Silts with a human touch: Transition from naturally- to anthropogenically-controlled fluvial dynamics revealed by OSL dating and heavy-metal analysis

While the dynamics of central European fluvial systems were originally controlled mainly by climatic perturbations, a transition occurred to anthropogenically-controlled systems during the Middle to Late Holocene. When exactly this transition of fluvial systems took place and to what degree different anthropogenic practices played a role is not yet fully understood. It is an important hiatus to address in our understanding of riverine landscapes and the search for sustainable future scenarios in light of the changing climate.

Floodplains constitute an ideal setting to address this issue, as their sediments record past river dynamics and human activities (e.g., settling, milling, mining, logging, agriculture). In contrast to the extensively studied Upper Rhine plain, it remains largely unclear how human alteration of fluvial systems climbed up the Rhine tributaries. Here, we use a combination of geophysical surveys, sedimentological investigations, luminescence dating, and heavy metal analysis at three sites to investigate how and when anthropogenic land use changes shifted the rhenian meso-scale Kinzig river from a natural to an anthropogenically-dominated fluvial system.

Our sedimentological analyses reveal three distinct phases (Late Pleistocene/Early Holocene till 9.00 ka BP, Mid/Late Holocene 9.00-0.82 ka BP, modern 0.81-0 ka BP) of floodplain accumulation, characterized by increasing accumulation rates (ca. 0.09 mm/a, 0.33 mm/a, 1.07 mm/a) and decreasing grain sizes. Concomitantly with the increase of mining activity that peaked between the 16^th and 18^th centuries, and heavy metal concentrations in the floodplain rise (enrichment factors of Ba, Pb, and Cu peaking at ca. 2.5, 4.0, and 3.0), suggesting a close link between these sediment contaminations and historic land use and mining records from the catchment.

Hence, cross-referencing the floodplain stratigraphy with catchment land use history allows for argumentation of a gradual shift from a somewhat natural to an anthropogenically altered system. It implies a time-conform response to later human settlement relative to the Upper Rhine plain and an intensification of anthropogenic impact in the floodplain stratigraphy over the last ca. 2500 years, in line with the unprecedented high floodplain accumulation rates relative to pre-human presence.