[meteorite-list] Dino Impact Gave Earth The Chill

[Ron Baalke]

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

Dino impact gave Earth the chill
BBC News
May 31, 2004

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. 

Italian, US and Dutch researchers studied rocks at El 
Kef in Tunisia which cover the Cretaceous-Tertiary (K-T) 
boundary, when dinosaurs - amongst other species - 
vanished from our planet. 

Sun block 

At the time of the dinosaurs, El-Kef was part of the warm 
western Tethys Sea. When the scientists studied the types 
of microscopic fossil creatures present in the Tunisian 
rocks, they found some surprising changes after the K-T 
boundary. 

Firstly, two new species of benthic foraminifera - simple 
animals that live near the sea floor - appeared. These 
newcomers were cold-water types found in more northerly 
oceans. 

Secondly, they found a curious difference in the
shape of a microscopic snail-like creature called
Cibicidoides pseudoacutus. This creature's shell is
said to coil in either a left or a right direction. 

In cold waters there are proportionally more
left-coiling individuals, while in warmer waters
this pattern is reversed. The researchers found a
proportional increase in left-coiling Cibicidoides,
after the K-T boundary. 

"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. 

Heat 'switch' 

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.