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Tree Rings Help Circle in on Dating Pre-History Events

Solar flares create spikes in the Carbon-14 of woody plants that can provide anchor years for more accurate dating of artifacts

(Dave Scriven /Flickr CC)
smithsonian.com

Recorded history only goes back so far. For Western society, the timeline breaks down before 763 B.C. In China, anything before 841 B.C. gets a bit dicey.

But researchers from Oxford University may have a solution, reports Tim Radford at The Guardian. By studying tree rings, they have figured out that years with particularly violent solar storms deposit up to 20 times the normal levels of the isotope carbon-14, which researchers use for carbon dating. They outlined their technique in The Proceedings of the Royal Society A.

Typically, the levels of C-14 in the atmosphere are relatively stable, and while plants and animals are alive, they incorporate C-14 into their bodies. But once they die, that C-14 slowly decays at a predictable rate. A different type of carbon is also incorporated into living creatures: C-12. But unlike C-14, this type of carbon doesn't change after the creature dies. So scientists can compare the ratios of C-14 to C-12 to approximate how old something is—give or take 100 years.

By looking at the elevated C-14 in tree rings caused by solar storms, researchers can figure out anchor dates that could help them date wood objects and timbers more accurately. In 2012, Japanese researcher Fusa Miyake identified the first C-14 spike in 775 A.D., which appears in tree rings found in Germany, Russia, the U.S. and New Zealand. Michael Dee and his team at Oxford have discovered another spike at 994 A.D. He tells Radford there are likely many more hiding in the vast collection of tree ring samples around the world.

"What we have is a decadal record going back more than 10,000 years," Dee tells Radform. "There must be more of these events and we will try and find where we should look for them." But that's a monumental task, he explains. Measuring every tree ring, which need to each be analyzed three times, would be cost prohibitive. "[A]nd there are a lot of years between now and 5,000 B.C.,” Dee says. Instead, he suggests using advanced mathematical algorithms to analyze previously collected data to look for new C-14 spikes.

The tree rings have the potential to finally tie “floating chronologies” to the current universal historical calendar. For instance, Dee points out that historians know the sequence of Egyptian dynasties and pharaohs pretty well. What they don’t know are the exact dates for when these rulers lived. The same goes for the Maya. They left behind a detailed record of their 2,500-year history, but lining that up with the current chronology has been tough. By finding a few universal anchor dates and tying them to archeological evidence, scientists could properly add that chronology to the world timeline.

And the technique is not just limited to trees. The researchers say it may be possible to detect C-14 spikes in ancient artifacts, including papyrus, linen garments and wooden timbers.

About Jason Daley

Jason Daley is a Madison, Wisconsin-based writer specializing in natural history, science, travel, and the environment. His work has appeared in Discover, Popular Science, Outside, Men’s Journal, and other magazines.

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