On the challenges of dating alluvial sediments with radiocesium: a caveat from the Wurm River, Central Europe

The activity of radiocesium in alluvial sediment is a widely used proxy for sediment age determination for the period after its first occurrence in the environment in 1952. In a continuous and undisturbed sediment archive, the results of this method are reliable as shown by various studies. However, depending on the specific characteristics of an archive provided by alluvial sediment, the results might be unreliable or biased. This article describes the challenges and ambiguous results obtained by the measurement of radiocesium in a soil profile that provides a time marker with a concrete structure of known age. The applicability of radiocesium for the study area was successfully tested on a floodplain profile. The results showed radiocesium peaks up to 22.5 Bq/kg that correspond to the years 1963 and 1986. As a result of different hydromorphological conditions and sedimentation rates at the sampling locations, the peaks occurred in different depths. In the floodplain, the 137Cs-peaks are significantly more distinguished than in the profile at the concrete structure. There, the radiocesium content is scattered, which queries the function of the structure as a chronological marker. The scattering might be caused by various syn-sedimentary and post-sedimentary processes and factors, such as scouring and redeposition of contaminated sediment, topsoil recycling by vegetation uptake and dieback, or input of eroded sediments from the hinterland forest soils. Thus, the concrete structure creates and at the same time destroys the possibility of using it as a chronological marker.