In related work, Sharpton has analysed gravitational data and found that the Chicxulub crater has multiple rings and may be 300 kilometres across—much larger than earlier estimates of 180 kilometres.
Contents Updated: Friday, April 28, 2000
The catastrophe that signalled the end of the Cretaceous period 65 million years ago may not have been a single event, say geologists in the US. Instead, the Earth may have been bombarded by comets or asteroids for hundreds of thousands of years. The dinosaurs and other species would then have been wiped out in steps rather than all at once.
Evidence is mounting that the principal event was the impact of an asteroid on the Yucatan coast, creating the Chicxulub crater. However, new studies indicate that the smaller Manson crater in Iowa was created at nearly the same time. The structure, 35 kilometres across and buried under 60 metres of soil, was acated by a body too srnall to have caused mass extinctions at the end of the Cretaceous.
Mick Kunk of the Us Geological Survey in Reston, Virginia, used argon-argon isotope dating to clarify the picture. He found that the Manson crater was 65.4 million years old, give or take 0.4 million years, whereas impact debris in Haitian rocks, near to Clicxulub, was 65.0 million years, give or take 0.2 million years. Although the dates are close, Kunk says they are not accurate enough to prove that Manson was created at the same time as the end of the Cretaceous.
However, magnetic fields "frozen" in Manson rocks fom the period do not match those in other rocks at the end of the Cretaceous. According to Maureen Steiner of the University of Wyoming, most Manson rocks have normal polarity, whereas the polarity of the Earth's feild is known to have reversed in the half-million year period at the end of the Cretaceous. Four samples fom Chicxulub all have reverse polarity.
Steiner stresses that her results are preliminary. "Right now, we don't know exactly what the data means," she says. The Manson impact could have occurred either 200,000 to 300,000 years before or after Chicxulub—that is, during the preceding or the following period of normal polarity. Alternatively, Manson could have been created during a brief and previously unknown interval of normal polarity during the reversed-polarity period. This would mean that it was formed closer in time to Chicxulub, though not at precisely the same time, backing up the idea that a series of impacts wiped out the dinosaurs.
One mystery is that some Manson samples do have reversed polarity, and that even those with normal polarity show signs of additional reverse polarity. This makes some observers sceptical of their meaning.
Other studies link Chicxulub with impact debris around the Caribbean that marks the end of the Cretaceous. Wayne Premo and Glenn Izett of the US Geological Survey in Denver estimated that zircon crystals from an impact layer in North America were 33 to 55 million years old. Kunk says this is too young for Manson. However, it meshes with ages for Chicxulub estimated by Buck Sharpton of the Lunar and Planetary Institute in Houston from the tectonic history of the Yucatan.
Manson-size impacts should occur every few million years, so it is suspicious that the Manson crater is so close to Chicxulub. Researchers also suspect that two Russian craters of about the same size as Manson—Kamensk (about 300 kilometres west of Volgograd) and Kara (on the Kara peninsula)—may he roughly the same age, although neither has been dated. If there was a barrage of impacts, it could explain the stepped pattern of extinctions reported by some palaeontologists.
In related work, Sharpton has analysed gravitational data and found that the Chicxulub crater has multiple rings and may be 300 kilometres across—much larger than earlier estimates of 180 kilometres.
Jeff Plescia of the Jet Propulsion Laboratory suggests it maybe the edge of a shattered zone which collapsed inward after the impact. Multi-ringed craters are known on the Moon: the inner ring corresponds to the central peak in smaller craters, while the intermediate 2l0 kilometre ring may be the edge of the original hole.