''Supernovae behind extinction of prehistoric giant shark''

WASHINGTON

A tsunami of cosmic energy from a star explosion 150 light years away may have triggered climate change and extinctions of large ocean animals such as the ancient giant shark Megalodon about 2.6 million years ago, scientist say.

According to a study published in the journal Astrobiology, an oddly bright light arrived in the prehistoric sky and lingered there for weeks or months. It was a supernova some 150 light years away from Earth.

Within a few hundred years, long after the strange light in the sky had dwindled, energy from the explosion reached our planet and pummeled the atmosphere, researchers said.

It has always been believed that supernovae should have affected Earth at some time or another, said Adrian Melott, professor at the University of Kansas in the US.

"This time, we have evidence of nearby events at a specific time. We know about how far away they were, so we can actually compute how that would have affected the Earth and compare it to what we know about what happened at that time," Melott said.

Previous research has revealed deposits of iron-60 isotopes in ancient seabeds—an evidence of the timing and distance of supernovae.

Iron-60 is radioactive, and if it was formed with the Earth it would be long gone by now, Melott said.

"So, it had to have been rained down on us," she said.

"If you look at iron-60 residue, there's a huge spike 2.6 million years ago, but there's excess scattered clear back 10 million years," she said, suggesting that there were two seperate supernovae that caused mass extinction.

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.

"The best description of a muon would be a very heavy electron—but a muon is a couple hundred times more massive than an electron," Melott said.

"They're 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," she said.

However, when a huge wave of cosmic rays hits, the number of muons is so large and their energy so high, there is an increased risk of mutations and cancer.

"We estimated the cancer rate would go up about 50 per cent 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," Melott said.

A supernova 2.6 million years ago may be related to a marine megafaunal extinction at the Pliocene-Pleistocene boundary where 36 per cent of the genera were estimated to become extinct.

The 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, researchers said.

A famously large and fierce marine animal inhabiting shallower waters may have been doomed by the supernova radiation, they said.

"One of the extinctions that happened 2.6 million years ago was Megalodon," Melott said.

"They 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," she said.