Detection of charred organic matter in soils from a Neolithic settlement in Southern Bavaria, Germany

Abstract

The amount and chemical structure of the aromatic carbon fraction in archaeological soil materials was investigated. Samples from several Neolithic pits and a ditch from a settlement in southern Germany were analysed together with the surrounding loamy soil. A suite of complementary methods including 13C CPMAS nuclear magnetic resonance (NMR) spectroscopy, dipolar dephasing NMR, combined fractionation/high-energy ultraviolet (UV) photo-oxidation/13C CPMAS NMR spectroscopy and analysis of benzenepolycarboxylic acids (BPCA) were used to investigate the aromatic organic matter composition of the pits. Most of the Neolithic samples gave the highest proportion of NMR spectral intensity in the aryl carbon region with a dominant peak at 130 ppm with very low, if any, signals for substituted aromatic carbons, such as in lignin or other phenols. Between 29% and 77% of soil organic carbon survived UV photo-oxidation, mostly present as aromatic structures. Dipolar dephasing NMR spectroscopy indicated a high degree of condensation for the aromatic rings while scanning electron microscopy showed that the material had a plant-like morphology. The analysis of BPCAs as specific markers provided further evidence for the presence of highly condensed aromatic structures in the organic matter of the Neolithic pits. The evidence therefore that the aromatic carbon is derived from charcoal is strong. This material either originated from soil material that has undergone vegetation fires or from other charred organic material, e.g., residues from Neolithic fire places.