Archaeology has gotten better at estimating the timeline of civilization. But there are limitations for the earliest cultures thousands of years ago, like the Maya and the Egyptians. Dating can’t get more precise than within a few centuries for those ancient peoples.

But a pair of highly-radioactive solar storms that occurred long ago left their indelible mark in trees growing at the time – and could provide important new touchstones for dating civilizations, argues an Oxford team in the journal Proceedings of the Royal Society A.

Levels of the isotope carbon-14 spiked during Miyake Events, in the years 775 and 994 A.D., and is encased like a “secret clock” within timber, papyrus, linen garments, baskets and other artifacts, the team says. The Miyake Events are thought to be solar storms that bombard the Earth with gamma rays and massive amounts of solar protons, spreading the irradiated particles worldwide.

“The spikes in 775 and 994 A.D. were almost vertical and of comparable magnitude all around the Earth,” said Michael Dee, the lead author, from Oxford’s School of Archaeology. “Such markers can be easily identified in known-age tree-rings and are fixed in time.

“In the past, we have had floating estimates for when things may have happened, but these secret clocks could reset chronologists concerning important world civilizations with the potential to date events that happened many thousands of years ago to the exact year,” Dee added.

The Miyake Events, named for Fusa Miyake of Nagoya University, were first identified in the journal Nature in 2013. The elevated Carbon-14 was linked to large pockets of Beryllium-10 found in Antarctic ice cores dating from the two events.

Now the Oxford team proposes a mathematical method to take those dates, filter out bogus dates, and get better chronological estimations.

The current dating for ancient civilizations is based on celestial phenomena, such as a solar eclipse during the ninth year of a particular Assyrian king.

But if the smaller solar radiation events are catalogued and properly put into a chronological protocol, dates more exact than within two or three centuries could revolutionize the understanding of civilization’s beginnings, they said.

“Miyake Events represent a new source of tie-points that could provide the means for anchoring early chronologies to the absolute timescale,” they contend.