Latest Dino Extinction Bit

["Richard W. Travsky" <rtravsky@uwyo.edu>]

http://news.bbc.co.uk/1/hi/sci/tech/3750765.stm

Evidence has been found for a global winter following the asteroid impact
that is thought to have killed off the dinosaurs 65 million years ago.

Rocks in Tunisia reveal microscopic cold-water creatures invaded a warm
sea just after the space rock struck Earth.

The global winter was probably caused by a pollutant cloud of sulphate
particles released when the asteroid vapourised rocks at Chicxulub,
Mexico.

The results are reported in the latest issue of the journal Geology.
...
"It's the first time we have found physical evidence for cooling at the
K-T boundary," said Dr Simone Galeotti of the University of Urbino, Italy.

Dr Galeotti and his colleagues think the most likely cause of the cooling
was a pollutant cloud of airborne sulphate particles, or aerosols, that
blocked out sunlight.

These would have been released when the asteroid collision vapourised
rocks rich in sulphate salts at Chicxulub. 

Matthew Huber of Purdue University in Indiana, US, calculated the global
impact of the winter.

"The results we got are fairly consistent with the impact winter
decreasing the amount of sunlight hitting the Earth by 90%. If you turn
off that heat source, the Earth will cool in a big way," he told BBC News
Online.

The oceans would have acted as a reservoir of heat to prevent the surface
temperature of the planet from cooling too much. However, this reservoir
is not infinite. If the sunlight was blocked out for long enough, the
oceans would eventually have frozen solid.

"It must have been dark long enough to cool the oceans, but not long
enough that the whole planet iced over - that's not what we see in the
fossil record," said Dr Huber.

This impact-induced darkness would have lasted between one and ten years
on land, but there is evidence for a cooling of up to 2,000 years at El
Kef.

Positive feedback mechanisms may have prolonged the cooling effect of the
impact winter in waters of intermediate depth - such as those at El Kef -
and deeper.

The Cretaceous-Tertiary extinction was a selective one; entire groups such
as dinosaurs and ammonites were killed off, while others were left
unaffected.

The latest research does not probe this mystery, but it does help fill in
the picture of what was happening to our planet following the impact at
Chicxulub.