The eastern Mediterranean was the setting of major transitions in the late Stone Age, including the widespread adoption of farming. But tracing the timeline of those transitions is difficult. Now, researchers have dated a late Stone Age (Neolithic) settlement in Greece using a combination of tree ring counting and radiocarbon dating.
The researchers measured radiocarbon in timber samples from Dispilio, where early farmers built houses on stilts along a lakeshore. A radiocarbon spike traced back to a known solar storm in 5259 BCE enabled the team to date the settlement with single-year precision.
“Thanks to the painstaking work of these authors, Dispilio is the first site from the Neolithic in the northern Balkans and Greece with an exact absolute timeline,” said Brita Lorentzen, an environmental archaeologist at the University of Georgia who was not involved with the study.
The results provide a chronological reference point for other prehistoric sites in the Mediterranean and may inform research about how early farmers interacted with their environment.
Tree Rings Record the Past
Archaeologists who study the time before written history must rely on other types of records. Where wooden artifacts are available, those records can be found in tree rings—a method called dendrochronology.
Each tree ring records the plant’s annual growth. Wet years promote growth and produce thick rings, whereas dry years produce thinner rings. Because neighboring trees experience similar weather conditions, their yearly rings create a local climatic barcode. Scientists can piece together partially overlapping barcodes to create tree ring chronologies spanning thousands of years. When a new wood sample is collected, its rings can be matched up with established chronologies to determine when the tree died.
The longest existing tree ring records in the United States come from California bristlecone pines and go back almost 9,000 years. In central Europe, tree ring records go back almost 13,000 years. But such long records are rare.
“Instead of one continuous chronology going from the present back to 5,000 BC, we may instead have several chunks,” Lorentzen explained. These chunks are said to float. Radiocarbon ages from the Neolithic have uncertainties of decades or more, so chunks of single-year tree ring records need additional constraints to narrow down the period they cover.
A Chronological Cheat Code
Because there is no continuous tree ring record for the Mediterranean reaching into the Neolithic, the new study’s authors initially faced a floating chronology. But cosmic rays illuminated the problem.
The researchers made use of so-called Miyake events: short, intense bombardments of cosmic rays likely caused by massive solar storms. At least five Miyake events have been discovered in Earth’s past, dating from the 8th millennium BCE to 993 CE. If such an event happened today, it could disrupt satellite and electrical signals and produce aurorae far outside the Arctic Circle.
“Combining tree rings and radiocarbon of Miyake events is a bit like getting the cheat codes in a video game.”
Conveniently for dendrochronologists, Miyake events cause rapid increases in atmospheric radiocarbon, which is incorporated into plants, including trees. Unlike the thick or thin patterns of tree rings, which depend on local climate conditions, the radiocarbon spikes produced by Miyake events are global. This eliminates the need for a regional dendrochronological record.
“Combining tree rings and radiocarbon of Miyake events is a bit like getting the cheat codes in a video game,” Lorentzen said. “Instead of build[ing] one continuous, long chronology extending back from the present, we can skip that and anchor a chunk of chronology like that from Dispilio to an exact point in time.”
To date wood from Dispilio, the study’s authors made use of a previously known Miyake event in 5259 BCE. Conventional radiocarbon dating suggested the trees had been felled in the mid-6th millennium BCE. The researchers then homed in on each tree ring and searched for the Miyake peak, which they found in multiple samples.
Connecting Climate, Farming, and Construction
Using the Miyake peak as a reference point, the researchers determined that Dispilio was occupied for at least 188 years from 5328 to 5140 BCE, with intermittent construction during that time.
“Each of the tree rings in this chronology provides an annual time capsule of the climate and environmental conditions.”
“This is the first time that we get annually resolved information on construction episodes on a 6th millennium BC site,” said Andrej Maczkowski, an archaeologist at the University of Bern, in Switzerland, and first author of the study. In the future, tree ring sequences from Dispilio could be used to precisely date other Neolithic sites in the Mediterranean.
Wood samples from Dispilio can also provide insights into prehistoric climate, Maczkowski explained. Because a tree ring’s width reflects annual rainfall, scientists can search the dendrochronological record for signs of drought and study how early farmers coped with such crises. Conversely, tree ring records can be combined with archaeological and climate information to reveal how agriculture affected the environment.
“Each of the tree rings in this chronology provides an annual time capsule of the climate and environmental conditions,” Lorentzen said. “So we can look very precisely at both human and climate activities over time and see how they fit together.”
—Caroline Hasler (@carbonbasedcary), Science Writer
Citation: Hasler, C. (2024), Cosmic rays shed light on Stone Age timelines, Eos, 105, https://doi.org/10.1029/2024EO240280. Published on 5 July 2024.
Text © 2024. The authors. CC BY-NC-ND 3.0Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.
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