Although we’re familiar with Earth’s major extinction events being caused by calamities such as large asteroid strikes or severe disruptions to the planet’s climate, researchers have uncovered evidence that the recently-discovered mass extinction of large marine mammals toward the end of the Pliocene 2.6 million years ago may have been caused by one or more nearby supernovae that bathed the Earth in deadly radiation.

We know that one or more of these massive stellar explosions took place 2.6 million years ago roughly 160 light-years from Earth, as evidenced by the presence of a large amount of the unstable isotope iron-60 found in fossilized rocks dating back to that period. There is virtually no iron-60 on Earth left over from the planet’s formation 4.6 billion years ago, having long since decayed down to a more stable isotope of nickel, meaning that the iron-60 present in that layer of the Earth’s rocks must have come from somewhere else. A supernova is the most likely source, as uncommon elements and their isotopes are created in the process of the star’s explosion, and such an explosion is also responsible for dispersing the newly-formed atoms throughout the cosmos.

The scars caused by this supernova event is still visible today in the local interstellar neighborhood, in the form of a 300-light-year-long structure called the Local Bubble, a region of space that’s been cleared of interstellar gases and dust by a series of novae. Our solar system sits near the edge of this Local Bubble, where the interstellar medium is only one-tenth as dense as the surrounding galaxy, and still emits a higher level of X-rays to this day.

Scientists have suspected for some time now that a supernova event played a part in the extinctions seen at the end of the Pliocene, but the discovery of a sudden spike in iron-60 levels at the same time offers strong evidence that this may have been the case.And although the novae that would have been involved in the formation of this void in space would have been mind-bogglingly violent, researchers suspect that it would have been an sharp increase in the amount of muon radiation bombarding the Earth that would have caused the massive die-off of the Pliocene’s marine megafauna, sea creatures that are large than 100 lbs.

Although we’re bombarded with them every day, muons typically don’t cause problems for Earth’s creatures because, for the most part, they pass harmlessly through most matter, including stone and water.

"They’re very penetrating. Even normally, there are lots of them passing through us," according to lead study author Adrian Melott, professor emeritus of physics and astronomy at the University of Kansas. "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," Melott continues. "Only a small fraction 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 percent 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."

This mass extinction of Pliocene-era marine animals saw the loss of one-third of the planet’s large sea creatures, with the diversity of marine mammals dropping by 55 percent. Although about a quarter of the animal species we have today arose after this extinction event, Dr. Catalina Pimiento, with the Paleontological Institute and Museum of the University of Zurich says that "the marine mega-faunal communities that humans inherited were already altered and functioning at a diminished diversity." 

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