Structural Characterization of Charcoal Exposed to High and Low Ph: Implications for ¹⁴C Sample Preparation and Charcoal Preservation

Abstract

ABSTRACT. Chemical and structural similarities between poorly preserved charcoal and its contaminants, as well as low radiocarbon concentrations in old samples, complicate 14 C age determinations. Here, we characterize 4 fossil charcoal samples from the late Middle Paleolithic and early Upper Paleolithic strata of Kebara Cave, Israel, with respect to the structural and chemical changes that occur when they are subjected to the acid-base-acid (ABA) treatment. Differential thermal analysis and TEM show that acid treatment disrupts the structure, whereas alkali treatment results in the reformation of molecular aggregates. The major changes are ascribed to the formation of salt bridges at high pH and the disruption of the graphite-like crystallites at low pH. Weight losses during the treatments are consistently greater for older samples, implying that they are less well preserved. Based on the changes observed in vitro due to pH fluctuations, various methods for removing contamination, as well as a mechanism for preferential preservation of charcoal in nature, are proposed.

Radiocarbon dating of charcoal and bone collagen associated with early pottery at Yuchanyan Cave, Hunan Province, China

“…Calcite, however, buffers the ground water to above pH 8, and this is often not conducive to the preservation of charred materials. In fact, the prescreening showed that the charcoal was E 6 2 12 10 T10 T11 21 10 18 5 T12 11 8 8 6 T4 12 3 21 4 T5 12 2 12 6 Totals 64 26 75 33 generally poorly preserved, especially in the eastern part of the cave, which today, at least, is much wetter than the western part (24). We also note that less than half the bones contained acid insoluble collagen.…”

“…The latter proves to be a good indicator of charcoal preservation (24) and in practice determines the yield of clean charcoal and hence whether or not the sample can be dated. Only samples that were well-preserved and free of detectable contaminants were dated.…”

“…While excavators of the cave sites have cited the cause as contamination from calcium carbonate in the karstic environment of the cave (2), this problem actually may be related to the presence of large amounts of calcite in the archaeological matrix of the caves. This can indirectly result in poor charcoal preservation (24). Here we apply a prescreening strategy for identifying the best-preserved bone collagen and charcoal samples.…”

Assessment of oxygen plasma ashing as a pre-treatment for radiocarbon dating

“…4 which a sample will be susceptible to dissolution/oxidation (Bird et al, 2002;Brock and Higham, 2009;Rebollo et al, 2008). In practice this means that some samples rapidly undergo virtually complete dissolution/oxidation, when only a small proportion of the material that is lost in pre-treatment is likely to be an exogenous contaminant.…”

A Puzzling 14C Result Obtained for a Carbonized Wood Sample Embedded in Volcanic Lava

“…It has been reported that the characteristics of a very old wood charcoal sample are different from those of modern wood charcoal according to its state of preservation (Cohen-Ofri et al 2006;Rebollo et al 2008). It can be assumed that these differences may lead to some difficulties when radiocarbon dating very old wood charcoal samples.…”