Prehistoric alteration of soil in the Lower Rhine Basin, Northwest Germany—archaeological, 14C and geochemical evidence

In Early Holocene, Chernozems were assumed to have covered the entire loess landscape of the Lower Rhine basin—today mirrored by the distribution of Luvic Phaeozems. These Luvic Phaeozems have characteristic dark brown (Bht) horizons accumulating clay and humus, inherited and translocated from their precursors Chernozem black humic A horizons. We examined Luvic Phaeozems along a 33-km-long and 2.0–2.5-m-deep gas pipeline trench in the Lower Rhine Basin, west of Cologne. Along this transect we discovered clusters of hundreds of regularly shaped pits. These pits were always connected to the Bht horizons of adjacent Luvic Phaeozems. The Luvic Phaeozem horizons and the pits were investigated by combining methods from (geo-) archaeology (geographical distribution within the landscape, shape of the pits, soil texture), geochemistry (content of carbon, nitrogen and black carbon), palaeobotany (species determination of charcoals) and AMS 14C measurements. We found that the Luvic Phaeozems occurred not only in the loess-covered landscape but also in the sandy Holocene floodplain, and their distribution could not be limited to certain slope positions or parent material. Carbon and nitrogen concentrations in the Luvic Phaeozem horizons and pits were larger than in the surrounding Luvisols, whereas the C/N ratios were small (< 10). Material found in the Luvic Phaeozem pits was clearly different from material found in prehistoric settlements. The pits investigated here never contained artifacts, and carbon and nitrogen concentrations and C/N ratios were smaller. We found charcoal particles, and black carbon contributed up to 46% of the total organic carbon. The AMS 14C ages of charcoals and black carbon indicated that fire occurred from Mesolithic (9500–5500 BC) to the Medieval Ages (500–1500 AD), and mainly in the Late-/End Neolithic period (4400–2200 BC). We conclude that (i) the Luvic Phaeozem pits and horizons are man-made, formed during several archaeological epochs between Mesolithic and Middle Ages, (ii) these pits must have been formed outside the actual prehistoric settlements (off-site) and may serve as a novel archaeological feature, (iii) the purpose of these pits at present is not clear and (iv) human activity has altered and ultimately formed the investigated soils of the Lower Rhine basin in prehistoric time.