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

Abstract

Radiocarbon dating of the carbonate remaining in calcined bones is widely regarded as a viable alternative to date skeletal remains in situations where collagen is no longer present. However, anomalously low δ13C values measured in calcined bones prompted questions about the origin of the carbon used for dating. The goal of this study was to quantify the magnitude of carbon isotope exchange between bone carbonate and environmental CO2 for bones calcined under natural conditions. Four archaeological bones ranging in age between the Neolithic and the Medieval period were combusted on a separate open fire for up to 4 hr and subsamples of calcined bones were taken every hour. All the bones experienced a significant increase in IRSF values and decrease in carbonate content and δ13C values. 14C ages measured in the carbonate fraction of well-calcined bones indicate that 67 ± 3% to 91 ± 8% of the carbon present in bone carbonate was replaced by carbon from the atmosphere of combustion. This finding confirms previous results obtained under laboratory conditions and has serious implications for 14C dating of calcined bones found in archaeological contexts. The 14C age obtained on a calcined bone will only reflect the true age of the bone sample if the age difference between the bone and the charcoal can be neglected. Our results show also that δ13C values of calcined bones can be used to estimate the degree of C exchange and control for postburial diagenetic alteration.