Tree rings may contain clues to the impacts of distant supernovae on Earth

Massive bursts of energy that occurred thousands of light years from Earth may have left traces in the biology and geology of our planet, according to new research by geoscientist Robert Brakenridge of the University of Colorado Boulder.

The study, published this month on International Journal of Astrobiology, examines the impacts of supernovae, some of the most violent events in the known universe. Within a few months, a single such eruption can release as much energy as the sun will during its lifetime. They are also bright, really bright.

“We always see supernovae in other galaxies,” said Brakenridge, senior researcher at the Arctic and Alpine Research Institute (INSTAAR) at CU Boulder. “Through a telescope, a galaxy is a small foggy spot. Then suddenly a star appears and it can be as bright as the rest of the galaxy.”

A very close supernova may be able to wipe out human civilization from the face of the Earth. But even from further afield, these explosions can still take a toll, Brakenridge said, flooding our planet with dangerous radiation and damaging its protective ozone layer.

To study these possible impacts, Brakenridge searched the planet’s tree ring records for fingerprints of these distant cosmic explosions. His findings suggest that relatively close supernovae could theoretically have triggered at least four disruptions to Earth’s climate over the past 40,000 years.

The findings are far from conclusive, but offer tantalizing clues that, when it comes to the stability of life on Earth, what happens in space doesn’t always stay in space.

“These are extreme events, and their potential effects appear to match tree-ring records,” Brakenridge said.

Radiocarbon peaks

His research hinges on the case of a curious atom. Brakenridge explained that carbon-14, also known as radiocarbon, is an isotope of carbon found only in small quantities on Earth. Not even in these parts. Radiocarbon is formed when cosmic rays from space bombard the atmosphere of our planet almost constantly.

“There is generally a constant amount year after year,” Brakenridge said. “Trees collect carbon dioxide and some of that carbon will be radiocarbon.”

Sometimes, however, the amount of radiocarbon that trees collect is not constant. Scientists have discovered a handful of cases where the concentration of this isotope inside tree rings increases suddenly and for no apparent earthly reason. Many scientists have speculated that these spikes of several years could be due to solar flares or huge expulsions of energy from the sun’s surface.

Brakenridge and a handful of other researchers have been keeping an eye on events much farther from home.

“We are seeing terrestrial events that demand an explanation,” Brakenridge said. “There are really only two possibilities: a solar flare or a supernova. I think the supernova hypothesis was dismissed too quickly.”

Watch out for Betelgeuse

He noted that scientists have recorded supernovae in other galaxies that have produced a stupendous amount of gamma radiation, the same type of radiation that can trigger the formation of radiocarbon atoms on Earth. While these isotopes are not dangerous on their own, a spike in their levels could indicate that the energy of a distant supernova has traveled hundreds to thousands of light years on our planet.

To test the hypothesis, Brakenridge turned to the past. He has put together a list of supernovae that have occurred relatively close to Earth over the past 40,000 years. Scientists can study these events by looking at the nebulae they left behind. He then compared the estimated age of those galactic fireworks with the record of the tree ring on the ground.

He found that of the eight closest supernovae studied, all appeared to be associated with unexplained peaks in the radiocarbon record on Earth. Consider four of these particularly promising candidates. Take the case of a former star in the constellation of Vela. This celestial body, which was once about 815 light-years from Earth, became a supernova about 13,000 years ago. Not long after, radiocarbon levels rose nearly 3% on Earth, a staggering increase.

The results are nowhere near a smoking gun, or a star, in this case. Scientists still have trouble dating past supernovae, making the timing of Vela’s explosion uncertain with a possible 1,500-year error. Furthermore, it is not clear what the impact of such a disruption may have been for plants and animals on Earth at that time. But Brakenridge believes the question is worth much more research.

“What keeps me going is when I look at the Earth record and say, ‘My God, the predicted and modeled effects seem to be there.’

He hopes humanity won’t have to see these effects on its own anytime soon. Some astronomers think they have caught signs that Betelgeuse, a giant red star in the Orion constellation, may be on the verge of collapsing and becoming a supernova. And it’s only 642.5 light-years from Earth, much closer than Vela.

“We can hope that’s not what’s going to happen because Betelgeuse is very close,” he said.