Researchers have shown that tree rings contain evidence of past solar storms, and could be used to more accurately date important events throughout history, such as the timeline of the Maya culture and ancient Egypt.
Just like a fingerprint, the distinctive signature of a solar storm could act as time stamp in timber, papyrus, baskets, and any other living plants or organic material from the past.
The idea is that if scientists can spot the radiation signature of a solar storm in a particular ancient object such as a papyrus scroll, they'll be able to accurately match it to when the event occurred in the tree ring record, giving the precise year that the item was made.
"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," said lead researcher Michael Dee, from the University of Oxford in the UK.
Solar storms are a natural parts of the Sun's cycle, and they occur when huge amounts of radiation blast out of the Sun in a stream that can reach Earth's atmosphere.
When that happens, all the intense radiation causes plants to store a whole lot more of the radioactive isotope carbon-14 in their cells, which is 20 times more carbon-14 than normal, to be precise.
Right now, researchers figure out the age of an object using carbon dating, which works by comparing the ratio between carbon-14 and regular carbon-12.
Because carbon-14 decays at a more or less predictable rate, by seeing how much of it is left compared to carbon-12, researchers can estimate within 50 to 100 years how old something is.
But now the team has put forward a more precise dating technique by looking for these carbon-14 spikes, and using the tree rings to count back the year it occurred.
Japanese scientist Fusa Miyake first figured out the link between ancient solar storms and unusually high levels of carbon-14 back in 2012, when he linked a spike in the radioisotope to a solar storm back in 775 AD (Anglo-Saxon chronicles records report an eerie "red crucifix" in the sky in 774 AD, which could have been a visual record of the activity).
The University of Oxford team has since identified a second event, called a Miyake event, dating back to 994 AD.
Now they're proposing that researchers can use this technique to tie down 'floating dates' of ancient Egypt, the Maya civilisation, or the Bronze Age to specific years.
While we currently have a rough idea of what happened in these cultures, it's hard to nail down specific dates because our calendars are so different these days.
"In fact, the earliest truly fixable date in the Americas is still taken to be the arrival of Columbus in 1492," the authors write in the Proceedings of the Royal Society A.
But what's interesting is that it's not just tree rings that would have the fingerprint of carbon-14, it's any plant that was alive at the time of the solar storm, such as reeds that would have been turned into papyrus, or cellulose fibres woven into linen.
The next step is for researchers to find more spikes in the tree ring record, and then isolate the year they happened.
In the past, researchers never noticed these carbon-14 spikes, because they'd been analysing tree rings a decade at a time, and the solar storm only affects on year of tree ring records.
But the team has now put forward a mathematical system that would let researchers re-examine existing data to efficiently look for the events. This information could teach us more about the history of solar activity, but it could also be used alongside written histories to create an extremely accurate record of the past.
"The key here is that we have long connected chronologies. In the Old Kingdom of Egypt, we have all the sequence of kings, and the order of kings is pretty well established. We have a reasonable handle on how long they were in power," Dee told The Guardian.
"If we found two or three spikes in the third millennium BC, not only would we be able to pin down the Old Kingdom's 400 to 500-year sequence, we'd be able to check that the years between the kingdoms add up; there's no missing years, because the tree ring record is absolutely established."