The “Bomb Peak” May Realign Date of an Ancient Volcanic Event

When the volcano on the Mediterranean island of Santorini erupted over three thousand years ago – give or take a few hundred – it spewed lava, rocks and ash over a huge region. The ash from that eruption is so prevalent in the archaeological record that it is used to date the strata above and below. Except few agree on the chronological date of the eruption, itself. That is why a single olive branch discovered in the ashes on Santorini has become the center of a recent controversy between archaeologists and chronologists. Whereas some archaeologists place that date in the second part of the 16^th century BCE, a study using radiocarbon dating and an analysis of the olive branch's growth rings placed this volcanic eruption around 1630-1620 BCE, nearly a 100 years’ difference. Researchers in the lab of Prof. Elisabetta Boaretto, of the Weizmann Institute of Science’s Scientific Archaeology Department, have now shown how different methods could produce dates that agree, after all.

The Weizmann scientists focused on the fact that the annual growth of the olive tree is not a simple story.  Although a cross section of olive wood does show somewhat concentric rings, the light and dark circles visible to the naked eye do not represent single years of growth. The darker rings may be a sort of staining, rather than signs of annual cycles. To get around this, the archaeological team studying the Santorini branch had made use of micro computer tomography (microCT), finding slight variations in density that logically seemed to represent yearly growth. But Weizmann's Prof. Boaretto, Dr. Yael Ehrlich and Dr. Lior Regev decided to go back to basics and ask: Do the variations noted in the microCT studies truly represent yearly growth rings, and, if so, would the last ring (and thus, its date) in the Santorini branch be a witness to the eruption?

Their answer was a resounding: sort of.

The team took a modern branch from an olive tree that was about 70 years old and dated its wood using several independent methods to study and date the growth of the rings. They applied microCT and high-resolution radiocarbon dating on various segments of a cross section. The researchers were looking to identify the “bomb peak” – a doubling of radiocarbon concentration in the atmosphere that occurred in the 60s due to the atmospheric nuclear weapons testing of those years. With the Dangoor research accelerator in her lab, Boaretto and her team conducted100 radiocarbon determinations in segments of 20 mg of wood along the growth radius of the branch. With these high-resolution measurements, the Weizmann researchers mapped the inclusion in the wood of the nuclear-testing radiocarbon to the level of single month in Israel, and they compared it with the global radiocarbon distribution from that period. On the same material, the team measured two other isotopes of carbon (¹²C and ¹³C), and these revealed seasonal changes in growth.

The highlight of the experiment, says Boaretto, was that their method was so precise, they could even identify parts of the year when the trees add their new mass – that is, olive trees do undergo seasonal growth through the spring and into the summer, a process that tends to contribute to the forming of rings that are deposited annually. Moreover, they found that the boundaries between years can generally be observed with microCT. Even when the wood was charred (which is how wood material is usually found in archaeology), the dividing lines could be made out.

In a second highlight of the group’s high-resolution method, however, they found that in the outermost layers, just beneath the bark, the growth that comprised something like the last 30 years in their samples had sort of run together in a single band. That could call ring-counting methods into question, as the ring boundaries telling them exactly how many years it had grown in that period were missing. On top of that, they found that olive trees, as they get older, can stop adding wood to some or parts of their branches even as they continue to add new growth elsewhere. The boundaries between the still-active and the dead parts are invisible to the eye, but the radiocarbon method revealed a difference of 40 years in the apparent age between two different samples, both taken around the circumference of the branch. In other words, a sample taken from the outermost area of a cross-section of a branch, which might be thought to represent the age of the tree at the time of sampling, could actually appear to be 40 or more years younger than the tree’s true age. This insight suggested to Boaretto and her team a new interpretation for the absolute date given for the “last” ring in the Santorini branch.

There is a distinct possibility that the radiocarbon date obtained from the branch does not represent the date of the eruption, but rather an older date

The Weizmann team suggested that, while the original microCT study of the Santorini branch may have been counting real growth rings, there is a distinct possibility that the radiocarbon date obtained from the branch does not represent the date of the eruption, but rather an older date. The area the sample was taken from may have stopped growing decades before the eruption. Modeling different possible scenarios, such as non-ring-forming growth and supposing a premature end to growth in the branch, the team showed that the resulting new date ranges overlap with that of the archaeological evidence.

Determining the date of the eruption is important to archaeological research: Whether it occurred before or after 1570 BCE would have large implications for other studies. This date is indeed related to the beginning of the so-called “New Kingdom” of Egypt – the period of extensive colonization and large pyramid building – and another significant signpost in the archaeological record. The original archaeological dating of the Santorini olive branch had put the eruption within the New Kingdom era; this chronological connection, or lack of connection, has significance for dating other findings in the Mediterranean region from this period.

“This is not the last word on the Santorini dating,” says Boaretto. “But it has highlighted, among other things, how we can use modern artifacts to check our dating methods, and why we need to continually return to the most basic questions. After setting the biological clock of an olive tree with radiocarbon, it is possible to better interpret a study in which the data comes from counting ancient olive tree rings.”