Researchers succeed for first time in accurately dating a 7,000-year-old prehistoric settlement using cosmic rays

Researchers at the University of Bern have for the first time been able to pin down a prehistoric settlement of early farmers in northern Greece dating back more than 7,000 years to the year.

For this, they combined annual growth ring measurements on wooden building elements with the sudden spike of cosmogenic radiocarbon in 5259 BC. This provides a reliable chronological reference point for many other archaeological sites in Southeast Europe.

Dating finds plays a key role in archaeology. It is always essential to find out how old a tomb, settlement or single object is. Determining the age of finds from prehistoric times has only been possible for a few decades.

Two methods are used for this: dendrochronology, which enables dating on the basis of sequences of annual rings in trees, and radiocarbon dating, which can calculate the approximate age of the finds by the decay rate of the radioactive carbon isotope ¹⁴C contained in the tree rings.

A team led by the Institute of Archaeological Sciences at the University of Bern has now succeeded in precisely dating timber from the archaeological site of Dispilio in northern Greece, where dating to the year had previously not been possible, to different building activities between 5328 and 5140 BC. The researchers made use of high-energy particles from space, which can be reliably dated to 5259 BC. Their research has been published in the journal Nature Communications.

Tree-ring chronologies and the ¹⁴C method have their limits

Dendrochronology uses characteristic patterns of broad and narrow annual growth rings in wood, which are influenced by climatic conditions. As a result, a wooden object can be dated by comparing the annual growth ring widths with already existing standard or regional chronologies.

"In Central Europe there is a tree-ring chronology that goes back almost 12,500 years into the past—to the year 10,375 BC. However, this chronology only applies to certain regions. There is no consistent chronology for the Mediterranean region," says the lead author of the study, Andrej Maczkowski from the Institute of Archaeological Sciences at the University of Bern.

Therefore, dendrochronological dating from this region must be classified as "floating" using radiocarbon dating. As long as a tree is alive, it absorbs the radioactive isotope ¹⁴C (radiocarbon) contained in the Earth's atmosphere through photosynthesis. When it dies, it no longer absorbs ¹⁴C; the isotope decays with a half-life of 5730 years.

A laboratory measurement method can then be used to determine how much ¹⁴C is still contained in a particular tree ring and thus calculate the tree's approximate time of death over the known half-life.

"However, the accuracy of such classifications is in the best case within the range of decades," says Maczkowski.

"Until recently, it was therefore believed that dendrochronological dating to the year was only possible if a continuous regional tree-ring chronology was available, which is the case for prehistoric periods in just three regions worldwide: this is the southwestern United States, the northern Alpine foothills and England/Ireland," explains Albert Hafner, Professor of Prehistoric Archaeology at the University of Bern and senior author of the study.