As early as the late '40s, American researchers began using the properties of natural carbon-14 radioactivity to date organic matter. In the 50s, Nobel Prize-winning chemist Williard Franck Libby successfully dated Egyptian samples.
Today, carbon-14 dating (also known as radiocarbon dating ) determines the time elapsed since the death of a living organism. This method has revolutionized archaeometry thanks to its ability to date wood, ivory, bone, teeth, flax, straw and other organic materials.
The principle of carbon-14 dating
Carbon-14 is a radioactive carbon isotope that disappears over time, so this method is based on measuring the amount of carbon-14 present in the body.
A living organism contains a constant quantity of carbon 14. When it dies, exchanges with the outside world cease and the quantity of carbon 14 decreases according to a known exponential law. We know that carbon-14 concentration is halved every 5730 years. Dating is therefore based on measuring the ratio of C14 to total carbon. It is possible to date organisms up to 60,000 years after their death; beyond that, the quantity of carbon 14 will be too low to be measured.
Measurement is carried out using mass spectrometry coupled to a particle gas pedal (AMS). This technique requires very little material (around 0.01g vs. 1g previously), little analysis time (less than an hour vs. several weeks previously) and provides more accurate measurements than in the past.
Calibrating results
The quantity of carbon 14 remaining in a material is proportional to its age. Radiocarbon age is expressed in BP (before present) years. The so-called "carbon-14 present" was established by Libby in 1950, and the radiocarbon age is calculated on the assumption that the concentration of C14 has been constant over time, which is not the case.
In fact, carbon-14 content varies according to a number of factors, such as solar activity, climate change and industrial activity. The results obtained must therefore be corrected, or calibrated. CIRAM laboratories use calibration curves to transform the BP age into a calibrated date interval, which is then associated with a probability percentage. Only this result has any scientific significance.
The recent past, the post-bomb effect
The nuclear bombs dropped on Japan in 1945 and atmospheric nuclear testing in the following years led to massive radiocarbon production from 1954 onwards, with concentrations almost doubling in 1964 and 1965. This makes it possible to draw a strict line between organisms that lived before and after 1954. This boundary is ideal for spotting recent fakes. The main problem encountered concerns living beings that lived after 1954, with abnormally high 14C concentrations far exceeding the "normal" concentration. This is why carbon-14 dating is so relevant for recent objects, even very recent ones. Indeed, for the post-1954 period, dating accuracy is only a few years away.
But the results are ambivalent. Concentration rises more rapidly until 1965, after the 1963 partial nuclear test ban treaty. After this date, radiocarbon content decreases again, with a gradual return to natural isotope ratios. Calibration is therefore complex, depending on the date of the materials to be analyzed. This phenomenon also biases the a posteriori probability distribution, as the "slope" before the maximum is "steeper" than after. This leads to a probability distribution that is artificially more favorable to the post-1964-1965 period.
