When researcher Charlotte Pearson examined a palm-sized piece of ancient tree, she noted that one of the rings seemed unusually small. Three years later, and armed with new methodology and technology, she discovered that this ring could mark the year that the volcano of Tera (the Greek island of Santorini) erupted in the time of the ancient Minoan civilization. The date of the eruption, which is one of the largest that humanity has witnessed, has been debated for decades.
Pearson, a professor of dendrochronology and anthropology at the University of Arizona, is the lead author of a paper, published in Proceedings of the National Academy of Sciences , in which she and her colleagues have used a new hybrid approach to assign calendar dates to a sequence of tree rings, which constrains the period during which Tera erupted within a given year. This allows them to present new evidence that could support an eruption date of around 1560 B.C.
Trees grow according to the conditions of their local environment. Each year, trees produce a new layer of concentric growth, called a tree ring, which can record information about rainfall, temperature, wildfires, soil conditions, and more. Trees can even record solar activity as it waxes and wanes.
When a sequence of tree rings of various ages overlap and add up, they can span hundreds or thousands of years, providing information about past climatic conditions and the context of concurrent civilizations.
The longest timeline in the world dates back 12,000 years. But in the Mediterranean, the problem is that we don't have a complete and continuous record going back to the time of Thera Pearson said. We have recorded the last 2,000 years very well, but there is a gap. We have tree rings from earlier periods, but we don't know exactly what dates the rings correspond to. This is what is called a 'floating timeline' .
Filling in this gap could help pin down the date of Tera's eruption and paint a climatic backdrop for the various civilizations that rose and fell during the Bronze and Iron Ages, which together spanned between 5,000 and 2,500 years.
Until you can put an exact year on events on a scale that makes sense to people – a year – it's not that powerful Pearson said. This study is really about taking the chronology of Peter Kuniholm (my co-author and dendrochronology lab research professor) who has put together over 45 years of work and dating it in a way that wasn't possible before .
The University of Arizona Dendrochronology Research Laboratory was established in 1937. This is the collection that founded the field of dendrochronology, and it is by far the largest in the world. Researchers come from all over to use our collection. It is filled with the remains of ancient forests and archaeological sites, which no longer exist, and contains samples of wood that were instrumental in the growth of the discipline of dendrochronology said Pearson.
The collection includes wood from the Midas Tumulus at Gordio in Turkey – a giant tomb of a man who was probably the father or grandfather of King Midas. From woods like these, Kuniholm has been constructing a tree-ring chronology of the Mediterranean for nearly half a century. Together, the Kuniholm records from the period B.C. they span more than 2,000 years, including trees that were growing at the time of Thera's eruption, making them key to research.
Despite the length of this chronology, it was never dated. To pin it down, the team decided to try something new.
When cosmic rays from space enter Earth's atmosphere, neutrons collide with nitrogen atoms to create a radioactive version of carbon, called carbon-14, which spreads throughout the planet. All living things on Earth, including tree rings, collect carbon-14, and because tree rings store a measure of carbon-14 for each year they grow, they maintain patterns that show how carbon-14 has changed. over time. These carbon-14 patterns in tree rings around the world should match.
Pearson and his team used the carbon-14 patterns captured in the Gordio tree rings to anchor the floating chronology to similar patterns from other calendar-dated tree-ring sequences.
It is a new way to anchor floating chronologies that makes use of the annual precision of tree rings said Pearson.
To validate their findings, the team turned to calendar-dated rings from high-elevation cherry pines of western North America that lived at the same time as the Gordio trees.
When large volcanic eruptions occur, the crustal tissue is often scarred by freezing during the growing season, creating a frozen ring said second author Matthew Salzer. We then compared the dates of those frozen rings with what was happening in Mediterranean trees, which respond to volcanoes by creating wider rings . And it worked. He showed that the wide rings in the Mediterranean chronology occurred in the same years as the frozen rings. We take that as confirmation that the dating was probably correct.
The team then considered using an X-ray fluorescence machine to scan the wood for chemical changes.
We scan the entire period in which Tera is known to have occurred , said Pearson, and we detected a very slight decrease in calcium, right where I noticed this smaller ring years ago . Although it is a slight fluctuation, it is significant and only occurs at one point in the years around 1560 B.C. We therefore suggest that it is a possible date for the Tera eruption said Pearson.
Something changed the chemistry of the environment in which the tree grew; acid deposition from a volcano is one possibility, forest fire is another, but since the date matches other tree ring markers for a major eruption, Pearson says it's worth exploring further.
I think to do good science you have to investigate everything and keep an open mind until enough data is gathered Pearson said. This is another little piece of the puzzle.