- 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.
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- Rocks in Tunisia reveal microscopic cold-water creatures
invaded a warm sea just after the space rock struck Earth.
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- The global winter was probably caused by a pollutant
cloud of sulphate particles released when the asteroid vapourised rocks
at Chicxulub, Mexico.
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- The results are reported in the latest issue of the journal
Geology.
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- 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.
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- Sun block
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- 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.
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- 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.
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- 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.
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- 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.
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- "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.
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- 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.
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- Heat 'switch'
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- These would have been released when the asteroid collision
vapourised rocks rich in sulphate salts at Chicxulub.
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- Matthew Huber of Purdue University in Indiana, US, calculated
the global impact of the winter.
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- "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.
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- 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.
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- "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.
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- 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.
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- 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.
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- The Cretaceous-Tertiary extinction was a selective one;
entire groups such as dinosaurs and ammonites were killed off, while others
were left unaffected.
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- 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.
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- © BBC MMIV
- http://news.bbc.co.uk/2/hi/science/nature/3750765.stm
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