Organogeochemical Investigations of Black Soils in Northwestern Germany Reveal Prehistoric Agricultural Burning.
Eileen Eckmeier1, Renate Gerlach2, Guido L.B. Wiesenberg3, Jan O. Skjemstad4, Ursula Tegtmeier5, Klaas Van der Borg6, and Michael W. I. Schmidt1. (1) Univ of Zurich, Dept. of Geography, Winterthurerstrasse 190, 8057 Zurich, Switzerland, (2) Rheinisches Amt fuer Bodendenkmalpflege, Endenicher Str. 133, 53115 Bonn, Germany, (3) Univ of Cologne, Dept. for Geology and Mineralogy, Zuelpicher Strasse 49a, 50674 Koeln, Germany, (4) CSIRO Land and Water, PMB 2, Glen Osmond, SA 5064, Australia, (5) Univ of Cologne, Instof Prehistoric Archaeology, Weyertal 125, 50923 Koeln, Germany, (6) Van de Graaff Lab, Utrecht Univ, Princetonplein 5, 3584 CC, Utrecht, Netherlands
We investigated black soils in the Lower Rhine Basin (NW-Germany) following a 33 km long pipeline trench and at 16 archaeological large-scale (0.5 to 5 ha) excavations. The black soils showed properties of Luvic Phaeozems, but occurred independent of environmental and edaphic factors (e.g. parent material). They were distributed like patches in the surrounding Luvisols and were always connected with man-made pits filled with dark soil material. Visible artefacts have not been found, but most of the examined 48 pit fillings contained macroscopic charcoal. Our main research questions were: (i) could the investigated black soils in the Lower Rhine Basin have been formed by an input of charred organic matter and (ii) can we demonstrate any human impact on the soil material via organogeochemical analyses? The proportion of charred material in total soil organic matter was examined by isolation and identification of black carbon in the soil material via UV photo-oxidation and 13C NMR. Isolated black carbon material and hand-picked macroscopic charcoal was dated by 14C AMS. Phosphorus (organic and inorganic) concentrations and the distribution of lipids (n-alkanes) were compared for the dark material and the surrounding soil material. The charcoal pieces were distributed homogeneously in the soil material. They derived from typical Neolithic deciduous wood species, like Quercus (oak) or Ulmus(elm). Large proportions of the soil organic matter taken from the black soil material consisted of charred organic matter, detected as black carbon (19 to 46 % of total organic carbon). The AMS radiocarbon ages of charcoal and black carbon indicated the presence of fires from 7530 – 7200 calBC to 675 – 780 calAD, or during the Mesolithic, Early Neolithic, Late/End-Neolithic, Bronze Age and the Middle Ages. In most cases the amount of phosphorous was higher in the black soil material than in the surrounding soil. The proportion of total lipids compared to total organic carbon was lower in the black soils than in the surrounding Luvisols. Some samples showed a particular high abundance of short-chain and even carbon-numbered n-alkanes.Temperate deciduous forests, which covered the studied area, could not be easily ignited by natural causes. Thus, mainly human induced fires are very likely the sources of the charred organic matter found in the dark soil material. This conclusion is consistent with the measured radiocarbon ages that clustered in archaeological periods where burning presumably was a part of agricultural activities both in Southern and Northern Germany. An increased input of organic matter, e.g. from manure or food waste was inferred by higher phosphorous concentrations in most pit fillings. The unexpected distribution of n-alkanes could result from a still unknown process of decomposition of plant material, which cannot be attributed to common microbial degradation processes in soils. Probably, these patterns are due to degradation during biomass burning. We conclude that the investigated black soils are relicts of prehistoric agricultural burning activities in Northwestern Germany. However, the process of incorporation of black carbon in soils remains unknown, and conversion rates of biomass carbon to black carbon for temperate deciduous forests are still not available. By investigating experimental prescribed burning in an archaeological field trial we are currently examining the production of charred organic carbon and its fate in soils after fire.