Cretaceous Die-Offs: A Tale of Two Comets?
Monastersky, Richard, Science News
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. …