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03/25/2013

Killer volcanoes and deadly humans: new angles on mass extinctions

Volcano

A series of devastating volcano eruptions in a short amount of time contributed to the die-off of the end-Triassic period.(AP Photo/ Salvatore Allegra)

The catastrophic event that killed off the dinosaurs -- probably an asteroid -- is the most famous mass extinction, familiar to schoolchildren everywhere.

But it is certainly not the only one. In fact, by most counts, there have been five major die-offs in the last 540 million years, when the number of species on Earth contracted by at least half in a short period of time.

This month, two unrelated studies shed new light on the subject of these cataclysmic mass extinctions.

The first tells us more about what caused the die-off that occurred 200 million years ago at the end of the Triassic, the one that created an ecological opening for dinosaurs to dominate the Earth. The second indirectly adds to the debate about a much more controversial die-off: the one some scientists say is happening right now, as homo sapiens drive ever-greater numbers of species to extinction.

Researchers have suspected for a long time that volcanoes were responsible for the extinction event at the end of the Triassic. But a new study in the most recent issue of Science provides strong evidence that a series of devastating eruptions occurred within a very short window of time. 

The researchers, including geologists at MIT and Columbia University, sampled rocks from Morocco, Nova Scotia and the New York City suburbs. The team found evidence that 200 million years ago, when the Earth consisted of a single continent, there was a massive, 2.5-million-cubic-mile-wide outpouring of lava -- four sudden spurts in a 600,000 year span, explosions that also split open a rift that became the Atlantic Ocean. 

Previous studies of the timing of these eruptions had a 1 to 3 million year margin of error, too loose to be able to say much. This week's science study narrowed that margin to a few thousand years, and showed that sediments below the lava layer contain fossils of organisms and spores that lived during the Triassic, like early crocodilians and tree lizards. Those above the lava layer do not. That, and other precise geological dating techniques, don't prove how volcanic eruptions killed off half the world's species -- but do provide an "ironclad" temporal link between the two occurrences.

The end-Triassic great extinction event must have taken 20,000 years at most, the new study shows -- the most accurate dating yet. But the scientists say the next step is to narrow that window even further.

In an unrelated study published in the journal The Holocene, scientists complicate our current understanding of the link between the arrival of the first humans in the Caribbean and species die-off in the region.

It's long been held that the arrival of humans in that part of the world coincided with the mass extinction of most of the region's birds. But the University of Florida-led study examined 5,000 bird fossils in Haiti and showed that the first arrival of humans caused a range of reactions -- from immediate species extinction to longer survival times.

"People arrive about 6,000 years ago and within a millennium or two, you lose the big, spectacular critters -- the ground sloths, the monkeys, the biggest rodents and some of the big extinct birds, like giant owls and eagles," lead author David Steadman, ornithology curator at the University's Florida Museum of Natural History, said in a statement.

"We have some bird species from our fossil site that, from a modern standpoint, are just as extinct as the others, but in fact, they almost were able to survive longer. That helps give us a gauge on what the future might bring."

The fossil analysis suggested that one now-extinct woodcock died off between 1350 and 1800, surviving the period when the first Amerindians came to the islands but not lasting beyond the European colonization that began in 1492.

The new research doesn't absolve homo sapiens -- it just complicates the question of what the exact mechanism of extinction was, which will help inform our understanding of other extinctions happening right now.

"When you take a look at what could've caused this, it really does just keep pointing to humans," Steadman said in the statement. "I just think it's habitat loss from people and introduction of non-native, invasive plants and animals. It's the same thing we're dealing with in Florida now -- who knows what the pythons are going to wipe out in the Everglades."

Much remains a mystery when it comes to global extinction events. But their catastrophic results when it comes to biodiversity are coming into ever-sharper focus.

 Kate Allen is the Star's science and technology reporter. Find her on Twitter at @katecallen.

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