Mass Extinction

Podcast

Mass Extinction

Transcript

Tracking history's biggest killer. I'm Bob Hirshon and this is Science Update.

250 million years ago, long before the dinosaurs, most of life on earth was completely wiped out. Some scientists have suspected that a giant asteroid was to blame.

But not so fast, says Cal Tech geochemist Ken Farley. He and his colleagues have analyzed rock samples from the time of the extinction. And they’ve found they came from earth, not outer space.

Farley:
In other words, we don't find any evidence for an impact, and as such then, it favors other kinds of explanations.

One possible culprit is a massive volcanic eruption that occurred around that time. The volcanoes would have belched huge amounts of carbon dioxide gas into the atmosphere–which may have changed the climate and disrupted life on earth. Although the question isn't settled, the new data are more consistent with this story. I'm Bob Hirshon for AAAS, the Science Society.

Making Sense of the Research

When you think of mass extinctions, you usually think of the dinosaurs. They're certainly the highest-profile group of organisms ever to disappear. But mass extinctions have occurred at several other times over the course of the earth's history, and the one discussed here–not the later extinction of the dinosaurs–was the most devastating ever. Life on earth wasn't quite as advanced back then, but the vast majority of what had developed by that point, on land and in the sea, suddenly disappeared.

It takes a pretty major event to cause a mass extinction. One candidate is an asteroid slamming into the earth. Many scientists believe an asteroid killed off the dinosaurs 65 million years ago, and so an asteroid would be a likely candidate for this earlier mass extinction as well.

If an asteroid caused the extinction, you would expect the rock layers from that time period to contain a lot of material from outer space. Scientists can tell an extraterrestrial rock from an earth rock by looking at its chemistry. Looking at rock samples from the time of the extinction, Farley's team searched for three chemicals–iridium, helium-3, and osmium-187–that are associated with asteroids, comets, and other extraterrestrial rocks. They found very little iridium and no helium-3 or osmium-187.

So their evidence suggests an asteroid was not to blame. It does not specifically prove what did cause the extinction. However, it's known that one of the largest lava eruptions in the history of the earth occurred in Siberia at around that time period. Coincidence? Probably not, says Farley. As you heard, because massive volcanic eruptions can change the earth’s climate, such an event certainly would have put a lot of pressure on the species that lived around that time. And according to the study's lead author, Christian Koeberl of the University of Vienna, Austria, the chemistry of the rocks suggests that the oceans at the time were starved of oxygen, which often results from high levels of carbon dioxide in the atmosphere, which in turn may have resulted from a volcanic eruption. Still, it will take a lot more evidence to nail down the real culprit of this historic blow to life on earth.

Now try and answer these questions:

1. What did this study show? What did it not show?
   
2. Prior to this study, why was an asteroid a good hypothesis for this extinction?
   
3. How did the findings support, or not support, the asteroid hypothesis?
   
4. Why is a volcanic eruption a good hypothesis now?
   
5. Why do scientists form hypotheses and test them, rather than just try and find the answer?

Going Further

The BBC's Extinction Files provides an overview of mass extinctions in earth's history.

Impact Geology, Chemistry, and Physics, on the site of the University of Bristol's (UK) Paleontology Research Group, discusses the how, what, and why of asteroid impacts, including the chemistry of extraterrestrial rocks.

The University of North Dakota's Volcano World contains a wealth of information about global vulcanism.