Multiple supernova explosions about 65 light-years away may have contributed to the ozone depletion and several subsequent extinction events at the Devonian-Carboniferous boundary, approximately 359 million years ago, according to a new paper published in the Proceedings of the National Academy of Sciences.
An artist’s rendition of a supernova explosion. Image credit: ESA / Hubble / L. Calcada / NASA’s Goddard Space Flight Center.
Devonian-Carboniferous boundary rock samples contain malformed plant spores that appear to be sunburnt by ultraviolet light — evidence of a long-lasting ozone-depletion event.
“Earth-based catastrophes such as large-scale volcanism and global warming can destroy the ozone layer, too, but evidence for those is inconclusive for the time interval in question,” said Professor Brian Fields, a researcher in the Department of Ecology & Evolutionary Biology and the Biodiversity Institute at the University of Kansas.
“Instead, we propose that one or more supernova explosions, about 65 light-years away from Earth, could have been responsible for the protracted loss of ozone.”
Professor Fields and colleagues explored several astrophysical causes for this ozone depletion, such as meteorite impacts, solar eruptions and gamma-ray bursts.
“But these events end quickly and are unlikely to cause the long-lasting ozone depletion that happened at the end of the Devonian period,” said Jesse Miller, a graduate student in the Illinois Center for Advanced Studies of the Universe and the Department of Astronomy at the University of Illinois.
A supernova, on the other hand, delivers a one-two punch: it immediately bathes Earth with damaging UV, X-rays and gamma rays; later, supernova debris slams into the Solar System, subjecting the planet to long-lived irradiation from cosmic rays accelerated by the supernova. The damage to Earth and its ozone layer can last for up to 100,000 years.
However, fossil evidence indicates a 300,000-year decline in biodiversity leading up to the Devonian-Carboniferous mass extinction, suggesting the possibility of multiple catastrophes, maybe even multiple supernova explosions.
“This is entirely possible. Massive stars usually occur in clusters with other massive stars, and other supernovae are likely to occur soon after the first explosion,” Miller said.
The key to proving that a supernova occurred would be to find the radioactive isotopes plutonium-244 and samarium-146 in the rocks and fossils deposited at the time of extinction.
“Neither of these isotopes occurs naturally on Earth today, and the only way they can get here is via cosmic explosions,” said Zhenghai Liu, an undergraduate student in the Illinois Center for Advanced Studies of the Universe and the Department of Astronomy at the University of Illinois.
“Plutonium-244 and samarium-146 decay over time,” Professor Fields said.
“So if we find these radioisotopes on Earth today, we know they are fresh and not from and thus the smoking guns of a nearby supernova.”
The scientists have yet to search for plutonium-244 and samarium-146 in Devonian-Carboniferous boundary rocks.
“The overarching message of our study is that life on Earth does not exist in isolation,” Professor Fields said.
“We are citizens of a larger cosmos, and the cosmos intervenes in our lives — often imperceptibly, but sometimes ferociously.”
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Brian D. Fields et al. Supernova triggers for end-Devonian extinctions. PNAS, published online August 18, 2020; doi: 10.1073/pnas.2013774117
