'Asteroid winter killed dinosaurs'
A devastating "impact winter" wiped out the dinosaurs after a six-mile-wide asteroid slammed into the Earth 66 million years ago, new research has shown.
The study is the first to provide physical evidence for the sudden plunge in global temperatures that followed the impact off Mexico's Yucatan Peninsula.
Its effects would have been identical to those of a "nuclear winter" - the chilling effect of dust blotting out the sun in the aftermath of a large-scale nuclear war.
According to the new research, the "Chicxulub" impact is likely to have lowered global sea surface temperatures by as much as 7C at a conservative estimate.
For creatures evolved to live in a lush, warm, green world, the result would have been catastrophic.
The "winter" is thought to have lasted no more than a number of decades, but that was enough to wipe out the dinosaurs, flying and swimming reptiles, and many other forms of life.
Only the most adaptable survived, leading to the reign of the birds and mammals that continues to this day.
The evidence comes from the Brazos River region of Texas, where sedimentary rocks the same age as the asteroid impact contain layers of broken shells.
They are thought to have been left there by a tsunami triggered when the massive space rock plunged into the ocean.
Unusually high concentrations of iridium produced as the asteroid vaporised are also found in the sediments.
An idea of post-impact sea surface temperatures was obtained by analysing fatty molecules from marine organisms preserved in the rocks.
These had a different signature before and after the event, also known as the Cretaceous-Paleogene (K-Pg) boundary impact.
They indicated a significant temperature drop compared with the period just before the asteroid struck, with some samples suggesting a fall of up to 7C.
The scientists, led by Dr Johan Vellekoop, from Utrecht University in the Netherlands, wrote in the journal Proceedings Of The National Academy Of Sciences: "The global impact winter, characterised by darkness and a dramatic cooling of ocean surface waters, perturbed a relatively stable, warm, latest Cretaceous climate and likely represented a major stress factor for life on Earth."
Computer simulations suggest a hellish environment in the first hours after the impact marked by earthquakes, tsunamis and global wildfires sparked by an intense heat pulse.
Next, dust and sulphur droplets would have collected in the atmosphere and blocked out the sun's rays.
The effect may have been made worse by heat-absorbing soot from burning organic matter, said the researchers.
The "dark phase" would have temporarily suppressed the photosynthesis of green plants and algae, causing a "global collapse of terrestrial and marine food webs".
According to the models, the amount of sunlight reaching the Earth's surface might have been reduced to around 20% of its normal level after the impact.
The contrast between still-warm oceans and cold air would have fuelled large storms and hurricanes, whipping up more dust and increasing the time it spent in the atmosphere.
Ocean life might also have been affected by the acidification of seawater caused by a fall-out of sulphurous particles raining down from the sky.
The researchers concluded: "The K-Pg boundary impact presents a unique event in Earth history because it caused global change at an unparalleled rate. This detailed portrayal of the environmental consequences of the K-Pg impact and aftermath aids in our understanding of truly rapid climate change."