"If it had killed off the cockroaches, no one would care," Earth Sciences Professor Page Chamberlain said about the giant comet or meteor he thinks caused the last great mass extinction of life on earth.
The cockroaches did not die 65 million years ago, nor did the mammals, but the dinosaurs and more than half the species on the planet did. Why they did remains a mystery.
Chamberlain and fellow Earth Sciences Professor Joel Blum are back in the national press after uncovering another clue to that scientific puzzle.
With the help of an international team and Dartmouth mass spectrometry specialist Michael Hingston, the two say they have confirmed Mexico as the site of a cataclysmic impact which happened at the time of the extinction.
Their findings will be reported in the next issue of Nature magazine.
Many researchers believe an impact of extraterrestrial origin caused the mass extinction, the last of several to take place throughout earth's history.
Chamberlain and Blum examined tiny glass spheres called microtektites that were found in Haiti, which they believe result from an impact.
Several sites around the globe have been suggested as the single "source crater" for the catastrophic event, most often a crater in Iowa and a crater near Chicxulub, Mexico, on the Yucatan Peninsula. The Chicxulub crater, 110 miles long, is completely underground. It was discovered by oil diggers and revealed with seismic testing.
Using radiogenic or unstable isotopes of strontium and neodymium, the professors proved the spherules from Haiti, previously dated at 65 million years old, were chemically identical to impact melt rock from Chicxulub, and different from such rock in the Iowa crater.
"We've been able to identify unequivocally the source crater," Chamberlain said. "It is Chicxulub, with no doubt at all."
Blum said the study supports the theory that the Chicxulub impact caused the mass extinctions of dinosaurs and other species.
"The species at the time were already stressed," Chamberlain said. "All you need when you have stressed species is some big change to wipe them all out.
The recent findings build upon a study by Blum and Chamberlain published last year in Science magazine. In that article they presented evidence suggesting an impact at Chicxulub caused the release of huge amounts of carbon dioxide, leading to global warming.
Blum and Chamberlain know there are dissenting voices in the national debate over what caused the mass extinction. They can not forget, because one of those voices comes from right down the hall.
"They're pretty well convinced of what the data mean," Professor Charles Drake said of the new study by his colleagues around the corner. "I'm not so sure."
Professors Charles Officer, John Lyons and Drake, all in Earth Sciences, published an article just over a year ago which pointed to flaws in impact theories.
Drake agreed the research team's radiometric data indicates an impact, but noted geologic data which shows that on top of the Chicxulub impact melts are rocks more than 65 million years old. He said any geological event that would cause the site to sink below older rocks would not have been caused by an impact.
Drake does not rule out an impact. He said the important question is whether such an impact had a major effect on life at the time. He said that in geologic time, "instantaneous" can be taken to mean anything from several months to 100,000 years.
According to Drake, who has worked on the impact theory for about 10 years, it is accepted that most of the large dinosaurs were already extinct by the time the mass extinction took place.
Drake says the best clues to the mass extinction in the fossil record come not from the dinosaurs but microscopic plankton, which evolve rapidly and existed in a vast number of different species, each plentiful in numbers. He notes that animal-like zooplankton became extinct rapidly but plant-like phytoplankton took 100,000 years to die off.
"The extinction was selective," he said. "If it wasn't, we wouldn't be around here talking about it. Some things died, others didn't. I have not heard of a mechanism which would persist and build up over time from an impact."
Chamberlain said the global warming hypothesis was "just conjecture." But he and Blum believe that global warming could have led to a gradual extinction over a period of 10,000 to 100,000 years. They believe this could indeed reconcile an impact with a gradual extinction suggested by the fossil record.
Last February Blum and Drake debated on the issue before the Dallas alumni club.
The impact theory was first proposed in the 1970s by a famous father-son team at Berkeley, Louis and Walter Alvarez. Their original theory suggested the extinction was sudden, in a matter of years, resulting from dust clouds blocking the sun, forming a theoretical basis for the idea of a nuclear winter. According to Drake, the Alvarez findings and subsequent debate revived the shrinking field of paleontology.
Drake said he was close friends with the late Louis Alvarez, the father. Chamberlain said he never met the elder Alvarez. Chamberlain and Blum, who are younger than Drake, know the younger Walter Alvarez well, keeping in touch through national seminars on impact theory.
"You look at the people pouring into 'Jurassic Park. That tells you that dinosaurs are grabbers," Drake said.
"If Alvarez had said a big meteor killed off the phytoplankton, nobody would have paid much attention to him."
On that point, all seem to be in agreement.
