Dating of burnt bones
In this article, CIRAM scientists explain the protocol used for Carbon-14 dating of bioapatite.
While Carbon-14 Dating is the ideal method for analyzing bones found in archaeological contexts, the method differs for charred bones or bones found in arid regions or acidic environments.
How do we date bones by Carbon-14?
C14 dating is the most common and effective method for dating all organic material. This technique, developed in the 1940s, relies on the radioactivity of carbon-14 (instability of carbon-14 that changes over time) contained in each organism. Measuring the concentration of carbon 14 makes it possible to determine the time elapsed since the death of a living organism. But if Carbon 14 dating, or radiocarbon dating, is relevant for classical bones, the method differs for burnt bones or bones found in arid environments. Because the latter are damaged, alternative protocols will need to be used to obtain relevant dating.
What is the difference between burned bones and bones?
"Conventional" bones contain 10 to 20 th their mass in collagen, whereas burned bones contain almost none, or the collagen has been severely degraded. It is precisely this protein that we analyze during Carbon 14 dating of bones.
For bones that have been burned or preserved in arid and/or acidic environments, the collagen is either of "poor quality" or in too low quantity. It is the C/N (carbon to nitrogen) ratio that provides the index of collagen quality. This atomic ratio must be between 2.9 and 3.6 to obtain a reliable dating of collagen. If this is not the case, our scientists use an alternative protocol and date the mineral part of the bone, the bioapatite.
Processing and extraction of charred bones
For radiocarbon dating, we always favor white bones that are burned at more than 500° C, because the structural carbonates are more resistant to soil contamination.
To avoid the presence of impurities in the samples, it is imperative to use an acid-etch purification method. This is because the reaction of carbonates with phosphoric acid releases CO2, which is then absorbed into a column filled with zeolite materials and then graphitized using hydrogen and an iron catalyst. This method is recognized, as studies attest to the agreement between Carbon 14 dating on bioapatite and C14 dating on associated coals.
C14 dating method and calibration
In order to deliver accurate measurements, we systematically validate our analytical protocols through the analysis of international standards (OxII, IAEA-C7, IAEA-C5). They serve as calibration and allow us to estimate our uncertainties at about 0.5 pMC, and between 0.1 and 0.2‰ for δ¹³C and δ¹5N.
Finally, our scientists separate the different carbon isotopes using mass spectrometry associated with a particle gas pedal (AMS) and measure the concentration of Carbon 14, 13 and 12.
Conventional age is expressed in years before 1950 (BP = before present). 1950 being the reference year for carbon-14 dating. The conventional age, or gross age, is then calibrated, using OxCal v4.4 software. The calibrated dates obtained are expressed to two sigmas, i.e. 95.4 es solutions are presented.
While the preparation before measurement differs from that with "conventional" bones, CIRAM laboratories have the resources and know-how to date calcined, collagen-poor bones.