Friday, December 11, 1998 Published at 18:16 GMT
Meteorite is possible ice age culprit
The frozen swirls of molten rock confirmed the impact
The discovery of a 3.3-million-year-old meteorite impact site in Argentina has revealed a potential cause for the series of ice ages that has periodically frozen the Earth since that time.
The new work has shown that a sharp dip in the ocean temperature occurred at the same time as dozens of species were suddenly wiped out and also when a thick layer of molten rock was formed.
A meteorite impact could explain all these, as the extreme pressure at impact would melt the rocks and create a cloud of dust or smoke which would block out the sun's warming light.
The impact's disturbance of the ocean circulation and atmosphere could have kick-started the global cycle of ice ages, believes Professor Peter Schultz, an impact specialist at Brown University and who led the work.
"This impact could be the trigger for the climatic change. All the coincidences are very intriguing and compelling, but we will need very careful stratigraphic studies to prove it."
Schultz believes the meteorite was at least one kilometre wide. More than 36 species of animals were blasted into extinction, including large armadillo-like creatures and a flightless meat-eating bird. The glassy layer of once-melted rock is up to six feet thick and the Atlantic ocean cooled by 2 deg C.
By comparison, the meteorite which caused the global extinction of the dinosaurs was 10 km wide, but an impact of this size is only expected every 50 or 100 million years.
One kilometre impacts will occur on average every 4 million years, but could still have very significant effects upon the Earth. "El Niño or a volcanic eruption produces small tweaks to the climate compared to what one of these impacts can do," says Professor Schultz.
In geological terms the impact near Mar del Plata, Argentina, happened a short time ago and so the evidence left behind is relatively undisturbed.
"The work is interesting and novel - usually people are looking at the biggest events but there is a spectrum in the size of meteorite impacts so it has been suggested there is a spectrum of extinctions."
There is great interest in predicting the effects of meteorite impacts on today's planet, says Benton, and the unearthing of relatively recent impacts are crucial to checking the theories.
The research, published in Science, comes on the same day that geologists are meeting in London to discuss "sub-critical" meteorite impacts.
These are impacts with regional, not global, effects but which occur much more frequently. Amongst the topics are whether impacts can trigger earthquakes and the evidence for a destructive blast wave and wildfires which swept the whole of the Middle East only 4000 years ago.
The Argentinian meteorite impact was suspected when scientists realised that the unusual glassy rock had formed at the same time as dozens of local species were wiped out and as the deep ocean cooled.
The clinching evidence came when a new exposure of the glassy rock was found recently. It contained swirls of frozen glass very similar to features seen in rocks at the site of known meteorite impacts.
No crater has been found despite evidence that it cannot have been far away. The large volcanic bombs - blobs of molten rock given aerodynamic shapes whilst flying through the air - are up to two metres in size meaning they cannot have been thrown far.
However, the coast has eroded by several kilometres since the impact so the crater itself may have been erased.