What’s in a colluvial deposit? Perspectives from archaeopedology

Abstract

Colluvial deposits are considered as sedimentary archives for the reconstruction of the sedimentation and climate history, past pedogenesis and phases of land use. However, the human contribution to the formation of colluvial deposits is mainly based on assumptions derived from the local chronostratigraphy and archaeology. For this reason, there is often a substantial gap in the identification of specific land use practices that promoted prehistoric soil erosion and sedimentation processes.We use an archaeopedological multi-proxy approach on a multi-layered colluvial deposit (six colluvial horizons) at the prehistoric site of Fürstenberg (Southwest Germany) to gain detailed insights into the sedimentation history, past pedogenesis and land use practices since the Neolithic. Soil and geochemical analyses such as X-ray fluorescence (XRF), X-ray diffraction (XRD), pedogenic oxides, pH-value and calcium carbonate content support the chronostratigraphy based on ages from optically stimulated luminescence (OSL) and AMS 14C dating. Further, biogeochemical analyses of phytoliths, charcoal spectra, black carbon (BC), soil organic matter (SOM) composition by using pyrolysis-field ionization mass spectroscopy (Py-FIMS), urease activity, steroid biomarker and heavy metals (HM) are used as land use proxies.The OSL and 14C ages and the local archaeological record indicate six phases of colluvial deposition that occurred through five phases of land use comprising the Early to Younger Neolithic, the Urnfield to Hallstatt period, the Iron to Roman Age, the High Middle Ages, and the pre-modern period. The soil and geochemical proxies correlate with a phase of geomorphodynamic stability between the Early to Younger Neolithic and the Urnfield to Hallstatt period. The high abundance of grass morphotypes since the Neolithic and the increase of Juniperus since the Urnfield to Hallstatt period indicate a persistent anthropogenic impact on the vegetation. Considerable amounts of burned OM (up to 676 g BC kg−1 SOC, also identified as thermally stabilized SOM compounds by Py-FIMS) in colluvial horizons provide information that fire clearing (e.g. slash-and-burn) was especially used to open and maintain the landscape until the Roman Age. The absence of phytolith species originating from cereals, the occurrence of Juniperus and the evidence from the analysis of urease activity and faecal biomarkers indicate that the slopes of the Fürstenberg were mainly used for livestock farming and wood procurement.Thus, the application of specific soil-biogeochemical proxies to colluvial deposits provide new information on the nature of past land use practices that caused the formation of colluvial deposits at that time. In general, archaeopedological multi-proxy analyses of colluvial deposits, integrating the local archaeological record, contributes to a refined understanding of how humans have shaped the landscape since the Neolithic.