Formation, Classification, and Properties of Soils at Two Relict Charcoal Hearth Sites in Brandenburg, Germany

Albrecht Bauriegel,Alexandra Raab,Anna Schneider,Thomas Raab,Florian Hirsch

doi:10.3389/fenvs.2018.00094

Abstract

Historical charcoal production can have significant effects on soil properties. We studied soils at former charcoal production sites (relict charcoal hearths, RCHs) and compared these soils with undisturbed soil next to the charcoal hearths and four typical soils on similar parent material located at distances between 10 and 70 km from the RCHs. In a landscape typical of the northern German lowland, we found Podsolige Braunerde (WRB: Brunic Arenosols (Protospodic)) outside of the RCHs and soils with a clearly different stratigraphy within the RCHs. The main feature of the soils at both of the studied RCHs is a heterogeneous, charcoal-bearing deposit that is approximately 30 cm thick. No indications of translocation or mineral transformation processes, which form distinct soil horizons after the deposition of anthropogenic material on the RCHs, are present. Except for the differences in color and total carbon content, the soil chemistry of the RCHs hardly differs from that of the soil outside of the charcoal hearth sites. The soil colors and magnetic susceptibility values strongly suggest that the RCH substrates and the underlying topsoil were affected by thermally induced transformation of iron (hydr-)oxides. Although the charring procedure normally requires approximately two weeks, the heating effect only reaches to a maximum depth of 8 cm into the buried soil below the charcoal hearths. The presence of reddish soil and an abrupt increase in magnetic susceptibility in the upper 2 cm of the soil below the charcoal hearths indicate the heat-induced transformation of iron (hydr-)oxides into maghemite. Brighter soil color and an increase in soil organic matter in the lower parts of the buried topsoil demonstrate the combustion of soil organic matter up to 5 cm depth below the RCH. According to the German Guidelines for Soil Mapping, the soils in the RCHs are classified as Regosols above Braunerde (WRB: Spolic Technosols (Arenic)). However, because the anthropogenic features of these soil sediments are disregarded in the German Guidelines for Soil Mapping, we suggest adapting the ‘M’ horizon to permit a jM horizon. Thus, the soils in the RCHs could be classified as Kolluviale Braunerde.

Highlights

The traditional production of charcoal in upright circular hearths (“Platzmeiler”; Groenewoudt, 2007) or modern brick-built kilns still plays an important role in the local economies of some parts of Africa and Asia (Chidumayo and Gumbo, 2013; Bolognesi et al, 2015), charcoal production for industrial processes is no longer economically relevant in industrial countries
We focus on two charcoal hearth sites (RCH 970 and relict charcoal hearths (RCHs) 1089) located ∼10 km east of the city of Peitz, Brandenburg, Germany (Figure 1a), where active open-cast lignite mining has prompted systematic, large-scale archeological excavations
The soil outside of the RCHs is a Podsolige Braunerde [WRB: Brunic Arenosol (Protospodic), Figure 2a] and the grain-size distribution of this soil is dominated by sand (Table 1)

Summary

Introduction

The traditional production of charcoal in upright circular hearths (“Platzmeiler”; Groenewoudt, 2007) or modern brick-built kilns still plays an important role in the local economies of some parts of Africa and Asia (Chidumayo and Gumbo, 2013; Bolognesi et al, 2015), charcoal production for industrial processes is no longer economically relevant in industrial countries. From prehistoric times until the nineteenth century, the charring of wood in charcoal hearths was the base for metallurgic processes in Europe (Groenewoudt, 2007; Pèlachs et al, 2009). The demand for charcoal for metal production peaked from the thirteenth to nineteenth centuries in Europe (Pèlachs et al, 2009; Deforce et al, 2013; Knapp et al, 2015; Carrari et al, 2017; Criscuoli et al, 2017; Dupin et al, 2017; Krebs et al, 2017; Schmidt and Levy, 2017) and in the middle of the nineteenth century in North America (Mikan and Abrams, 1995; Straka, 2014; Raab T. et al, 2017). The platforms were built on man-made terraces These platforms were typically ∼8–10 m in diameter and were often used repeatedly (Knapp et al, 2015). The residues of the charring process, such as small charcoal pieces, ash, and the material used to seal the charcoal hearth and burnt soil, remained on the platform and formed a new layer covering the former surface (Kemper, 1941; Bond, 2007; von Kortzfleisch, 2008)

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