Unveiling the enigma: Decoding human influence in soils with poor development. Case of study: Celtic oppidum Bibracte

The Late Iron Age Bibracte oppidum, established in the 2nd century BCE, features a distinct soil cover that does not align with typical long-term soil development or human activity patterns. These soils are affected by erosion, but it made them, on the other hand, unique concerning the preservation of information about the past. Single methods applied, as sedimentological, micromorphological and geochemical signals combined with the Optically Stimulated Luminescence (OSL) dating, brought only limited distinguishing patterns, but their combinations together with principal component showed to be crucial for the understanding the formation processes of the site. Significant variations in geochemical composition were observed, particularly in cultural layers, attributed to ashy deposition, pollution, and pedogenesis. Pedogenic magnetic particles, indicated by an increase in frequency dependent magnetic susceptibility, suggest changes in soil formation processes. Colluvial processes triggered naturally or anthropogenically 2 – 4 thousand years ago reveal, that the soil cover underwent repeatedly heavy erosion. Subsequently, some of these sediments were buried under anthropogenic terraces. This period spans from the Late Neolithic to the early Gallo-Roman Period, coinciding with the presence of the oppidum Bibracte during the Late Iron Age. Portable X-ray fluorescence device achieves the best results when combined with other geochemical and mineralogical analyses. This integrated approach ensures a more comprehensive understanding of the soil profile and the underlying site formation processes.