Unearthing a Solar Storm in Ancient Alpine Trees

Scientists have discovered a 14,373-year-old piece of history written within the pine trees on the banks of the Drouzet River in the French Alps.

The researchers, led by Edouard Bard (College of France), recorded an abrupt increase in a radioactive form of carbon, known as radiocarbon, within the rings of the pines. The spike was recorded within a single year’s growth. The team published the study in the Philosophical Transactions of the Royal Society A, proposing the most likely cause for that spike to be a significant solar energetic particle (SEP) event — in other words, a massive solar storm.

The Sun Leaves Its Mark

When a solar flare erupts, high-energy particles often escape along with radiation. Upon reaching Earth’s atmosphere, these energetic particles cause nuclear reactions, converting nitrogen-14 to carbon-14 (also known as radiocarbon). Trees then absorb that extra radiocarbon, preserving it within the currently growing layer of wood. This process of absorption allowed a 14,373-year-old solar storm to carve its signature into the Scots Pines in what is now the French Alps.

Over the past 25 years, Bard’s team has been conducting field campaigns to collect and analyze subfossil trees, whose fossilization process was incomplete. The information in these trees’ rings offers a glimpse back in time. The team has sampled more than 500 trunks in the French Alps, 172 of them along the Drouzet River. 

The team conducted dendrochronological analysis, a dating method based on counting tree rings, and compared it against carbon dating, a method based on the radioactive decay of carbon-14 to provide a reflection of the trees’ ages and past environment.

“It is of course not possible to go back to 14,000 years ago with just a single tree,” Bard explains. “Instead, long chronologies are made of many trees that are crossmatched by using recognizable patterns of successive tree ring widths. These patterns are linked to hydroclimatic changes in a particular region.”

What interested the team was instances in which radiocarbon dating deviated from dendrochronological dating. The years when this happened may have hosted fluctuations in atmospheric radiocarbon content. But there is only one known fluctuation that would occur within a single year —  a sudden increase caused by solar activity.

German Pines and Greenland Ice

The researchers compared the information gleaned from the French Alps trees with a well-dated reference derived from German pines to achieve a higher precision in the obtained timeframe, putting the event at exactly 14,373 years ago.

To further confirm that a solar storm caused the sudden rise in radioactive carbon, the scientists looked at ice cores taken from Greenland. The levels of beryllium-10, another radioactive element produced when high-energy cosmic rays interact with our atmosphere, matched the anomalies found in the Drouzet trees, albeit with less precision.

“I think that an SEP event, as the authors state, is the most likely cause of the spike,” says Dr. Gavin Schmidt (NASA Goddard Institute for Space Studies), who was not involved in the study. That puts this event on a short list of other so-called Miyake events, sudden spikes of atmospheric radiocarbon levels that leave a notable signature in tree-ring records. The other known events occurred in 774 AD, 993 AD, 660 BC, and 7175 BC.

“We now have five examples of this phenomenon ,” Schmidt adds, “and so it seems likely that this event has a common cause.” This latest discovery, however, is by far the most ancient of the five and may provide valuable insights into past solar activity and its effects on Earth.

The Road Ahead

The only modern solar storm that’s comparable to the Miyake events is the Carrington event in 1859, which disrupted telegraph stations while causing global auroras. A Carrington event would be bad for us, but the event 14,373 years ago was 10 times stronger still, Bard says.  

“A Miyake event would be quite destructive for our connected society,” he adds. Effects would ripple through the electric power grid, computer connections, satellite communications, and even high-altitude travel. “We should pursue the scientific research on Miyake spikes, but also prepare backup solutions in order to avoid complete blackout of our infrastructures.”

Scientific research is still ongoing to analyze more tree rings in order to discover more of those events and understand their frequency and possible consequences. Ultimately, this kind of study could help us better guard our modern world against exceptionally strong solar storms.