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Ancient radioactive tree rings could rip up the history books

Boffins mull a new way of historic dating

An archaeologist and an astrophysicist have discovered a new method of timekeeping that could reset key historic dates by inspecting ancient radioactive tree rings.

Researchers from the University of Oxford, Michael Dee and Benjamin Pope, published their results today in Proceedings of the Royal Society A.

“The discovery of past spikes in atmospheric radiocarbon activity, caused by major solar energetic particle events, has opened up new possibilities for high-precision chronometry,” the paper said.

Academics believe that powerful solar storms caused bursts of radiation that showered down on Earth in 775 to 994 CE.

The two spikes, known as Miyake events, can be detected in the unusually high levels of radioactive carbon-14 in tree-rings that grew over that time period.

Carbon-14 - an unstable isotope of carbon - ends up in plant tissue through photosynthesis. When plants are alive the carbon-14 to carbon-12 ratio remains constant, but after they die the ratio begins to decline. A measurement of the ratio, therefore, provides a way to estimate how much time has elapsed since the plant was alive.

The Miyake events stand out amongst the normal carbon-14 levels detected in the atmosphere and can be dated because the tree-rings retain the carbon-14 level from the year in which they grew.

Archives mapping out the growth year of every tree-ring are known, making the Miyake events in tree-rings time markers.

The time markers will be present in every living tree and other during that time period, allowing the date to be marked for timber used in ancient buildings or other artefacts from plants.

Each ring represents one year. Photo credit: Shutterstock

The spike for 775 CE has already been found in tree-rings in Germany, Russia, the US and New Zealand. Dendrochronology - tree-ring dating - and carbon-14 dating can be combined to date an event to a particular calendar year, the researchers said.

Michael Dee, lead author of the paper and researcher from the School of Archaeology at the University of Oxford, said: "In the past, we have had floating estimates of when things may have happened, but these secret clocks could reset chronologies concerning important world civilisations with the potential to date events that happened many thousands of years ago to the exact year.”

But there are challenges. For a more accurate dating method, more events - like the Miyake events - must be uncovered in the dendrochronological archives, and pinned down to an exact year. Secondly, carbon-14 measurements from plants should be marked in successive calendar years.

“If one of the samples overlies a known spike, the entire sequence can be automatically allocated to exactly one point in time,” the paper said.

It is unknown how common the Miyake events are and if anymore spikes can be found. Current dendrochronology archives are only available in ten-year blocks rather than year by year.

The lack of data means that archaeologists have to rely on sparse evidence for dating the history of Western civilisation before 763 CE, and Chinese history is only widely agreed from 841 CE.

They often have to rely on ancient records of rare astronomical events such as the Assyrian eclipse, a solar eclipse which occurred on the 15 June 763 CE to estimate the age of historical events.

Radiocarbon dating currently provides the best estimates, but are only accurate to within 200 to 300 years. If more Miyake events could be found, then tree-ring data taken from archaeological items and carbon-14 data could be used to determine when historic events occurred exactly. ®

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