Astronomers seeking to chart the history of supernovas that once lit up our skies and flooded our planet with radiation ought to take a look at tree rings, it's been suggested.
Robert Brakenridge, a geoscientist at the University of Colorado Boulder’s Institute of Arctic and Alpine Research, reckons stars exploding relatively close to Earth may have left fingerprints in our forests and jungles, which could be used to build a timeline of epic eruptions.
The trick, we're told, is to pay close attention to the amount of carbon-14 found inside the circular layers of trunk that form as trees grow. Carbon-14, aka radiocarbon, is produced in Earth's atmosphere when neutrons, smashed off atoms by incoming cosmic rays, merge with nitrogen. Some of that carbon isotope is absorbed into the trees as they grow, the quantity of which can be measured.
You can see where this is going: higher-than-expected levels of carbon-14 in a tree ring may point to an increase in radiation caused by exploding faraway stars, which kick out extreme levels of cosmic rays, causing a surge in radiocarbon that's absorbed by the plants. Brakenridge thus reckons spikes in radiocarbon detected in tree rings may have been created when the Earth was showered with more cosmic rays than usual. These radiation-heavy periods mark when massive old stars erupted nearby as supernovas, he argued.
This theory has been mulled before: for example, in 2005, a team in Japan investigated the effect of supernova on radiocarbon levels in trees. They found "no evidence" linking the two, and suggested any variation in the isotope is due to standard solar rays rather than dramatic explosions. Brakenridge thinks the scientific world should take a look again.
“We’re seeing terrestrial events that are begging for an explanation,” he said on Wednesday. “There are really only two possibilities: a solar flare or a supernova. I think the supernova hypothesis has been dismissed too quickly.”
I think the supernova hypothesis has been dismissed too quickly
Crucially, he said he can trace four radiocarbon increases, detailed in a paper published last week in the International Journal of Astrobiology, to four separate supernovas. The oldest explosion dates back more than 13,000 years when a giant star in the Vela constellation located 815 light-years away burst, and seemingly ramped the level of carbon-14 on Earth by three per cent.
“These are small radiocarbon isotope anomalies, peaks in concentration, in tree rings at particular years as dated by tree-ring scientists,” Brakenridge told The Register. “Tree rings in many trees are annual, so we can reliably identify to the year how old the ring is.”
In other words, the carbon-14 spikes in trees could pinpoint the date a sun detonated more accurately than studying the heavens, assuming you can get an old-enough sample.
“Astronomers can only approximately date the times of the supernovas from their remnant nebula," Brakenridge said. "If a secure connection of a specific event to a recorded carbon-14 anomaly [can be pinpointed], the energy of the supernova, and its actual age, can be tightly constrained. Supernova energies vary widely: so this helps get at the basic physics of the explosion."
The study's sample size of four is small, and so you may not want to jump to conclusions, though Brakenridge is hopeful the dating technique can be used in the future. “My paper only starts this process, it proposes strong candidates for specific known supernova events and known carbon-14 anomalies,” he told us. ®