Two dating techniques

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1 Jan 2024

Dendrochronology

During its lifetime, a tree thickens by one or two growth rings every year, its circumference varying according to the tree's reactions to its environment. This phenomenon affects both trunk and branches.

Cross-section of a trunk

This cross-section of a trunk shows the ring system of the heartwood. Neither bark nor sapwood is available here, but differences in ring thickness are clearly visible.

If we study the rings of a living tree and compare their sequence with those of an older tree of the same species, and continue the comparison with even older trees, we can reconstruct a long history by superimposing the ring patterns. To do this, the heartwood of a living tree is removed and the rings compared with those of a beam from an old building, which in turn are compared with those of an older frame, and then with pieces of wood found during excavations. In this way, we can establish a reliable chronology going back several millennia.

However, there may be several "floating" sequences, i.e. not set in the main chronology. In such cases, research must be carried out on other samples. When a piece of wood is found on an archaeological site, its ring system is compared with the known sequence.

Precise dating

If the wood still has its sapwood and bark, we can generally determine the date it was felled, which does not necessarily correspond to the date it was used. However, in the past, wood was often used green, so the two dates may be practically concomitant. For the moment, this method has been applied to a limited number of species, such as :

Oak in Northern Europe

The pine

Douglas fir in the United States

The main purpose of dendrochronology is to verify or correct carbon-14 dating: by combining the two techniques, we can establish the most accurate dating possible.

Ring measurement

A computer counts the rings, measures their thickness, then compares the pattern with known sequences, specific to a particular species and region.

Carbon-14 dating

The first scientific and reliable method of absolute dating in archaeology is carbon-14 dating. It is based on the fact that all living elements contain carbon, of which there are both stable and radioactive types. Carbon-14 is the radioactive form. When an organism dies, its carbon-14 content decreases, as atomic particles are lost at a constant rate. This rate can be measured using scientific equipment, enabling the object under study to be dated.

Rate of decrease

This rate was determined in the USA by Willard Frank Libby (1908-1980): he established that in the space of 5,568 years, an object had lost half its quantity of carbon 14.

Willard Frank Libby (1908-1980)

Since then, a number of corrections have been made to this study, including an increase in time to 5,730 years. The discovery that the atmospheric production rate of carbon 14 is not constant over time necessitated the application of a correction factor (calibration). The dates determined are first transferred without calibration, then rectified by adaptation to calendar years, taking into account atmospheric fluctuations. Thus, for example, a carbon-14 date of 2000 years BC corresponds approximately to the calendar year 2350 BC.

High precision

Early dating methods only measured radioactive emissions from the objects under study. But using particle acceleration dating, the radioactivity of the entire sample is measured, enabling greater precision to be achieved with a smaller quantity of material. Carbon-14 dating is best known for its application to single objects, but is mainly used to date layers of archaeological occupation. Several samples are taken from a well-defined archaeological perimeter, ensuring that they are not contaminated by modern materials. This dating method can be applied to any formerly living element, e.g. wood, charcoal, bone, shell, etc. Once the quantity of radioactive carbon has been measured, the date of the object's death can be established, i.e. the year in which its carbon-14 content began to decline.