Trees are like sentinels that preserve a record of shifting climates. Their growth rings hold that history and dendrochronology studies those rings. Scientists can determine the exact ages of trees and correlate their growth with climatic and environmental changes.
But they also record the effects of more distant changes, including the Sun’s activity.
Carbon is life’s primary building material. It’s the backbone of life on Earth because it forms bonds with itself and other atoms in a wide array of compounds. Carbon is present in almost 10 million different compounds. Trees are made of mostly cellulose, a carbon-rich organic compound with the formula (C6H10O5n.
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But all carbon isn’t created equal. It exists naturally in three different isotopes: Carbon 12 (12C, carbon 13 (13C), and carbon 14 (14C). Carbon always has 6 protons, but the different isotopes have different numbers of neutrons. 14C is different than the other isotopes because it’s a radioisotope. That means it decays, and since 1949, scientists have been using 14C to find the ages of ancient things through radiocarbon dating.
Trees absorb carbon daily as they take in carbon dioxide and expel oxygen. Trees don’t discriminate between the different isotopes of carbon. So when scientists analyze the chemical contents of trees’ growth rings, they find all three naturally occurring carbon isotopes.
Some events cause a bump in the amount of 14C in Earth’s atmosphere. Scientists can date tree rings accurately, so by finding which tree rings have more 14C, scientists can determine when Earth experienced a carbon-14 bump.
This all goes to the heart of a new study published in the Proceedings of the Royal Society A. The paper is “Modelling cosmic radiation events in the tree-ring radiocarbon record.” Dr. Benjamin Pope from the University of Queensland’s School of Mathematics and Physics led the study.
Carbon-14 has unearthly origins.
When cosmic rays from the Sun and more distant stellar objects strike Earth’s upper atmosphere, the rays interact with nitrogen, which is abundant in Earth’s atmosphere. The interaction creates 14C, which mixes freely in the atmosphere, where it’s taken in by living things. The amount of carbon-14 is relatively constant as long as the Sun behaves “normally.”
But when the Sun is highly active, it releases more energy towards Earth’s atmosphere, creating more 14C. Trees that are alive when the Sun experiences an outburst of energy keep a record of that outburst by absorbing more of the carbon isotope into their tissue. And since trees grow seasonally, one ring at a time, each ring is a record of stellar activity.
Some of the Sun’s behavior is still a mystery. Miyake Events occur on the Sun and create “spikes” in the amount of carbon-14 in Earth’s environment. The researchers in this study used the link between carbon-14 and tree rings to understand Miyake Events. The effort involved advanced statistics and special software.
The Miyake Event is also called the Charlemagne event because Charlemagne ruled western Europe at the time. It’s also called the 774–775 carbon-14 spike. It produced the most significant rise in 14C that we know of.
“These huge bursts of cosmic radiation, known as Miyake Events, have occurred approximately once every thousand years, but what causes them is unclear,” Dr. Pope said in a press release. “The leading theory is that they are huge solar flares.”
If a solar flare caused the carbon-14 spike, it was the most powerful flare ever known, but still within the Sun’s capabilities. The flare wasn’t catastrophic for life and may have gone largely unnoticed at the time.
But it would’ve been ugly if the same event occurred in our modern technological times. Modern technology, especially satellites, would bear the brunt of the effects. In our world, we rely on satellites for communications and navigation. Add the risk of damage to electrical infrastructure, and these events are nothing to take lightly.
“We need to know more because if one of these happened today, it would destroy technology, including satellites, internet cables, long-distance power lines and transformers,” Dr. Pope said. “The effect on global infrastructure would be unimaginable.”
This is where tree rings come into the picture.
The paper’s first author Qingyuan Zhang is an undergraduate math student at U of Q. He developed software for the research that analyzes tree ring data.
“Because you can count a tree’s rings to identify its age, you can also observe historical cosmic events going back thousands of years,” Mr. Zhang said. “When radiation strikes the atmosphere, it produces radioactive carbon-14, which filters through the air, oceans, plants, and animals, and produces an annual record of radiation in tree rings.”
“We modeled the global carbon cycle to reconstruct the process over 10,000 years to gain insight into the scale and nature of the Miyake Events,” said Zhang.
They’re called Miyake events because Japanese scientist Fusa Miyake was the first author of a paper describing them. There’ve been six Miyake Events. The oldest known one occurred in 7176 BC, and the Charlemagne event is the most recent.
The most widely-held theory is that extremely powerful solar flares created the spikes in carbon-14. But in her 2012 paper, Miyake and her co-authors argued that solar flares can’t be responsible. This study also challenges the idea that solar flares are responsible.
“But our results challenge this,” Mr. Zhang said. “We’ve shown they’re not correlated with sunspot activity, and some actually last one or two years. Rather than a single instantaneous explosion or flare-up, what we may be looking at is a kind of astrophysical ‘storm’ or outburst.”
An astrophysical storm doesn’t sound very pleasant. Earth is at the mercy of the Sun, and the Sun’s usually placid behavior allows our civilization to thrive. So finding evidence of powerful events that we don’t understand and can’t predict is troubling.
People in 774-775 noted unusual things in the sky, though they knew nothing about radiation, carbon, or astrophysics.
The Anglo-Saxon Chronicle said, “This year also appeared in the heavens a red crucifix, after sunset;” What might that have been? (The same chronicle also says, “… wonderful serpents were seen in the land of the South-Saxons,” so a grain of salt is prudent.) The Chinese talked about an aurora in 776, the only one they mentioned during the 770s. They also recorded an unusual thunderstorm in 775. Were these observations related to whatever happened to the Sun?
A 2015 paper examined the 774–775 carbon-14 spike. It concluded, “These events probably had no optical counterpart, and a short gamma-ray burst, giant flare of a soft gamma-ray repeater or a terrestrial ?-ray flash could all be candidates.” If that paper was correct, nobody on Earth would’ve noticed a thing.
The research team found that whatever solar activity drove the 14C spikes isn’t correlated with the solar cycle. The spikes’ amplitude is also not dependent on latitude. Both of these things have been hypothesized as causative. “… we find no clear relation in timing to the solar cycle, or in amplitude to latitude as had previously been claimed,” the authors write in their conclusion.
When the researchers realized that the events were prolonged rather than acute, they explored the idea that atmospheric mixing could have played a role in extending them by holding the carbon aloft in the upper atmosphere well after the event ended. They also wondered if something about the trees themselves could explain the carbon-14 spike.
But those explanations were unsatisfactory. “They do not show a consistent relationship to the solar cycle,” the paper states, “and several display extended durations that challenge either astrophysical or geophysical models.”
The prolonged nature of the spikes is confounding. “On the other hand,” the authors write in their conclusion, “if the prolonged radionuclide production has an astrophysical origin, this will be hard to reconcile with an impulsive production model of one large energetic particle burst, whether of solar energetic particles or from a stellar remnant.”
So it remains a mystery, for now at least. And an undesirable one. How can we predict one if we don’t know what they are?
“Based on available data, there’s roughly a one percent chance of seeing another one within the next decade,” said Dr. Pope. “But we don’t know how to predict it or what harms it may cause.”
“These odds are quite alarming and lay the foundation for further research.”