Buried dark soil horizons and archaeological features in the Neolithic settlement region of the Lower Rhine area, NW Germany: Formation, geochemistry and chronostratigraphy

The properties and age of buried humic and clay-rich dark soil remains (Bht-horizons and pits) at archaeological excavations in the Lower Rhine Basin (NW Germany) were investigated. These Bht-features were formerly described as a component of Luvic Phaeozems and as relics of chernozemic soils.Field observations challenge this interpretation. The Bht-horizons in the Lower Rhine Basin occur in a patchy distribution independent of relief position and climatic condition, and they are mostly connected with artefact-free but human-made pits (off-site features) consisting of the same Bht-material. The presence of charred organic matter (pyrogenic carbon) and its radiocarbon ages suggested that these Bht-horizons are not relics of naturally formed soils but rather archaeological features. Pyrogenic carbon, produced during agricultural burning practices (e.g. slash and burn) in the Younger to End Neolithic (4400–2200 BC), led to the characteristic dark colouring of the soil material.Previously, it was generally presumed that the parent material of those dark soils is Pleistocene loess. New IRSL and OSL ages (around 6.4–4.3 ka) indicate that the Bht-horizons in the Lower Rhine Basin formed in colluvial sediments which also date to the Younger to End Neolithic. Thus, the history of human induced soil erosion in the Lower Rhine area started more than 1000 years earlier than assumed before. It was most likely connected to Neolithic burning practices (slash and burn) which subsequently triggered soil erosion, and then influenced the formation of dark Anthrosols by eluviation and illuviation of the partly charred dark humic material. These Anthrosols are an example of strong human impact on soil forming processes since the onset of agriculture.