Geochemical and mineralogical investigation of domestic archaeological soil features at the Tiel-Passewaaij site, The Netherlands

Archaeological soil features can be defined as areas of staining in ancient cultural soil horizons and are frequently used in surveys to locate sites and activity areas. Visual observation of these features, however, provides only limited information on their origin and the processes leading to their formation and this limits site interpretation. This paper presents the results of geochemical and mineralogical analyses of domestic archaeological soil features in order to assess their composition, formation pathway and origin. The greenish soil features studied here belong to a house plan from a Roman farmhouse located at the Tiel-Passewaaij site in the Netherlands. The soils were investigated using a range of techniques, including X-ray Fluorescence (XRF) spectroscopy, sequential soil extractions, Fourier-transformed infrared (FT-IR) spectroscopy, X-ray Diffraction (XRD) and microprobe analysis. The results indicate that the studied soil features can chemically be distinguished from off-site soils e.g. based on their phosphate and carbonate content. Mineralogical analysis of aggregate samples from soil features further reveal that quartz and feldspar minerals are coated with Ca–Fe–P phases that closely resemble anapaite and mitridatite mineral forms. The formation of these mineral phases probably resulted from periodic reductive dissolution of Fe-oxide phases and subsequent precipitation of Fe–P phases, with inclusion of Ca from anthropogenic carbonates or groundwater. The combined presence of (Ca)-carbonates, P-phases and some anthropogenic trace elements (Cu, Zn) suggests that the greenish soil features stem from manure inputs.