Holocene floodplain stratigraphy reflecting natural and anthropogenic induced sediment dynamics in a mining-affected catchment in the Eastern Harz Mts., Germany

The formation of fine-grained overbank deposits is, among other factors, linked to human-induced land cover changes and water management, such as mill dams, ponds and ditches. Therefore, floodplain sediments and architecture can provide information about past sediment dynamics. In former mining areas, the variability of risk element concentrations with the sediment depth is a valuable proxy for environmental conditions and anthropogenic impacts.

The study area is the 180 km² large upper and middle catchment of the Selke River, which is well-known for mining activities since the 13^th century. The Selke River is a typical river at the transition between the uplands of the Harz mountain range and the loess-dominated lowlands.

Based on archaeological and historical records, sites for drill core drilling and soil profiles were identified, representing floodplain characteristics upstream, in between, and downstream of known mining and smelting sites. The sampled sediments are analysed for grain size, radiocarbon ages from charcoal fragments, elemental concentrations, and slag.

Initial results indicate at least a biphasic nature of the overbank deposits, reflecting the onset of mining/smelting activities in the study area. In particular, changes in the risk element concentrations, the presence of slag fragments and their properties characterise anthropogenically affected sediments. The first radiocarbon ages date the earliest anthropogenically affected (e.g. polluted) sediments to the 10^th to 11^th century. In addition, variations in the risk element concentrations both vertically and horizontally within the cross-sections indicate intercalation with different phases of erosion and aggradation. Distinct changes in the grain size distribution support this complex floodplain architecture.