Approximately 2.6 million years ago (Pliocene epoch), a tsunami of cosmic energy from a massive supernova or a series of them about 150 light-years away reached Earth and pummeled the atmosphere, touching off climate change and triggering the mass extinction of large marine animals, according to University of Kansas Professor Adrian Melott and co-authors.
“Recent papers revealing ancient seabed deposits of a radioactive isotope of iron called iron-60 provided strong evidence of the timing and distance of supernovae,” said Professor Melott, lead author of a paper published in the journal Astrobiology.
“Because iron-60 is radioactive, if it was formed with the Earth it would be long gone by now. So, it had to have been rained down on us.”
“There’s some debate about whether there was only one supernova really nearby or a whole chain of them. I kind of favor a combo of the two — a big chain with one that was unusually powerful and close.”
Other evidence for a series of supernovae is found in the very architecture of the local Universe.
“We have the Local Bubble in the interstellar medium. We’re right on its edge. It’s a giant region about 300 light-years long,” Professor Melott said.
“It’s basically very hot, very low-density gas — nearly all the gas clouds have been swept out of it.”
“The best way to manufacture a bubble like that is a whole bunch of supernovae blows it bigger and bigger, and that seems to fit well with idea of a chain.”
Whether or not there was one supernova or a series of them, the supernova energy that spread layers of iron-60 all over the world also caused penetrating particles called muons to shower Earth, causing cancers and mutations — especially to larger animals.
“Muons are very penetrating. Even normally, there are lots of them passing through us. Nearly all of them pass through harmlessly, yet about one-fifth of our radiation dose comes by muons. But when this wave of cosmic rays hits, multiply those muons by a few hundred,” Professor Melott said.
“Only a small faction of them will interact in any way, but when the number is so large and their energy so high, you get increased mutations and cancer — these would be the main biological effects.”
“We estimated the cancer rate would go up about 50% for something the size of a human — and the bigger you are, the worse it is. For an elephant or a whale, the radiation dose goes way up.”
The end-Pliocene extinction was concentrated in coastal waters, where larger organisms would catch a greater radiation dose from the muons.
“Damage from muons would extend down hundreds of yards into ocean waters, becoming less severe at greater depths,” the researchers said.
“High energy muons can reach deeper in the oceans being the more relevant agent of biological damage as depth increases.”
“Indeed, large and fierce marine animals inhabiting shallower waters may have been doomed by the supernova radiation.”
One of the extinctions that happened 2.6 million years ago was Carcharocles megalodon, the largest shark that ever lived on Earth.
“Carcharocles megalodon was about the size of a school bus,” Professor Melott said.
“This species just disappeared about that time. So, we can speculate it might have something to do with the muons. Basically, the bigger the creature is the bigger the increase in radiation would have been.”
This article was originally published by Sci-news