Tree Rings Hide a 7,000-Year Old Solar Mystery

A Cosmic Connection to Trees

bristlecone pines
Bristlecone pines can live to great ages, and their tree rings contain surprising hints about celestial events involving our Sun more than 7,000 years ago. Credit: A.J.T. Hull. A.J.T. Hull. used by permission.

High on a mountain in California, nestled deep in a bristlecone pine forest, lies evidence of a long-gone cosmic event that occurred in the year 5480 B.C.E. Hidden in the tree rings of those pines are clues to something that happened on the Sun, a blast that sent levels of cosmic radiation soaring out to space. What was it? It turns out the answer involves cosmic rays and earth's atmosphere, along with some very ancient trees.

Dating the Trees

The story begins with scientists at Nagoya University in Japan, working with U.S. and Swiss researchers. They studied the carbon-14 atoms found in bristlecone pines that were alive more than 7,000 years ago. Those ancient trees made faithful recordings of something that happened way back then, just as trees have done throughout history. Due to the way carbon-14 is made in our atmosphere, they suspected some kind of outburst from the Sun was involved in the presence of that element.

The science of using trees to figure out events from long in the past is not new. Trees can reflect droughts and floods in their rings. If you know what to look for, you can also find evidence of more "cosmic" events. Those can give interesting insights into totally unrelated objects, such as musical instruments.

For example, the so-called "Little Ice Age" conditions brought dramatically cooler temperatures to parts of Europe for several hundred years beginning in the year 1400.

The worst temperature dips occurred for a few decades beginning in the year 1645. That coincided with a reduction in the number of sunspots during a time astronomers call the Maunder Minimum. The Sun was pretty quiet during that period. The connection between low solar activity and changed weather is still being investigated.

However, what is well known is that the lower temperatures affected the growth of certain trees. The trees were much denser, with with very narrow rings.

Interestingly enough, these trees were the source of wood for Stradivarius violins and other stringed instruments, which have a beautiful, distinctive sound. It's an interesting link to the Sun that no one suspected until they studied the wood in those instruments and traced them back to trees affected by the climate conditions. That link shows that living with a star can be quite complex, indeed.

How the Carbon-14 Gets into the Trees

Active outbursts from the Sun don't just disappear into space. They leave behind evidence. In Earth's case, solar cosmic rays blast through the atmosphere, creating carbon-14 atoms (which are what we call an "isotope" of carbon). Trees and planets "suck in" the air containing the carbon-14. Eventually, they produce oxygen, which goes back into the air. The carbon-14 stays behind in the tree rings. If the tree lives long enough, as the bristlecone pines do, then the evidence of a sudden event producing large amounts of carbon-14 is just waiting to be discovered.

Earth's Atmosphere and Cosmic Rays

Our atmosphere is a chemical mix of mostly nitrogen, with small amounts of oxgyen.

Carbon dioxide is there in trace amounts, and it is known as a greenhouse gas. It traps heat radiating back out from Earth, which makes our planet more habitable. It's a delicate balance; too much carbon dioxide and other greenhouse gases can keep the planet too warm, which is what contributes to global warming.

The process from the Sun to tree rings is a complex one. As solar cosmic rays pour into our atmosphere, they smack into atoms of nitrogen. That causes secondary cosmic rays called neutrons. When the neutrons collide with other nitrogen atoms, they create carbon-14 atoms, which are radioactive. A given atom of the stuff has a half-life of 5,700 years. That's the time it takes for half of the atoms of carbon-14 to decay completely to another form. If you've ever studied chemistry, you've probably heard these terms before.

Carbon-14 dating is an indispensable means of determining the ages of materials that contains the isotope.

Searching out the Evidence

To understand what might have happened to the bristlecones, the team measured the levels of carbon-14 in several sets of wood samples and found a huge change in the amount of it buried among rings created in the year 5480 B.C.E. That was a major clue that something happened. But what? It had to be something sudden, and from outside the planet. The best explanation of the uptick in carbon-14, was some kind of strong outburst from the Sun. It could have been coupled with a change in magnetic activity. It could have unleashed a lot of cosmic rays that sped toward Earth. Once they hit the atmosphere, they created larger than normal amounts of carbon-14. The trees did their thing, and today, 7,000 years later, scientists are finding the evidence.

Solar activity has been a hallmark of our star since its birth. At times, it has been very active — particularly 4.5 billion years ago just as it formed. It also went through quiet periods throughout history. Solar physicists study it constantly to map its activity and understand why the Sun does what it does. They know it can affect our planet in many ways, from space weather to regular weather.The more data about solar activity they gather, the more they'll be able to predict what might do next. However, in the case of the pine tree rings, they can also find data right here on Earth to explain just what might have happened back when human cultures were just starting to take root and spread across the continents of our planet.