New Evidence in Debate Over ‘Great Dying’ : UCSD Findings Buoy Theory That Colliding Comet Caused Extinctions
UC San Diego scientists examining ancient rocks have detected organic molecules that they believe could only have been deposited when an extraterrestrial object, probably a comet, crashed into the Earth 65 million years ago.
Some researchers say the finding lends powerful support to the hotly debated theory that a cosmic collision threw up a worldwide dust cloud that hampered photosynthesis and set off a mass extinction of dinosaurs and many other animals and plants.
Other scientists, however, say there are puzzling questions in the report, and that the cause of the extinctions is still in doubt.
The detection of two extraterrestrial amino acids in rock layers deposited about the time of the mass extinction represents “the final nail in the coffin” of volcanism as an explanation for the mass extinction, said Jeffrey L. Bada, professor of marine chemistry at UCSD’s Scripps Institution of Oceanography.
“We have provided completely independent molecular evidence that there’s no question that a large object hit the Earth 65 million years ago,” Bada said. He advances no opinion on whether the impact actually caused the dinosaurs to die off.
Walter Alvarez, the UC Berkeley geology professor who remains among the strongest proponents of the impact theory, sees the amino acid as strong evidence for it. “I can’t imagine how you can explain it any other way,” he said.
Bada and graduate student Meixun Zhao report the results in today’s issue of the scientific journal Nature.
However, their findings appear unlikely to change the minds of those who look to intense volcanism, rather than an object from outer space, to account for the geological evidence of a cataclysm 65 million years ago.
“It’s interesting. But you’ve got people who are true believers and
people who are skeptics. If you’re a true believer, you’ll grasp at any straw that supports your case. If you’re a skeptic, you too will grasp at any straw that supports your case,” said Charles L. Drake, professor of earth sciences at Dartmouth College and a major proponent of the volcanism theory.
Also, some scientists regard the Bada-Zhao conclusions as premature because the amino acids were found in larger amounts than would be expected after a cosmic collision and because the deposits were above and below--but not in--the geologic layer that marks the extinctions.
The key evidence for the impact theory has been deposits of a rare element, iridium, in sedimentary rock layers that were laid down about 65 million years ago, about the time that dinosaurs disappeared. The idea was first published in 1980 by Alvarez; his late father, Luis; Helen Michel and Frank Asaro.
However, critics have contended that intense volcanism could both have deposited the iridium and extinguished species. Others argue that, whichever cataclysm one hypothesizes, it did not solely cause the mass extinction because the extinction was already in progress at the time.
Unlike iridium, the two amino acids detected by Bada and Zhao could not have come from massive volcanic eruptions because the molecules are destroyed by heat, the researchers say in their Nature report. Furthermore, the amino acids are common not in the Earth but in meteorites, they said.
Tedious Methods
Working over the last four years, they developed tedious purification methods to isolate billionth-of-a-gram quantities of the two amino acids. They worked with 1- to 2-ounce samples of rocks collected by the Alvarezes at a known iridium site in Denmark.
They found the molecules, isovaline and alpha-aminoisobutyric acid, or AIB, present in quantities up to 2,000 times higher than in both deeper and shallower rocks taken from the same site in Stevns Klint, Denmark.
The amino acids were not found in rocks taken from sites where there is no iridium layer.
These molecules were ideal targets for confirming the impact theory because they are exceedingly rare on Earth and are not known to be formed during geologic processes, but are common in meteorites, Bada said. Neither substance can even be used by plants or animals, because their structure varies from that of the amino acids in proteins that are the workhorse molecules in all living things.
The AIB and isovaline were found in such large quantities that the most likely explanation for their origin would be a comet, because it consists mainly of water that could harbor the amino acids and their precursor molecules, whereas a rocky asteroid could contain fewer of both, Bada said.
However, it is this “embarrassment of riches” that cosmo-chemist Edward Anders of the University of Chicago says is the biggest obstacle to this Bada-Zhao hypothesis.
For delicate amino acids to have survived an impact in such quantities, a major part of the object would have to have survived the collision, implying a soft impact that would seem unlikely, Anders said.
Furthermore, previous analysis by Anders and others at Chicago found no noble gases in the rocks around the iridium layer, yet they would have been more likely to have survived an impact than amino acids would have, he noted. Noble gases are inert elements common throughout the universe.
‘Responsible Way’
Bada suggests that the amino acids may have been re-synthesized during the impact.
Also, the AIB and isovaline were found not in the iridium-containing layer but in rocks that were a foot to a yard above or below the iridium. Bada said diffusion of the organic molecules after deposition can account for that, an explanation that deep-sea sedimentologist Wolfgang Berger at Scripps said is plausible.
Anders, who thinks the Bada-Zhao conclusions may be premature, nevertheless said he has no doubts about Bada’s methods.
“He is going at it in a very responsible way,” Anders said. “He is not a trigger-happy person who comes up with an interesting result and makes a big splash. He has sat on this data and worked on this problem for several years trying to verify it in every possible way.”
Furthermore, if the amino acids prove to have come from a comet, the researchers will have shown something that international expeditions to study Comet Halley in 1986 were unable to do, Anders noted.
