Trees are like sentinels that keep a record of changing climate conditions. Their growth rings support the history and arboreal studies of these rings. Scientists can determine the exact age of trees and relate their growth to climate and environmental changes.
But they also record the effects of more distant changes, including the activity of the Sun.
Carbon is the main building block of life. It is the backbone of life on Earth because it forms bonds with itself and with other individuals in a wide range of associations. Carbon exists in nearly 10 million different compounds. Trees are mainly composed of cellulose, a carbon-rich organic compound of the formula (C6H10THE5) n.
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But not all carbon is created equal. It exists naturally in three different isotopes: carbon 12 (12C), carbon 13 (13C), and carbon 14 (14DO). Carbon always has 6 protons, but different isotopes have different numbers of neutrons. 14C is different from other isotopes because it is a radioisotope. This means it decomposes, and since 1949, scientists have been using it 14C to find the ages of ancient things through radiocarbon dating.
Trees absorb carbon every day as they take in carbon dioxide and expel oxygen. Trees do not discriminate between different isotopes of carbon. So when scientists analyze the chemical content of tree growth rings, they find all three natural isotopes of carbon.
Certain events cause an increase in the amount of 14C in the Earth’s atmosphere. Scientists can date tree rings precisely by finding which tree rings have more 14C, scientists can determine when Earth experienced a carbon-14 hit.
All of this goes to the heart of a new study published in the Proceedings of the Royal Society A. The paper is ‘Modeling 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 otherworldly origins.
When cosmic rays from the Sun and more distant stellar objects hit Earth’s upper atmosphere, the rays interact with nitrogen, which is abundant in Earth’s atmosphere. Interaction creates 14C, which mixes freely in the atmosphere, where it is taken up by living things. The amount of carbon-14 is relatively constant as long as the Sun behaves “normally”.
But when the Sun is very active, it releases more energy into Earth’s atmosphere, creating more 14C. Trees that are alive when the Sun experiences an energy burst keep a record of that burst 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 in the Sun and create “spikes” in the amount of carbon-14 in Earth’s environment. The researchers in this study used the relationship between carbon-14 and tree rings to understand the 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. Also called carbon-14 spike 774–775. It caused the most significant rise in 14C that we know.
“These massive bursts of cosmic radiation, known as Miyake Events, occur about once every thousand years, but what causes them is unclear,” Dr. Pope said in a press release. “The leading theory is that these are massive solar flares.”
If a solar flare caused the carbon-14 spike, it was the most powerful flare ever known, but still well within the Sun’s capabilities. The flare was not life-threatening and may have gone largely unnoticed at the time.
But it would be bad if the same event happened in our modern technological age. Modern technology, especially satellites, will bear the brunt of the impacts. In our world, we rely on satellites for communications and navigation. Add in the risk of damage to electrical infrastructure, and these events are not to be taken 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 impact on global infrastructure would be unimaginable.”
This is where the tree rings come into the picture.
The paper’s first author Qingyuan Zhang is an undergraduate mathematics student at the U of Q. He developed software for the research that analyzes tree-ring data.
“Because you can count the rings of a tree to determine its age, you can also observe historical cosmic events going back thousands of years,” said Mr Zhang. “When the radiation hits the atmosphere, it produces radioactive carbon-14, which filters into 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 Miyake Events,” Zhang said.
They are called Miyake events because Japanese scientist Fusa Miyake was the first author of a paper describing them. There have been six Miyake events. The earliest known 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 cannot be responsible. This study also challenges the idea that solar flares are responsible.
“But our results challenge that,” Mr. Zhang said. “We’ve shown that they don’t correlate with sunspot activity, and some actually last a year or two. Rather than a momentary burst or explosion, what we may be looking at is a kind of astrophysical ‘storm’ or explosion.”
An astrophysical storm doesn’t sound very pleasant. The 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 noticed unusual things in the sky, although they knew nothing about radiation, carbon or astrophysics.
The Anglo-Saxon Chronicle said: “This year also a red cross appeared in the heavens after sunset. What could that have been? (The same chronicle also says, “…wonderful serpents were seen in the land of the South Saxons,” so salt is wise.) The Chinese spoke of an aurora in 776, the only one they reported during the 770s. They also recorded an unusual storm. in 775. Were these observations related to what happened to the Sun?
No one is sure.
A 2015 paper examined the 774–775 carbon-14 spike. He concluded, “These events probably had no optical counterpart, and a brief gamma-ray burst, a giant flare of a soft gamma-ray repeater, or a terrestrial γ-ray flare could all be candidates.” If this paper was correct, no one on Earth would have noticed a thing.
The research team found that any solar activity drove the 14C peaks are not correlated with the solar cycle. The width of the spikes also does not depend on latitude. Both of these things have been assumed to be causal. “…we find no clear relation in timing to the solar cycle, or in latitude to latitude, as has been previously argued,” the authors write in their conclusion.
When the researchers realized that the events were protracted rather than acute, they explored the idea that atmospheric mixing could have played a role in their extension by keeping carbon high in the upper atmosphere long after the event ended. They also wondered if something about the trees themselves could explain the carbon-14 spike.
But these explanations were not satisfactory. “They do not show a consistent relationship with the solar cycle,” the paper states, “and several show extended durations that challenge either astrophysical or geophysical models.
The protracted nature of the spikes is confusing. “On the other hand,” the authors write in their conclusion, “if the sustained production of radionuclides has an astrophysical origin, this will be difficult to reconcile with an impulsive model of the production of a large burst of energetic particles, either by solar energetic particles or by a stellar residue”.
So it remains a mystery, for now at least. And an unsolicited one. How can we predict one if we don’t know what it is?
“Based on the available data, there’s about a one percent chance we’ll see another within the next decade,” Dr. Pope said. “But we don’t know how to predict it or what damage it might cause.”
“These possibilities are quite alarming and lay the groundwork for further investigation.”
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