Cretaceous die-offs: a tale of two comets?

Hot on the trail of a prehistoric killer, geologists have used the chemical equivalent of fingerprints to exonerate one suspect while shoring up the case against another in Earth's greatest murder mystery- the mass extinction that ended the Cretaceous period and wiped out the last living dinosaurs.

At the Lunar and Planetary Science Conference in Houston last month, several teams of researchers reported on studies concerning two impact craters that date to the boundary between the Cretaceous (K) and Tertiary (T) periods 65 million years ago. Analysis of a crater buried near Manson, Iowa, suggests that the impact there left no widespread trace in the sediments of the time and therefore did not cause any of the global havoc. Instead, mounting evidence links the Chicxulub crater beneath Mexico's Yucatan Peninsula with the K-T catastrophe.

"I think people who have been supporters of Manson have realized that Manson is probably not responsible for virtually anything we see in the K-T boundary sediments and that everything is fitting into place for Chicxulub:' says Joel D. Blum of Dartmouth College in Hanover, N.H.

Scientists first raised the idea of K-T impacts in 1979, after finding a thin layer of clay containing high concentrations of the element iridium in 65-million-yearold sediments, Because iridium is much more abundant in comets and asteroids than in Earth's crust, the scientists proposed that the clay layer represents the failout from a thick dust cloud created when an extraterrestrial body walloped Gravity data from Chicxulub show the buried crater. White dots indicate proposed outer rim, which would make the crater nearly 300 km across.

Earth at the end of the Cretaceous.

Further impact evidence came when researchers studying the K-T boundary sediments found slivers of quartz bearing fractures created by a severe shock wave. The K-T sediments also yielded pieces of "tektite" glass, which forms when an impact sends up a spray of molten rock droplets that solidify as they fall.

When geologists went searching for the crater left after the K-T crash, they focused first on the Manson structure. At 35 kilometers wide, Manson is one of the biggest impact craters on Earth, though most scientists have considered it too puny to account for the amount of iridium present in K-T boundary sediments. In the last two years, geologists have drilled into the Manson crater to obtain samples that would resolve its age and relationship with the K-T event.

Blum and his colleagues used the isotopic ratios of several elements to compare drill samples from Manson with pieces of tektite glass found in K-T boundary sediments from Haiti. This chemical fingerprinting technique revealed that the Manson impact could not have created the Haitian glass. "The Manson samples are about as different as you can get:" Blum says.

The same technique shows a "striking similarity" between the Haitian glass and rocks from Mexico's Chicxulub crater, suggesting that the Haitian samples formed from the Yucatan rock, says Blum. Although oil company geologists discovered the Chicxulub structure in the late 1970s, only within the last two years have researchers recognized the formation as a K-T crater.

After the identification of Chicxulub, some geologists who had previously worked on the Manson crater wondered whether K-T boundary sediments contain evidence of both impacts. Indeed, in the Rocky Mountains, sediments dating to this time show two distinct clay layers, possibly from the two impacts.

New evidence, however, argues against the idea that Manson left a widespread imprint. Wayne R. Premo and Glen A. Izett of the U.S. Geological Survey in Denver came to this conclusion by dating zircon crystals found in K-T sediments from Colorado, a technique first used last year by another team. These crystals - remnants of the rock originally hit by the impactor -- are roughly 550 million and 330 million years old. Because the rocks at Manson are much older, the zircons suggest that the Iowa impact left no appreciable mark in the Colorado sediments, says Premo.

Maureen B. Steiner of the University of Wyoming in Laramie reports that early analysis of the magnetic orientation in the Manson drill samples indicates the Manson crash occurred roughly 200,000 years before or after the Chicxulub impact, On the basis of this evidence, Eugene M. Shoemaker of the USGS in Flagstaff, Ariz., suggests the two strikes most likely came from comets, perhaps knocked loose from the Oort cloud at the edge of the solar system.

In terms of global effects, geologists believe the Chicxulub crash did almost all of the damage because it was so much larger than the Manson hit. Early analysis of the buried Mexican crater indicated it was 180 km across, which would make it the largest known crater on Earth. But Virgil L. Sharpton of the Lunar and Planetary Institute in Houston reports that a new study of gravity measurements hints that the Chicxulub crater spans almost 300 km, making it one of the largest in the solar system. Scientists think such a crash blocked out sunlight and chilled Earth for several years before spawning a global heat wave that lasted perhaps a millennium.