The Kyrenia Ship is an ancient merchant vessel that sank off the coast of Cyprus in the 3rd century BCE. Through fresh analysis, a team led by Sturt Manning at Cornell University has placed tighter constraints on the age of the shipwreck. The researchers achieved this through a combination of techniques that improve the accuracy of radiocarbon dating, and reversing wood treatments that make dating impossible.
In the late 1960s, a diving expedition off the coast of Kyrenia, Northern Cyprus, uncovered the wreck of an ancient Greek merchant ship. With over half of its hull timbers still in good condition, the wreck was remarkably well preserved, and carried an archaeological treasure trove of valuable coins and artefacts.
“Ancient shipwrecks like these are amazing time capsules, since their burial in deeper water creates a near oxygen-free environment,” Manning explains. “This means that we get a remarkable preservation of materials like organics and metals, which usually do not preserve well in archaeological contexts.”
Following the discovery, the Kyrenia ship was carefully excavated and brought to the surface, where its timbers were treated to prevent further decay. In accordance with preservation techniques at the time, this involved impregnating the wood with polyethylene glycol (PEG) – but as archaeologists attempted to determine the age of the wreck through radiocarbon dating, this approach soon created problems.
To perform radiocarbon dating, researchers need to measure the amount of carbon-14 (14C) that a sample contains. This isotope is created naturally in the atmosphere and absorbed into wood through photosynthesis, but after the tree is cut down, it gradually decays into more stable isotopes (mainly 12C and 13C). This means that researchers can accurately estimate the age of a sample by measuring the proportion of 14C it contains, compared with 12C and 13C.
However, when samples from the Kyrenia ship were treated with PEG, the wood became contaminated with far older, petroleum-derived carbon. “Initially, it was not possible to get useful radiocarbon dates on the PEG-conserved wood,” Manning explains.
Recent archaeological studies indicate that the Kyrenia ship had likely sunk between 294 and 290 BCE. But radiocarbon dating using the most up-to-date version of the radiocarbon “calibration curve” for this period – which accounts for how concentrations of 14C in the atmosphere vary over time – still didn’t align with the archaeological constraints.
“With the current internationally approved methods, radiocarbon dates on some of the non-PEG-treated materials, such as almonds in the cargo, gave results inconsistent with any of the archaeological assessments,” says Manning.
To address this disparity, the researchers employed a combination of approaches to improve on previous estimates of the Kyrenia ship’s true age. Part of their research involved analysing the most up-to-date calibration curve for the period when the ship sank, and comparing it with wood samples that had been dated using a different technique: analysing their distinctive patterns of tree rings.
Tree-ring patterns vary from year to year due to short-term variations in rainfall, but are broadly shared by all trees growing in the same region at a given time. Taking advantage of this, Manning’s team carried out radiocarbon dating on a number of samples that had already been dated from their tree ring patterns.
“We used known-age tree-rings from the western US and the Netherlands to redefine the atmospheric radiocarbon record in the northern hemisphere over the period between 400 and 250 BCE,” Manning explains. Variations between atmospheric concentrations of 14C differ between Earth’s hemispheres, since the northern hemisphere contains far more vegetation overall.
In addition to revising the radiocarbon calibration curve, the team also investigated new techniques for cleaning PEG from contaminated samples. They tested the techniques on samples dating from around 60 CE, which had undergone radiocarbon dating before being treated with PEG. They showed that with the appropriate sample pretreatment, they could closely reproduce these known dates.
By combining these techniques, the researchers had all the tools that they needed to constrain the age of the Kyrenia ship. “With a technique called Bayesian chronological modelling, we combined all the tree-ring information from the ship timbers, the radiocarbon dates, and the ship’s archaeological time sequence – noting how the ship’s construction must predate its last cargo and sinking,” Manning describes.
New technique uses food residues to date prehistoric pottery
“The date for the ship is most likely between 293 and 271 BCE: confirming other recent arguments that the original late 4th century BCE date for the ship needs a little revision,” he says.
By constraining this date, Manning’s team hopes that the work could enable researchers to better understand where the Kyrenia ship and its numerous artefacts fit within the wider chronology of ancient Greece. In turn, their discoveries could ultimately help archaeologists and historians to deepen their understanding of a fascinating era in history.
The researchers report their findings in PLOS ONE.
