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

Researchers from the University of Bern have succeeded for the first time in identifying a prehistoric settlement of early farmers in northern Greece that is more than 7,000 years old.

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

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

Two methods are used for this: dendrochronology, which allows dating based on series of annual rings in trees, and radiocarbon dating, which can calculate the approximate age of the finds based on the decay rate of the radioactive carbon isotope. ¹⁴C in the annual rings.

A team led by the Institute of Archaeological Sciences at the University of Bern has now managed to accurately date timber from the Dispilio archaeological site in northern Greece, where dating to the year was previously not possible, to various construction activities between 5328 and 5140. BC. The researchers used high-energy particles from space, which can be reliably dated to 5259 BC. Their research has been published in the journal Nature communication.

Tree ring chronologies and the ¹⁴C methods have their limits

Dendrochronology uses characteristic patterns of wide and narrow annual rings in wood, which are influenced by climatic conditions. This allows a wooden object to be dated by comparing the annual ring widths with pre-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,” said the study’s lead author, Andrej Maczkowski of the Institute for Archaeological Sciences of the University of Bern.

Therefore, dendrochronological dating from this region must be classified as “floating” using radiocarbon dating. As long as a tree lives, it absorbs the radioactive isotope ¹⁴C (radiocarbon) found 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.

How much can then be determined using a laboratory measurement method ¹⁴C is still in a given tree ring and thus calculates the estimated time of death of the tree during its known half-life.

“However, the accuracy of such classifications is at best in the range of decades,” says Maczkowski.

‘Until recently it was therefore believed that annual dendrochronological dating was only possible if a continuous regional tree-ring chronology was available, which is the case for prehistoric periods in only three regions worldwide: these are the southwestern United States, the northern Foothills of the Alps and England/Ireland,” explains Albert Hafner, professor of prehistoric archeology at the University of Bern and senior author of the study.

Paradigm shift thanks to Japanese physicist

In 2012, a solution to the problem emerged: Japanese physicist Fusa Miyake discovered that a huge influx of cosmic rays, presumably due to solar flares, can cause a rise in the atmosphere. ¹⁴C content, which is deposited in the annual rings of the relevant years. These peaks can be accurately dated from long tree-ring chronologies, and because they are global events they are important anchor points, especially in regions without consistent annual tree-ring chronologies.

“Miyake recognized the first anchor points of this kind and thus brought about a paradigm shift in prehistoric archaeology,” says Hafner. Today, a dozen of these Miyake events are known as far back as 12,350 BC, and the two major events in 5259 and 7176 BC were only discovered in 2022 by researchers at ETH Zurich.

Such events of comparable magnitude have not been recorded in recent centuries. If an event of this magnitude, such as in 5259 BC, occurs today, it will likely have a disastrous effect on telecommunications and electronics.

Miyake event makes dating in Dispilio possible

The research team of the EXPLO project led by the University of Bern has managed to establish an annual tree ring chronology spanning 303 years, ending in 5140 BC, by analyzing 787 pieces of wood from the archaeological site of Dispilio at the Orestida Lake in Northern Greece. . The identified settlement phases show various housing construction activities over 188 years between 5328 and 5140 BC. This precise dating is possible because a well-known Miyake event occurred during this period in 5259 BC.

Researchers at ETH Zurich were able to detect a peak in radiocarbon content during this period by radiocarbon dating of several individually defined tree rings. It was therefore a matter of reproducing this peak – which is reflected worldwide in the tree-ring chronologies of Siberian larch, American pine and European oak – on the tree-ring chronology of Dispilio in Greece and connecting it to the anchor point 5259 BC. .

“The Balkans is therefore the first region in the world to benefit from this paradigm shift and to successfully determine absolute dating independently of a consistent calendar,” says Hafner.

Maczkowski adds: “We expect that other chronologies for the region from this period can now be linked to the ‘Dispilio Chronologie’ in quick succession. This paves the way for the development of a regional dendrochronology for the southern Balkans.”

The Balkans have the oldest lake settlements in Europe, with sites dating back to just after 6000 BC. The region played a key role in the expansion of agriculture in Europe.