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Alexandra Witze, Earth in Action

Bad days for dinosaurs began long before the last of them died

2:22pm, October 1, 2012
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SN Prime | October 1, 2012 | Vol. 2, No. 37

Sometimes a bad day doesn’t know when to stop. It turns into a bad week, then a bad month, and maybe even the worst year you’ve ever had.

If you’re familiar with that sort of day, then you should have a lot of sympathy for the dinosaurs. They suffered through one of the worst times on Earth for hundreds of thousands of years — and then died out anyway.

As any 8-year-old can tell you, dinosaurs went extinct when a giant space rock slammed into the Earth 65 million years ago. It’s a great story, full of fiery apocalypse. But it’s so extraordinary that scientists took more than a decade to accept it. Only after 1991, when geologists reported traces of a gigantic crater off the Yucatán coast dated to the same era, did most of their colleagues agree that death had indeed come from above.

Now there’s a second twist to the dinosaur-killer saga. Not only did a meteorite get them — so too did volcanoes. Huge eruptions, in what is now India, spewed out climate-altering gases that choked life on Earth. The meteorite arrived soon afterward to deliver the second, knockout punch.

Geologists have long had a sneaking suspicion that volcanoes may have played a role in this particular mass extinction, which also killed three-quarters of all other species. But only now is a full chronicle emerging of what those last bad days on Earth were like.

The answers come from Antarctica. Sixty-five million years ago the frozen continent was a lot warmer, and creatures such as huge spiral ammonites thrived in the oceans nearby. When they died they sank to the ocean floor, there to be encased by layer after layer of mud settling atop their cephalopod corpses.

Most places with fossil evidence for the dinosaur die-off contain only narrow bands of rock preserving the transition — a sort of CliffsNotes version of geologic time. But off the tip of the Antarctic peninsula lies an island called Seymour, which contains a near-encyclopedia of the mass extinction. Thick mud layers document moment after moment as animals wound toward their end days.

So off to Seymour went Thomas Tobin, a graduate student at the University of Washington. From the rocks he pulled evidence that shows how warm ocean temperatures got, up to and through the dinosaur extinction. And Tobin found some knockout numbers.

Temperatures soared by at least 5 degrees Celsius in two pulses between about 68 million and 65 million years ago. Those moments coincide with three main pulses of volcanic activity in India, Tobin and his colleagues report September 15 in Palaeogeography, Palaeoclimatology, Palaeoecology.

In just the few hundred thousand years before the meteorite hit, India’s volcanoes spewed out enough lava to cover the state of Texas. That’s enough to build up huge amounts of heat-trapping gases like carbon dioxide in the atmosphere. The rising thermostat started to weaken ammonites, dinosaurs and other creatures used to cooler climes, Tobin suggests. When the meteor-ite hit, it finished off the strong that had survived.

At least that’s the story according to mainstream geologists. Another new study argues that there may not have been any critters left by the time the extraterrestrial blow landed.

Gerta Keller, a geoscientist at Princeton, is one of the last holdouts against the space-rock extinction theory. For years she has argued that animals were on the decline well before the meteorite impact. Now, she’s looked at 10 wells drilled through four huge lava flows from the Indian eruptions, the last flow dating to or just before the meteorite impact. Each flow, she and her colleagues found, knocked out more and more plankton and other sea creatures — until, in the end, none were left. The work appears in the August Earth and Planetary Science Letters.

So which is the real heavyweight champion, then — volcano or meteorite? A neutral referee would probably come down in favor of the meteor-ite. But both new studies show how extinctions are complex things.

“Our results imply that what has been called the … mass extinction was composed of a more complicated series of separate events than any single cause can explain,” Tobin’s team writes.

The dinosaur extinction is only the latest of five mass extinctions in Earth’s history. At least two others have been linked to volcanic eruptions, while other possible culprits include giant oceanic belches and ice ages that caused sea levels to plunge. So the story of death may be much weirder than scientists had thought.

Just like most of the rest of life.

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