Archaeologists believe they have identified a new way of putting
accurate dates to great events of prehistory. Rare and spectacular storms on the sun appear to have left their
mark in forests and fields around the planet over the past 5,000
years.

Michael Dee, of Oxford University’s research laboratory for
archaeology and the history of art, thinks evidence of such solar
storms could help put precise years to some of the great
uncertainties of history: the construction of Egypt’s Great
Pyramid of Giza, the collapse of the ancient Mayan civilization
in Central America, and perhaps even the arrival of the Vikings
in the Americas.

Every tree maintains its own almanac in the form of annual growth
rings. For decades dendrochronologists have been using tree-ring
evidence and radiocarbon dating to build a timetable of events
that confirm historical accounts, even those predating the first
written chronicles.

Carbon dating works by comparing the ratio between two isotopes
of the element carbon, C-14 and C-12, present in old samples of
organic material. Because of constant bombardment by cosmic rays,
ratios of C-14 in the upper atmosphere are more or less constant,
and since radioisotopes decay with time, at a predictable rate,
the ratio of C-14 to the stable form C-12 is a guide to the age
of any timber in a cathedral roof or a stone age burial site, for
example.

The technique is imprecise, with an error range of 50-100 years,
and also expensive. However, the discovery of unusually high
levels of C-14 – up to 20 times the normal level – laid down in
during especially violent solar storms may enable scientists to
date material much more accurately. Every tree growing at the
time of such a sun storm, anywhere in the world, would have
preserved a record of it.

In 2012, the Japanese scientist Fusa Miyake identified a
dramatically raised level of C-14 in one set of growth rings that
is known to date from 775AD. Since then, what the Oxford team
call a second Miyake event – a consequence of a catastrophic
extraterrestrial discharge of energy – has been identified from
the year 994AD.

Dee and his co-author Benjamin Pope propose a new science,
astrochronology, to harness this solar storm evidence, in an
article in the journal Proceedings of the
Royal Society A . The technique could very precisely tie
so-called “floating chronologies” of ancient Egypt , Mayan
civilisation or the bronze age to fixed dates in the universal
calendar. The Mayan day numbering system spans a thousand years
and is well established – but researchers have so far been unable
to tie any event in that to any date in the Gregorian calendar of
Europe.

“In fact, the earliest truly fixable date in the Americas is
still taken to be the arrival of Columbus in 1492,” the authors
write.

Where checks have been made on tree rings, these have been on a
decadal basis – which is why no-one noticed the rare single-year
anomalies of the past. Such celestial violence may also have been
witnessed: the Anglo-Saxon Chronicle records an eerie “red
crucifix” in the sky in 774AD. Another spectacular solar storm in
1859 led to aurorae visible in Hawaii and the Caribbean.

The scientists propose cutting-edge mathematical techniques to
re-examine all the existing data and identify hints of more
possible solar storm spikes.

“What we have is a decadal record going back more than 10,000
years,” Dee said. “There must be more of these events and we will
try and find where we should look for them. You can’t just
measure every single tree ring because it would cost tens of
millions of pounds, because each tree ring has to be measured in
triplicate essentially and there are a lot of years between now
and 5,000 BC.”

If the researchers do identify another spike, they expect it to
be duplicated in surviving plant tissue everywhere in the world
from that year: in the reeds that became papyrus, in the flax
that was preserved as linen, in the timbers that shore up ancient
graves. Spikes in tree rings from 775AD have been found in
Germany, Russia, the US and New Zealand. The astrochronologists
have a potential record far more accurate than a human scribe.

“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 said.

“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 400-500 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.”