[meteorite-list] Double Whammy Link To Dinosaur Extinctions

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:31:09 2004
Message-ID: <200404011814.KAA04556_at_zagami.jpl.nasa.gov>

http://news.bbc.co.uk/1/hi/sci/tech/3582767.stm

Double whammy link to extinctions
By Paul Rincon
BBC News
April 1, 2004

The chances that asteroid impacts and huge bouts of volcanism coincide
randomly to cause mass extinctions may be greater than previously
imagined.

UK researchers conducted statistical tests to determine the probability
of such catastrophic events happening at the same time in Earth history.

They found massive releases of lava and space collisions should have
overlapped three times in the last 300 million years.

Details will be published in a future issue of the geological journal
Lithos.

The work has been done by Dr Rosalind White and Professor Andy Saunders
of the University of Leicester.

The probabilities they calculated assumed there was no causal link
between the two phenomena - that impacts from space did not set off the
volcanism.

Smoking guns

Flood basalts, as the term suggests, are formed by massive outpourings
of lava from beneath the Earth. Hundreds of thousands of cubic km of
material can be spewed on to the surface in short geological
timescales.

These eruptions have, like space impacts, been blamed for some of
Earth's mass extinctions due to the environmental changes they may
trigger.

The Leicester authors contend that because impacts and flood basalts
occur more frequently than mass extinctions, it is unlikely the two
phenomena bring about mass extinctions on their own.

However, mass extinctions may be triggered when the two events occur
together, they argue.

There is evidence of both phenomena happening at the same time 65
million years ago, when the dinosaurs disappeared from the fossil
record. The impact that created the 180km-wide impact crater at
Chicxulub in Mexico is generally thought to have played a major
part in this extinction.

But some scientists think the flood basalts known as the Deccan
Traps in India are an alternative "smoking gun". The gases released
in this volcanic event would have resulted in major climate changes.

Impact trigger

Firstly, the Leicester researchers determined the probability that
a flood basalt would coincide with a Chicxulub-sized crater.

They found the probability of this happening at least once over a
period of 300 million years was 57%.

Once the researchers reduced the size of the impact slightly, the
probabilities increased sharply.

For craters exceeding 100km, the probability of at least three
co-occurrences between flood basalts and impacts was 46%. For
craters exceeding 60km, the probability of three or more was 97%.

The researchers point out that impacts and flood basalt volcanism
have been implicated in three major mass extinctions in the past.

Some researchers, such as Professor David Price, of University
College London, argue that asteroid impacts can themselves bring
on catastrophic volcanism.

Professor Price says there is geochemical evidence to suggest an
impactor started the Siberian trap flood basalts, which are
associated with the end-Permian mass extinction 251 million years
ago.

"I wouldn't be very convinced by the robustness of any statistical
analysis when you're dealing with just half a dozen events in the
last billion years," Professor Price told BBC News Online.

'Kill mechanism'

He also points to evidence uncovered by Professor Mike Coffin, of
the University of Texas at Austin, which shows the Ontong Java
Plateau - a submarine flood basalt in the western Pacific Ocean
thought to be the world's largest - was initiated by the impact
of a 20km object from space.

The Leicester researchers say the idea of impacts causing massive
volcanism is interesting, but no causal link has been proven.

"In the case of the Ontong Java Plateau, we would expect such a
large impact to have left lots of signs in the geological record,
but none have been found," Dr White said.

Professor Tony Hallam, of the University of Birmingham, commented:
"The idea that [flood basalts and impacts] could be purely
coincidental is rather a negative one - a null hypothesis - but it
strikes me as quite reasonable."

Dr White and Professor Saunders propose in their paper that the
"kill mechanisms" associated with flood basalts or impacts by
themselves are not sufficiently powerful to cause the worldwide
collapse of ecosystems - a point disputed by many other scientists.
Received on Thu 01 Apr 2004 01:14:06 PM PST