Sample preparation for radiocarbon dating of charred materials : the decontamination process of ABOx-SC method

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  • 炭化物の14C年代測定のための試料処理 : ABOx-SC法における外来炭素の除去過程について

Abstract

Charred material is widely used for radiocarbon (14C) dating in archaeological and palaeoenvironmental studies. Reliable 14C dating requires appropriate chemical treatment to remove post-deposition contamination, especially humic acid, from the charred material. This study compared two pre-treatment methods: Acid-Base-Acid (ABA) treatment and Acid-Base-Oxidation with Stepped-Combustion (ABOx-SC) treatment, in order to examine the effectiveness of decontamination. 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 charred material's molecular structures, which were produced by the original pyrolysis. Oxidation treatment can preferentially remove humic acid contaminants, which remained after NaOH treatment. Therefore, the ABOx treatment can more effectively and efficiently remove exogenous humic acid than the ABA treatment. Furthermore, stepped combustion procedure can remove remained contaminants after chemical treatment, such as exogenous organic component and atmospheric CO2 absorbed during samples treatment. Therefore, the ABOx-SC treatment of the charcoal samples can lead a most reliable 14C age for the "old" (especially >30 ka) samples, which are susceptible to contaminants.

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