The dinosaur demise debate

“Everybody knows” that the dinosaurs were killed off 65 million years ago by a giant meteor that slammed into the Yucatan Peninsula. But as is often the case in science, what “everybody knows” may be wrong.

The asteroid impact theory has been dominant for 20 years, but there have always been doubters. They admit a huge meteor 16 kilometres in diameter hit the Yucatan peninsula 65 million years ago: there’s a giant crater buried under 800 metres of rock and a thin, iridium-rich layer of clay all around the planet to prove it. But they don’t believe that single impact killed off the dinosaurs.

Instead, these “gradualists” believe the evidence shows many species were in decline long before the Yucatan meteor hit. It–or a series of meteor impacts over a few dozen millennia–may have delivered the coup de grace, but they believe a major role may also have been played by volcanic eruptions.

Of course, they’re not talking about tourist-attracting Hawaiian-sized eruptions. They’re talking about flood-basalt eruptions, the largest eruptions of lava ever seen on the planet, producing as much as 2,000 cubic kilometers of lava, capable, over a lifetime measured in hundreds of thousands of years, of burying a million square kilometers or more under many metres of rock.

Earth consists of a thin solid crust over a vast reservoir of molten rock, known as the mantle. Flood basalt eruptions are thought to be caused by “mantle plumes” that break through the crust. These plumes originate hundreds or even thousands of kilometers inside the Earth, rising because they’re hotter than the surrounding mantle and richer in lighter elements.

There’s a direct correlation between the age of at least three known flood basalt eruptions and a major extinction event. One, which formed the Deccan Traps (steppes) in India, corresponds to the time of the disappearance of the dinosaurs.

Can a flood basalt eruption disrupt the environment enough to cause extinctions? At a conference in Wales last week, Princeton University paleontologist Gerta Keller presented evidence that a flood basalt eruption on the ocean floor millions of years before the dinosaurs disappeared was enough to cause the extinction of many species without any meteor impact being involved. She based her conclusions on fossils of tiny organisms taken from undersea core samples from the Indian Ocean.

Flood basalt eruptions may have been accompanied by huge outbursts of gases. Sulfuric acid in the atmosphere could have cooled the climate; carbon dioxide or sulfur dioxide could have warmed it. Severe acid rain may have killed lakes and forests. Undersea eruptions could have altered ocean chemistry, circulation and oxygenation.

The debate between the gradualists and those who believe in a single catastrophic asteroid strike might sound like a rather dry academic discussion–but far from it. Careers, reputations and personalities are involved. Two years ago, a major project was launched in the Yucatan to retrieve samples of the buried crater wall. In 2003, to the dismay of gradualists, the first samples were entrusted to Dr. Jan Smit, a geologist at the Free University of Amsterdam and a leading supporter of the meteorite hypothesis. A year later, many scientists were still waiting for their promised samples. When they finally arrived, they were often too small for experiments.

Dr. Keller pressed Dr. Smit for better samples, and finally got them. At a conference in Nice, she reported finding fossils of marine plankton that not only survived the impact, but thrived for hundreds of thousands of years afterward. In the prestigious science magazine Nature, she accused Dr. Smit of withholding samples so he could present his own interpretation of them unchallenged. Dr. Smit blames his own busy schedule and poor communication. He also dismisses Keller’s results, claiming some of her “fossils” are nothing of the sort.

The results of more studies will appear next year. Meanwhile, says Dr. Norman MacLeod, keeper of paleontology at London’s Natural History Museum, young scientists are refusing to get involved in this field because “no matter what they say it will offend someone and damage their careers.”

But then, these sorts of debates are not unusual in science. The focus of the conference in Wales Dr. Keller attended last week was mantle plumes. Mantle plumes have been a central feature of mainstream geological theory since the early 1970s. And guess what? There’s an increasingly acrimonious argument underway as to whether they really exist at all.

Science is anything but a collection of dull facts: it’s a living, breathing, growing and very human enterprise.

That’s what makes it fascinating.

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