http://www.bbc.co.uk/news/science-environment-2417
Double meteorite strike 'caused dinosaur extinction'
By Howard Falcon-Lang
BBC News
August 27, 2010
The dinosaurs were wiped out 65 million years ago by at least two
meteorite impacts, rather than a single strike, a new study suggests.
Previously, scientists had identified a huge impact crater in the Gulf
of Mexico as the event that spelled doom for the dinosaurs.
Now evidence for a second impact in the Ukraine has been uncovered.
This raises the possibility that the Earth may have been bombarded by a
whole shower of meteorites.
The new findings are published in the journal Geology by a team lead by
Professor David Jolley of Aberdeen University.
When first proposed in 1980, the idea that a meteorite impact had killed
the dinosaurs proved hugely controversial. Later, the discovery of the
Chicxulub Crater in the Gulf of Mexico, US, was hailed as "the smoking
gun" that confirmed the theory.
Double trouble
The discovery of a second impact crater suggests that the dinosaurs were
driven to extinction by a "double whammy" rather than a single strike.
The Boltysh Crater in the Ukraine was first reported in 2002. However,
until now it was uncertain exactly how the timing of this event related
to the Chicxulub impact.
In the current study, scientists examined the "pollen and spores" of
fossil plants in the layers of mud that infilled the crater. They found
that immediately after the impact, ferns quickly colonised the
devastated landscape.
Ferns have an amazing ability to bounce back after catastrophe. Layers
full of fern spores - dubbed "fern spikes" - are considered to be a good
"markers" of past impact events.
However, there was an unexpected discovery in store for the scientists.
They located a second "fern spike" in a layer one metre above the first,
suggesting another later impact event.
Professor Simon Kelley of the Open University, who was co-author on the
study, said "We interpret this second layer as the aftermath of the
Chicxulub impact".
This shows that the Boltysh and Chicxulub impacts did not happen at
exactly the same time. They struck several thousand years apart, the
length of time between the two "fern spikes".
Uncertain cause
Professor Kelley continued: "It is quite possible that in the future we
will find evidence for more impact events".
Rather than being wiped out by a single hit, the researchers think that
dinosaurs may have fallen victim to a meteorite shower raining down over
thousands of years.
What might have caused this bombardment is highly uncertain.
Professor Monica Grady, a meteorite expert at the Open University who
was not involved in the current study, said "One possibility might be
the collison of Near Earth Objects".
Recently, Nasa launched a program dubbed "Spaceguard". It aims to
monitor such Near Earth Objects as an early warning system of possible
future collisons.
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