Carbon Exchanges between Bone Apatite and Fuels during Cremation: Impact on Radiocarbon Dates

Abstract

An important advance in the radiocarbon dating of archaeological material occurred in the late 1990s, with direct dating of cremated human remains. A crucial part of the argument was the demonstration that comparable results could be obtained from paired dates of charcoal and calcined bone from the same contexts. Recent studies, however, have noted the influence of carbon from the fuel sources, raising a question over the interpretation of the paired charcoal/bone dates.Here, fleshed modern animal joints were burned with "old" fuel of known age, providing experimental evidence under natural conditions, demonstrating a clear effect of the fuel source on the carbon isotopic composition of calcined bone. In most situations in which branchwood was used as fuel, dates on calcined bone should not show any significant offset, as the wood will be of a similar age to the cadaver. For cases in which old wood, coal, or peat are used as fuel, we expect an offset of some decades/centuries, potentially up to millennia. We observed, however, that the amount of 14 C intake from the fuel is extremely variable (from 39 to 95%). A strong correlation between age offset and δ 13 C values suggests that the latter might be useful in identifying large inputs from 14 C-depleted fuels. A level of caution is recommended when 14 C dating calcined bone in cases where fuels with an inbuilt age may have been used in the cremation process.

Bayesian Modeling of Wood-Age Offsets in Cremated Bone

“…Hüls et al (2010) cremated modern bone in a sealed furnace filled with 14 C-free CO 2 obtained from fossil fuel, and by dating the CB found that 53-86% of carbon in re-crystallized bio-apatite was derived from the cremation atmosphere. This was confirmed by open-air experiments under natural conditions, where Zazzo et al (2012) using archaeological bone and recent wood measured a carbon exchange of 48-91%, while Snoeck et al (2014), using modern bone and old wood of known-age, measured a carbon exchange of 39-95%. Carbon exchange between the bone and the combustion atmosphere will cause a calendar date offset between the calibrated 14 C measurement and the main event of interest (the date of cremation), which we will in the present paper refer to as a wood-age offset.…”

“…Beyond igniting the fires, no additional wood was added (Henriksen 2016). Snoeck et al (2014) conducted similar experiments, albeit with fleshed modern bone. The new contribution of our experiment is the dating of multiple bone fragments from the same bone, from the same pyre, thereby documenting variable uptake of exogenous carbon during cremation.…”

“…Cuts of cattle (Bos taurus) and sheep (Ovis aries) reared in Denmark were cooked and defleshed prior to cremation (Table 2). Although we assume that prehistoric humans were not defleshed before cremation, skin and flesh will combust at Table 1 Pyre fuel for experimental pyres with dendro dates and values of F 14 C and 14 C age (Hua et al 2013;Levin et al 2013;Reimer et al 2013;Hammer and Levin 2017 (Zazzo et al 2009;Snoeck et al 2014). Collagen from the bones themselves might however contribute to the carbon composition of the pyre atmosphere, but must be burnt out before recrystallization can begin, and should therefore have little effect on the CB 14 C age.…”

A Reassessment of the Routine Pretreatment Protocol for Radiocarbon Dating Cremated Bones

“…Furthermore, even if some organic carbon were to be released during the reaction, it could only have originated from endogenous carbon from the body, fuel, or atmospheric carbon (Hüls et al 2010;Van Strydonck et al 2010;Zazzo et al 2012;Snoeck et al 2014b). At the time of cremation, if the body and fuel are contemporary (which is the underlying assumption for 14 C dating of cremated bone, though there are some exceptions, e.g.…”

Calcined bone provides a reliable substrate for strontium isotope ratios as shown by an enrichment experiment

“…Both studies, then, hint at the potential for reliable 87 Sr/ 86 Sr measurements on calcined bone, but both leave several unanswered questions. Is there any exchange of strontium between calcined bone and its surrounding environment, either during the burning process itself (as has been demonstrated for carbon e.g. 24–26 ), or during a subsequent exchange in the burial environment?…”