64 Scientific American, December 2021grow. Because each ring within a tree’s trunk corresponds to
a single year of growth, scientists can determine the precise
dates of any isotope spikes caused by increased solar activi-
ty: the more carbon 14 there is in one ring, the more solar
particles were hitting our atmosphere at that time. Such rings
“allow us to reconstruct patterns of radiocarbon through
time,” says Charlotte Pearson of the Laboratory of Tree-Ring
Research at the University of Arizona, who is a co-author on
Brehm’s paper. “One of the key things that drives those fluc-
tuations is the activity of the sun.”
By studying the concentrations of beryllium 10 and chlo-
rine 36 in ice cores, scientists can make similar though
slightly less accurate measurements. Together the two
methods can provide a precise account of historical events.
We have tree-ring data for most of the Holocene—the cur-
rent geologic epoch, which began about 12,000 years ago.
Poring over them to search for carbon 14 spikes is time-con-
suming, however. Looking at just a single year typically
requires weeks of analyzing and cross-correlating multiple
tree-ring samples. “There’s 12,000 years of the Holocene to
do, and we’ve done 16 percent of it,” says Alexandra Bayliss,
head of scientific dating at Historic England and a co-au-
thor on the paper. “It’s a matter of time and money.”
Brehm and his team got lucky. For the event in 7176 b.c.,
they first saw preliminary evidence for a beryllium 10
spike in ice cores. The researchers followed up with tree
rings and saw a corresponding spike of carbon 14. For the
event in 5259 b.c., Bayliss had noticed a gap in archaeo-
logical data around this time period. Studying carbon 14
data in tree rings from this era, the team found another
spike. “We found this huge in crease” for both dates, Brehm
says, each similar in magnitude to the spikes Miyake found
in the samples that clinched the a.d. 775 event.
At first scientists were unsure what had caused these
spikes, and some even thought solar events were unlikely.
A 2013 study led by Brian Thomas of Washburn University,
however, showed solar flares were the probable culprit.
“There were people making suggestions [that the 775 spike]
could be from a supernova or even a gamma-ray burst,” says
Thomas, who was not involved in the latest paper by Brehm
and his colleagues. “But they’re just too rare to cause this
kind of frequency. It doesn’t fit as well as the solar explana-
tion.” Such large, frequent spikes, he argues, were more like-
ly the result of increased solar activity—possibly accompa-
nied by a geomagnetic storm similar to the Carrington
Event but far more powerful. “The Carrington Event isn’t
even detectable” in tree rings and ice cores, Bayliss notes,
which suggests it was minuscule by comparison.
Even so, the exact correlation between spikes in solar
particles and the intensity of any accompanying geomag-
netic storm remains unclear. “A big particle event is often
associated with a geomagnetic storm, but it doesn’t nec-
essarily have to be,” Thomas says. It may even be that geo-
magnetic storms like the Carrington Event do not cause
spikes in carbon 14 at all—that would explain its absence
from tree-ring and ice-core data dated to the event. There
are hints, however, that at least the event in 775 was
accompanied by powerful auroras, recorded in China,
pointing to a strong geomagnetic storm alongside this