 A cloud of sulphate particles may have blocked out the sun's warmth
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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.
 The impactor theory is the favoured candidate for the demise of the dinosaurs
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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.
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.
