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Changes of Chemical Structure and Composition of Charcoal by Radiocarbon Pretreatments: Decontamination by ABA and ABOx Treatments

Published online by Cambridge University Press:  11 May 2016

Shinji Tomiyama
Affiliation:
Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8601, Japan.
Masayo Minami*
Affiliation:
Institute for Space-Earth Environmental Research, Nagoya University, Nagoya 464-8601, Japan.
Toshio Nakamura
Affiliation:
Institute for Space-Earth Environmental Research, Nagoya University, Nagoya 464-8601, Japan.
Koichi Mimura
Affiliation:
Graduate School of Environmental Studies, Nagoya University, Nagoya 464-8601, Japan.
Hiroyuki Kagi
Affiliation:
Geochemical Research Center, Graduate School of Science, The University of Tokyo, Tokyo 113-0033, Japan.
*
*Corresponding author. Email: [email protected].

Abstract

Charcoal is widely used for radiocarbon dating in archaeological and paleoenvironmental studies. Reliable 14C dating requires appropriate chemical treatment to remove postdeposition contamination from the charcoal samples. This study assesses two pretreatments: acid-base-acid (ABA) and acid-base-oxidation with stepped combustion (ABOx-SC). In addition to 14C, the effects of the treatments on the chemical structure and composition of charcoal were studied using Fourier transform infrared spectroscopy (FTIR) and C/H/O elemental analysis. Samples of pine wood charred in the laboratory at 270, 300, 400, 500, and 600°C, and environmental samples of charred pine wood from pyroclastic flow deposits in southern Kyushu, Japan, were tested. The laboratory-charred samples showed that NaOH treatment removed highly hydrophilic organic components derived from endogenous and exogenous organic materials in the samples and that oxidation treatment caused the oxidative degradation of molecules in samples starting from its edges. The ABA-treated environmental charcoal yielded younger 14C dates than the ABOx-treated samples, probably owing to the effects of remaining organic contaminants bound to the edges of the aromatic molecular structures produced by the original pyrolysis. Meanwhile, it was found that ABA-SC treatment can reduce contaminants as effectively as ABOx-SC treatment. This implies that the stepped combustion (SC), not the chemical oxidation, is the key to reduce contaminant residue left after ABA and ABOx treatments. The results in this study indicate that the investigation of the structural and compositional changes of charcoal during its pretreatment is useful for assessment of the reliability of the 14C ages.

Type
Research Article
Copyright
© 2016 by the Arizona Board of Regents on behalf of the University of Arizona 

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