Current Issue

June 6, 2012

Vol. 112, No. 14


The Dissenter

Most experts think an asteroid killed the dinosaurs. Not Gerta Keller

By Joel Achenbach ’82
Published in the June 6, 2012, issue

Gerta Keller in her office in Guyot Hall.
Gerta Keller in her office in Guyot Hall.

THE SEPIA-TINTED PHOTOGRAPH shows a farm family in Switzerland a few years after the end of World War II. There’s a mother, a father, and a swarm of children. The family eventually will have a dozen kids, six boys and six girls, plus 18 cows and a dozen pigs and some workhorses. In the photo they pose next to a wagon. The girls wear aprons to protect their hand-sewn clothes from getting soiled. They’re poor. It’s a ­pastoral but cramped existence, the horizon limited by poverty and, for the girls especially, a social code that tells them they can dream of little more than being a nurse or a seamstress. 

In the photo, most of the children look wary; some smile timidly. But one little girl, on the left side of the frame, smiles broadly for the camera. She occupies a special position, at the edge of the group, sitting on the wagon, and one can sense her independence.

“I was always a bit separate from the rest,” says Gerta Keller.

Keller, left, in a family photo taken after World War II.
Keller, left, in a family photo taken after World War II.

Professor Keller, that is. Her rags-to-riches story has wound up improbably on the third floor of Guyot Hall at Prince­ton. It has been a very long journey by way of England, North Africa, Australia (where she was shot and nearly killed by a bank robber), and San Francisco during the heyday of Flower Power. There were many way stations in the middle of nowhere, among outcroppings, riverbeds, quarries — amid the rocks that tell the story of the planet’s history. Keller is a geologist and paleontologist. 

She’s also a catastrophist. Her world isn’t one of gradual, uniform change. It’s a lot more dramatic than that. 

Keller, who never has let other people tell her what to do, is today one of the most controversial figures in her field. She’s an indefatigable critic of the most widely accepted theory of what wiped out the dinosaurs 65 million years ago and brought the Cretaceous Period to an end. She has a rival hypothesis, and is determined to prove she’s right.

She’s once again separate from the rest. 

On a terrible day about 65.5 million years ago, a mountain-sized object from space crashed into the Earth near what is now the tip of the Yucatan Peninsula. The ­so-called Chicxulub (CHEEK-shoe-lube) impact, named for a Yucatan village, excavated a 110-mile-wide crater and sent a tsunami the height of a Manhattan skyscraper crashing onto the shores of the Gulf of Mexico. The impact ejected enough dust into the stratosphere to darken the planet and temporarily shut down photosynthesis. The effects were devastating, and global. 

Half the genera on the planet disappeared, including the dinosaurs (or, as we are compelled to say now that we know a chicken is a distant cousin of T. rex, “the non-avian dino­saurs”). This was the most recent of the five great mass extinctions in the fossil record. The discovery of the Chicxulub catastrophe has obvious implications for our own species, which, though currently in a period of efflorescence, could discover one day that it has no more permanent sovereignty on the planet than did the giant reptiles that ruled for 150 million years.

The impact hypothesis for what’s called the K/T extinction (K for the German word for Cretaceous, T for Tertiary) dates to 1980. Geologist Walter Alvarez *67 had been doing fieldwork in the late 1970s in Gubbio, Italy, studying an outcropping that featured a thin clay layer marking the boundary between the Cretaceous and Tertiary rock formations. It was one of the best-known sites for seeing the change in the fossil record that occurred 65.5 million years ago. Below, the older rock contains fossils of numerous species of foraminif­era, which are planktonic marine organisms. Above the clay layer, in younger rock, the forams, as they’re commonly called, are smaller, and represent only a few species. 

Alvarez wanted to know how long it had taken for the clay layer to be deposited. An enduring mystery was whether this change in the fossil record reflected a sudden die-off or something more gradual. Sedimentary strata can be deceptive: It’s hard to know how much time is represented by any particular layer. There can also be gaps in sedimentation. Alvarez’s father, Nobel laureate physicist Luis Alvarez, suggested that it would be possible to estimate the time it took for the clay layer to be deposited by studying the quantity of iridium in the clay. Iridium is a rare element on Earth but steadily rains upon the planet from space in small quantities. 

The father-son Alvarez team made a startling discovery: The clay layer contained an anomalously large quantity of iridium. This suggested a new idea: The iridium came from a large asteroid that had struck Earth. 

The Alvarezes and two colleagues published their earthshaking paper, “Extraterrestrial Cause for the Cretaceous-Tertiary Extinction,” in the journal Science in 1980. There remained one obvious hole in the impact theory: Where was the crater? As it happened, in the late 1970s, geologists working for a Mexican oil company studying gravity maps off the coast of the Yucatan discovered what appeared to be a buried, eroded crater. They didn’t know if it was from a volcano or an impact. They said little about it. Throughout the 1980s, the mystery of the crater’s location bedeviled scientists, but they gradually began homing in on the Gulf of Mexico after finding what appeared to be tsunami debris in southern Texas. Finally, in 1990, they put it all together, and Chicxulub soon became known as the Crater of Doom.

Keller doesn’t dispute that there was a giant impact event at Chicxulub. But ...

“I’m an impactor, too,” she says. “It comes down to the age of the impact. Very simply, the age. It is much older than the mass extinction.”

It’s all about the timing, the sequence of events. The Chicxulub impact, she has argued, predates the extinction by roughly 150,000 to 300,000 years. She bases this on fieldwork that has taken her to 150 K/T boundary sites around the world. 

So what caused the extinctions? Volcanoes, for starters, she says. Specifically, the Deccan Traps. The Deccan Traps (“traps” comes from the Swedish word for stairs) comprise a vast area of lava flows, much larger than the state of Texas and in some places more than two miles thick, on the Indian subcontinent. The main eruption occurred very close to the end of the Cretaceous period, pumping enormous quantities of sulfur dioxide, carbon dioxide, and chlorine gases into the atmosphere and causing dramatic climate change. Add to that the impact events — plural. Chicxulub wasn’t the only impact, Keller says. In her scenario, the planet endured not just one but a series of catastrophes that caused the mass extinction.

She is fighting an intellectual war in two distinct theaters — the Gulf of Mexico and India. She spends a great deal of time along the Brazos River in Texas, a key K/T boundary site. At multiple sites around the Gulf of Mexico, she says, tiny spherules from the impact are below the K/T boundary, which suggests that the impact and the extinction were separated in time. She notes that there are several locations where the spherules appear to be right at the K/T boundary, but says this is the result of erosion that has reworked the sediments. It’s a debate that is literally and figuratively granular: Most of us have no way to know if she’s right.

She says of the proponents of the Chicxulub theory: “It’s a beautiful scenario that they made. Only that it’s all wrong.”

I ask her if she thinks the Chicxulub impact had caused any extinctions whatsoever. She pauses, and looks at me steadily. 

Then she says: “Not a single species.”

Post Comments
3 Responses to The Dissenter

Jamie Spencer '66 Says:

2012-06-04 12:15:23

You are lucky to have so good a writer as Joel. His was one of the best-written pieces I can recall. And we are all lucky to have an iconoclast of Gerta's stature on campus, too.

Ide Trotter *57 *60 Says:

2012-06-06 15:11:28

I think Jamie said it well. Long live scientific controversy. Some day these matters will come into clearer perspective, thanks to the grit and tenacity of the dogged Gerta's of science, may their tribe increase.

Dassanayake Pannila Says:

2013-02-11 10:00:24

If dinosaurs could survive long years like 33,000 after the impact, how could not so powerful volcanic activity cause them to oblivion? 33,000 means 330 generations if the life span of one is 100.
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