Experimental Study on the Origin of Cremated Bone Apatite Carbon

Abstract

Bones that have undergone burning at high temperatures (i.e. cremation) no longer contain organic carbon. Lanting et al. (2001) proposed that some of the original structural carbonate, formed during bioapatite formation, survives. This view is based on paired radiocarbon dating of cremated bone apatite and contemporary charcoal. However, stable carbon isotope composition of carbonate in cremated bones is consistently light compared to the untreated material and is closer to the δ13C values seen in C3 plant material. This raises the question of the origin of carbonate carbon in cremated bone apatite. That is, does the isotope signal reflect an exchange of carbon with the local cremation atmosphere and thus with carbon from the burning fuel, or is it caused by isotopic fractionation during cremation?To study the changes in carbon isotopes (14C, 13C) of bone apatite during burning up to 800 °, a modern bovine bone was exposed to a continuous flow of an artificial atmosphere (basically a high-purity O2/N2 gas mix) under defined conditions (temperature, gas composition). To simulate the influence of the fuel carbon available under real cremation conditions, fossil CO2 was added at different concentrations. To yield cremated bone apatite properties similar to archaeological cremated bones, in terms of crystallographic criteria, water vapor had to be added to the atmosphere in the oven. Infrared vibrational spectra reveal large increases in crystal size and loss of carbonate upon cremation. The isotope results indicate an effective carbon exchange between bone apatite carbonate and CO2 in the combustion gases depending on temperature and CO2 concentration. 14C dates on archaeological cremated bone apatite may thus suffer from an old-wood effect. Paired 13C and 14C values indicate that in addition to this exchange, isotope fractionation between CO2 and carbonate, and admixture of carbon from other sources such as possibly collagen or atmospheric CO2, may play a role in determining the final composition of the apatite carbonate.

Radiocarbon Dating of Calcined Bones: Insights from Combustion Experiments Under Natural Conditions

“…In calcined bones with ô 13 C values lower than -25%o, we show that 67 ± 3% to 91 ± 8% of the carbon present in bone carbonate come from C0 2 present in the atmosphere of combustion. This finding confirms earlier results based on laboratory experiments (Hüls et al 2010;Van Strydonck et al 2010). …”

“…The comparison of the ô 13 C of the charred (black) and calcined (white) portions of the same bone indicates that these modifications are highest in the calcined portions (Olsen et al 2008). Recently, laboratory experiments performed on modern bones showed that bone 14 C activity was also modified during calcination (Hüls et al 2010;Van Strydonck et al 2010). Calcination of modern bone in the presence of old C0 2 resulted in a decrease in bone 14 C activity, which paralleled the decrease in 5 13 C values (Hüls et al 2010).…”

“…These results demonstrate that carbon isotope exchange occurs during calcination between bone carbonate and the carbon present in the environment of combustion. Several parameters seem to control the intensity of C isotope exchange (T, humidity, %C0 2 ), but the largest carbon exchange (86%) was obtained on apatite heated at 800 °C under dry conditions (Hüls et al 2010). Inorganic carbon dated in calcined archaeological bones would therefore at least partly come from the environmental C0 2 , which might in some cases have a different 14 C activity than carbon originally present in the bone itself.…”

“…Recently, laboratory experiments performed on modern bones showed that bone 14 C activity was also modified during calcination (Hüls et al 2010;Van Strydonck et al 2010). Calcination of modern bone in the presence of old C0 2 resulted in a decrease in bone 14 C activity, which paralleled the decrease in 5 13 C values (Hüls et al 2010). These results demonstrate that carbon isotope exchange occurs during calcination between bone carbonate and the carbon present in the environment of combustion.…”

Chronology of the Danish Bronze Age Based on 14C Dating of Cremated Bone Remains

“…However, bicarbonate exchange reactions with fossil bones commonly result in 14 C ages too young (Tamers and Pearson 1965;Hassan et al 1977;Hedges and Millard 1995;Surovell 2000), which, if true, pushes the start of period III further back in time. Recent results suggest that the carbon isotope composition of cremated bones probably reflects the CO 2 of the burning atmosphere (Hüls et al 2010;van Strydonck et al 2010). This indicates the possibility of cremated bone samples being affected in a similar fashion as charcoal samples by the "old wood" effect.…”