The hull of the ship Kyrenia during excavation. The hull of the Kyrenia on the seabed off northern Cyprus during underwater excavations in the late 1960s. Credit: Image provided by Kyrenia Ship Excavation team for use with this document, CC-BY 4.0
Cornell researchers have refined the estimated sinking period of the Kyrenia wreck to 286-272 BC by overcoming dating issues such as removing contaminants and revising the radiocarbon calibration curve, advancing both historical understanding and broader scientific research.
Historic shipwrecks often evoke dreams of sunken riches waiting at the bottom of the ocean to be reclaimed.
For Cornell researchers trying to date the famous Hellenistic-era shipwreck of the Kyrenia, which was discovered and restored off the northern coast of Cyprus in the 1960s, the real treasure was not the gold coins but the thousands of almonds found in jars among the cargo.
The almonds, combined with newly cleaned wood samples and the team’s expertise in modeling and radiocarbon dating, led the Cornell Tree-Ring Laboratory to identify the most likely timeline for the sinking of Kyrenia between 296-271 BC, with a high probability that it occurred between 286. – 272 BC.
The team’s work was recently published in a journal PLOS ONE. Lead author is Sturt Manning, Distinguished Professor of Arts and Sciences in Classical Archeology in the College of Arts and Sciences.
The Kyrenia has a legendary legacy as the first large ship from the Greek Hellenistic period, which was found in 1965 with a largely intact hull. It was excavated in 1967-69, along with a cargo that included hundreds of ceramic vessels, then reassembled off-site and scientifically studied.
“Kyrenia was one of the first to find that this type of rich evidence from the classical world could be found largely intact more than 2,000 years later on the seabed, if you could find it,” said Sturt Manning. “It was a bit of a watershed moment, the idea that you could actually dive and excavate and bring up a classical-era ship and thus directly explore this ancient past. Shipwrecks are unique time capsules and you can gain amazing protection.”
The hull of the Kyrenia remains shortly after being reassembled from wood recovered from excavations on the seabed. Credit: Image provided by Kyrenia Ship Excavation team for use with this document, CC-BY 4.0
Over the past six decades, Kyrenia has provided archaeologists and historians with key insights into the development of ancient ship technology, construction practices and maritime trade. To date, no less than three Kyrenia replicas have been built and launched, and these reconstructions have yielded considerable information about the ancient ships and their sailing performance. However, the timeline of Kyrenia’s origins and the exact date of its sinking have always been vague at best. Initial efforts to date the ship were based on its artifacts found, such as pottery on board and a small batch of coins, which initially led researchers to estimate that the ship was built and sunk in the later 300s BC.
“Classical texts and harbor finds already tell us that this era was significant for expanded maritime trade and connections across the Mediterranean – an early period of globalization,” Manning said. “But the discovery of the Kyrenia, less than 15 meters long, probably with a crew of four, dramatically made it all very immediate and real.” It has yielded key insights into the practicality of the early part of the millennium of intense maritime activity in the Mediterranean, from the Greeks to Late Antiquity.
The first volume of the Kyrenia Ship Project’s final publication, published last year, argued that the date of the wreck was slightly later, closer to 294-290 BC, but the primary evidence – a poorly preserved, almost illegible coin – was not. watertight.
Manning’s team, which included co-authors Madeleine Wenger ’24 and Brita Lorentzen, ’06, Ph.D. ’15, was trying to secure a date.
Dangers of polyethylene glycol
The biggest obstacle to precisely dating Kyrenia was another artifact, one from the 20th century: polyethylene glycol (PEG). Dredgers and conservationists often applied a petroleum-based compound to waterlogged wood to prevent it from decaying after it was pulled from the oxygen-deprived ocean environment.
“PEG has been the standard treatment for decades. The problem is that it’s a petroleum product,” Manning said, “which means if you have PEG in the wood, you have this ancient fossil carbon contamination that makes radiocarbon dating impossible.”
Manning’s team worked with researchers at the University of Groningen in the Netherlands to develop a new method for purifying PEG from wood and demonstrated it on PEG-treated Roman-era samples from Colchester, England, which already had established dendrochronological (tree ring sequence) data.
“We removed the PEG from the wood, radiocarbon dated it and showed that in every case we got a radiocarbon age consistent with the actual (known) age,” Manning said. “We basically removed 99.9% of the PEG.”
They used this technique to remove PEG from a Kyrenia sample that Manning and his colleagues had tried to accurately date 10 years ago, but failed. The team has also now dated a small, twisted piece of wood that was salvaged from Kyrenia in the late 1960s, but was too small to be included in the reconstruction and thus avoided the PEG treatment. It subsequently sat in a glass of water in a museum for 50 odd years.
The data showed that the latest preserved tree rings from these trees grew in the mid-4th century BC. Since the samples did not contain bark, the researchers could not determine the exact date when the original trees were felled, but they could say that the date was probably after about 355-291 BC.
Organic evidence
Working with Kyrenia’s original excavation team, the researchers examined its various artefacts, including pottery and coins, focusing on organic materials, including an astragalus (the ankle bone of a sheep or goat once used for games and divination rituals in several ancient cultures) and thousands of fresh of green almonds found in some large amphorae, i.e. ceramic vessels. These “short-lived” sample materials helped define the date of the ship’s last voyage.
The team used combined statistical modeling with dendrochronology of the wood samples to obtain a level of dating that was much more accurate than previous efforts. Modeling identified the most likely date range for the last voyage to be between 305-271 BCE (95.4% probability) and 286-272 BCE (68.3% probability) – several years more recent than current estimates.
But there was one big hitch along the way. The new data did not match the international radiocarbon calibration curve, which is based on tree rings and is used to convert radiocarbon measurements to calendar dates for the Northern Hemisphere.
Manning took a closer look at the data behind the calibration curve, which has been collected by dozens of labs and hundreds of scientists over many decades. He found that the period between 350 and 250 BC had no modern accelerator mass spectrometry (AMS) radiocarbon data. Instead, the calibration curve in this period relied only on a few measurements made in the 1980s and 1990s using an older type of radiocarbon dating technology. With collaborators in the US and the Netherlands, the team measured year-old samples of redwood and oak of known age to recalibrate the curve for the period 433–250 BC. This not only helped to explain the large jump in radiocarbon production caused by the minimum of solar activity centered around 360 BC, but also led to important revisions of the curve around 300 BC – improvements that were crucial to the dating of Kyrenia.
Manning expects the new findings will not only shed light on the timeline of Kyrenia and its cargo, but also help researchers use a calibration curve for very different projects.
“This revised 400-250 BC curve now has implications for other problems researchers are working on, whether in Europe, China or elsewhere in the northern hemisphere,” he said. “Half the people who cite this paper in the future will cite the fact that we revised the radiocarbon calibration curve during this period, and only half will argue that the Kyrenia shipwreck is really important and has a much better date.”
Reference: “Revised 350-250 BC radiocarbon calibration curve affects high-precision dating of the Kyrenia ship” by Sturt W. Manning, Brita Lorentzen, Martin Bridge, Michael W. Dee, John Southon, and Madeleine Wenger, 26 June 2024. PLOS ONE.
DOI: 10.1371/journal.pone.0302645