Researchers consider whether supernovae killed off large ocean animals at dawn of Pleistocene

December 11, 2018, University of Kansas
Researchers consider whether supernovae killed off large ocean animals at dawn of Pleistocene
Muons showering Earth may have spelled curtains for Megalodon, a school-bus-sized shark, 2.6 million years ago. Credit: Wikimedia Commons.

About 2.6 million years ago, 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, a tsunami of cosmic energy from that same shattering star explosion could have reached our planet and pummeled the atmosphere, touching off climate change and triggering mass extinctions of large ocean animals, including a shark species that was the size of a school bus.

The effects of such a supernova—and possibly more than one—on large ocean life are detailed in a paper just published in Astrobiology.

"I've been doing research like this for about 15 years, and always in the past it's been based on what we know generally about the universe—that these supernovae should have affected Earth at some time or another," said lead author Adrian Melott, professor emeritus of physics & astronomy at the University of Kansas. "This time, it's different. 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—it's much more specific."

Melott said recent papers revealing ancient seabed deposits of iron-60 isotopes provided the "slam-dunk" evidence of the timing and distance of supernovae.

"As far back as the mid-1990s, people said, 'Hey, look for iron-60. It's a telltale because there's no other way for it to get to Earth but from a supernova.' 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. 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."

Melott's co-authors were Franciole Marinho of Universidade Federal de Sa?o Carlos in Brazil and Laura Paulucci of Universidade Federal do ABC, also in Brazil.

According to the team, 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," Melott said. "We're right on its edge. It's a giant region about 300 long. 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. When we do calculations, they're based on the idea that one supernova that goes off, and its energy sweeps by Earth, and it's over. But with the Local Bubble, the kind of bounce off the sides, and the cosmic-ray bath would last 10,000 to 100,000 years. This way, you could imagine a whole series of these things feeding more and more cosmic rays into the Local Bubble and giving us cosmic rays for millions of years."

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 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. But when this wave of cosmic rays hits, multiply those muons by a few hundred. 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 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."

A supernova 2.6 million years ago may be related to a marine megafaunal extinction at the Pliocene-Pleistocene boundary where 36 percent of the genera were estimated to become extinct. The extinction was concentrated in , where larger organisms would catch a greater radiation dose from the muons.

According to the authors of the new paper, damage from muons would extend down hundreds of yards into ocean waters, becoming less severe at greater depths: "High energy muons can reach deeper in the oceans being the more relevant agent of biological damage as depth increases," they write.

Indeed, a famously large and fierce marine animal inhabiting shallower waters may have been doomed by the supernova radiation.

"One of the extinctions that happened 2.6 million years ago was Megalodon," Melott said. "Imagine the Great White Shark in 'Jaws,' which was enormous—and that's Megalodon, but it was about the size of a school bus. 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."

The KU researcher said the evidence of a , or series of them, is "another puzzle piece" to clarify the possible reasons for the Pliocene-Pleistocene boundary extinction.

"There really hasn't been any good explanation for the marine megafaunal extinction," Melott said. "This could be one. It's this paradigm change—we know something happened and when it happened, so for the first time we can really dig in and look for things in a definite way. We now can get really definite about what the effects of radiation would be in a way that wasn't possible before."

Explore further: Research increases distance at which supernova would spark mass extinctions on Earth

More information: Adrian L. Melott et al, Hypothesis: Muon Radiation Dose and Marine Megafaunal Extinction at the End-Pliocene Supernova, Astrobiology (2018). DOI: 10.1089/ast.2018.1902

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3 / 5 (4) Dec 11, 2018
Terrific & terrifying article. The research seems to fill in some of the unanswered questions about that period of the Earth's history.

Of course the data & observations all need to be confirmed & verified before general acceptance.

The question that comes to my mind is: At what distance, from our system, were the Supernovae that produced the Bubble, 2,6 to 10 million years ago?

Those stars were moving around & so was our Solar System.
Is the Sun & the Bubble stars traveling in proximity as we orbit the Milky Way?

Cause if we are moving in separate directions? The conjunctures of the researchers would rather fall apart?
1 / 5 (3) Dec 11, 2018
Muon average lifetime 2.2 microseconds.

'Hey, look for iron-60. It's a telltale because there's no other way for it to get to Earth but from a supernova.'

No, our own Sun is quite capable of blasting Earth with iron ions, and will do so again.
Da Schneib
3.6 / 5 (5) Dec 11, 2018
So let me ask this: if it killed large marine life preferentially, how about large terrestrial life? Wouldn't large terrestrial mammals be more vulnerable?
4 / 5 (2) Dec 11, 2018
Da Schneib, my thoughts exactly. For example, there were (apparently various) hominins roaming a large area of the planet, and of course MUCH larger land animals as well. How were they impacted?
5 / 5 (8) Dec 11, 2018
The shark is just the apex predator teaser in the article.

The theory is that the mass marine extinction of the Pliocene might have been due to cosmic rays from the supernova that must have left the Iron-60.

What about largescale terrestrial life?

Terrestrial life gets hit by cosmic rays all the time, although less than 1% are HZE particles, and so presumably has evolved at least some means to fix that excess damage and cancers. One possibility is that the huge number of Iron-60 and other HZE particles from the supernova penetrated more deeply into water and thus exposed more ocean life at those depths to cosmic rays. Ocean life that evolution had not yet needed to protect.

It would be interesting to see a study on the relative hardiness of, say, deep sea bacteria and terrestrial bacteria when exposed to cosmic rays.
5 / 5 (3) Dec 11, 2018
Incidentally, the reason it's worth considering the outsize effects of the HZE particles, rather than just the enormous burst of muons, is because of the prevailing NASA theory of cosmic ray shielding:

Basically goes like this, the heavier and/or faster cosmic rays transfer more energy to you if you get hit by their byproducts after spallation with some type of shield, than if they just go right through you unimpeded. In other words, you are better off getting hit by one arrow than ten arrows if they're all still going fast enough to go through you. Not sure I buy that, when you consider all the slower arrows that are stopped by the shield, but there it is.
5 / 5 (3) Dec 11, 2018
if we were moving apart from the stars in question, there still could have been a supernova. And, what do you mean by "conjuncture." That word doesn't seem to apply.

the lifetime of a muon is infinite at the speed of light, and very long at nearly the speed of light, like in a cosmic ray. . .
Same with Iron-60. Ok, but then they both hit the earth and they both "stop" and then the half-life clock starts ticking.
5 / 5 (2) Dec 11, 2018
\Damn it, Max! You found my missedspelling.

I am not disagreeing with the evidence for the existence of possible nova that produced the Bubble.

But from 2.5 million to 10 million years ago? How close was our Star System to the active events that produced the Bubble?

That would influence my decision as to whether or not to accept these "conjunctures".

Another way to consider these issues? What if the Bubble novae's were transiting too far from Earth to have caused lethal damage?

Yet there is evidence that some sort of misfortune from our Local Stellar neighbors did a paddywhack on the Earth in that time period.
That leads me to speculate {without proof} that there may have been a closer nova. The remains of which are still undetermined. Maybe already discovered but indeterminate?

As for why the decimation of Maritime species?
What if the radiation blasts were when the Earth's rotation exposed mainly ocean?
3.7 / 5 (3) Dec 11, 2018
Maybe megalodon not extinct, just moved to really deep waters of Challenger Deep and survived. Maybe he really really big now, like 400 ft with jaw that can swallow a school bus.
3 / 5 (2) Dec 11, 2018
We should be on alert and look out for nearby supernovas. If a supernova that occurs only 150 light years away from us, its radiation might be on the way to us.

But, we can detect it only when radiation reaches us. There is no supernova early warning system in place yet as far as I know. We may get wiped out much sooner than climate would do to us.
Da Schneib
5 / 5 (3) Dec 11, 2018
Luckily there aren't any good supernova candidates near us.
5 / 5 (1) Dec 11, 2018
2.5 to 3.5 million years ago the gap between North and South America closed and initiated dramatic changes in ocean currents. That also seems to me to be a likely candidate to cause marine megafauna extinction.

Co-incidentally, approximately 3 million years ago was also the start of the current ice age.
Da Schneib
3 / 5 (2) Dec 11, 2018
Handwaving detected. These folks aren't dealing in millions, they're dealing in thousands. Of course the dimwits from the trailer park are a little fuzzy about the whole thousands vs. millions thing. Never mind billions.
3 / 5 (1) Dec 11, 2018
Handwaving detected. These folks aren't dealing in millions, they're dealing in thousands. Of course the dimwits from the trailer park are a little fuzzy about the whole thousands vs. millions thing. Never mind billions.

@Da Schneib
The first line in the report starts with: "About 2.6 million years ago, an oddly bright light arrived... "
Da Schneib
5 / 5 (1) Dec 11, 2018
Yeah, that's 2,600 thousand years. Not 3,500. Which is like 40% more. And stuff.
1 / 5 (1) Dec 11, 2018
Megalodon died from climate this case it was human induced global cooling.

Not really, but it was global cooling. Megalodon disappears slightly after the fairly stable pliocene climate goes volatile and cold. Circumploar oceanic current cessation appears to be a good explaination for why our modem climate is so cold and variable.
5 / 5 (1) Dec 11, 2018
No, our own Sun is quite capable of blasting Earth with iron ions, and will do so again.

Oliver Manual is that you?
Da Schneib
5 / 5 (1) Dec 11, 2018
Just to be completely clear, what these guys are saying is that a number of large ocean predators died out around the transition between the Pliocene and Pleistocene. They are hypothesizing, based on a hypothesis that a number of O- and B-type stars became SNe in an OB association that was passing some 160 light years from the Sun, that this was a contributing factor (not a primary cause) of some of the marine megafauna extinctions around the right time.

The article preprint is available on arXiv at:
Da Schneib
5 / 5 (1) Dec 11, 2018
And there's a problem with this hypothesis: a bunch of the *smaller* mysticetes died out about this time, along with some of the smaller (but mid-sized) odontocetes. What was left is the larger ones we see today: blue whales, sperm whales, and humpbacks, for example.

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