.