Magnetic and geochemical record of soil impacted by 300 years of Early medieval settlement

Abstract

Iron oxide minerals in soils can be influenced by various environmental conditions, which can be detected through magnetic susceptibility (χ) and its frequency dependence (χFD%). Human activities can have a long-term impact on soil properties. If we look at the soil as an archive of these activities, we can talk about the soil as a library storing information. However, the interaction between iron oxide minerals and microparticles originated from past settlement is not well understood, especially in areas where the cultural layer may have been disturbed by recent agriculture. To address this gap, we combined measurement of χ, elemental composition, and particle grain-size. Other soil parameters, such as pH, soil organic carbon, and micromorphological description were also evaluated. We examined the topsoil and soil profiles of the acropolis and bailey of the Early Medieval Prague–Královice hillfort, as well as of a control site. The exceptional aspect of the hillfort is that it was intensely occupied for 300 years. We evaluated data using Spearman correlations and principal component analysis. The results showed that pedogenic magnetic particles and clay–humus complexes adsorb elements derived from medieval settlement, resulting in enrichment of χ, χFD%, P, Ca, Mn, Cu, Zn, and Pb at the acropolis compared to the bailey and control. Magnetic parameters do not correlate with iron (Fe) values measured by pXRF as Fe is in a para- and/or antiferromagnetic form. The acropolis reflects the strongest links between magnetic parameters and P, Zn, and Pb. Despite relatively extensive soil erosion in the bailey, increased concentration of the elements P and Mn is retained in sand fraction and correlates with the χ. Our findings provide important insight into non-destructive archaeology and soil science, demonstrating how magnetic susceptibility can be a useful proxy parameter for identifying disappeared medieval settlements with poorly preserved archaeological features.