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By Deborah Netburn, Los Angeles Times

LOS ANGELES — The asteroid that wiped out the dinosaurs also caused a temporary but devastating “impact winter” — darkening the sky, cooling the Earth and inhibiting photosynthesis, new research suggests.

Sixty-six million years ago, a 6.2-mile-wide asteroid known as Chicxulub struck the Earth off the Yucatan coast, setting off a series of catastrophic events that led to one of the world’s worst mass extinctions.

Computer simulations suggest that in the hours immediately after the impact, life on Earth was rattled by massive earthquakes and tsunamis, as well as global wildfires.

Then, dust and soot rose into the atmosphere, absorbing sunlight and keeping it from reaching the Earth’s surface. Plants had trouble getting enough light to photosynthesize, causing a wide-scale collapse of the food web. At the same time, the surface of the planet began to cool.

Because water holds onto heat longer than land or air, there were initially significant temperature differences between the atmosphere and the oceans that led to large storms and hurricanes.

The impact winter did not last long, however. Over a few months or possibly a few decades, the dust and soot fell out of the atmosphere and rained down onto the land and oceans, allowing sunlight to warm the planet once again.

It’s a compelling story, but one that has been difficult to prove — until now. Writing in the Proceedings of the National Academy of Sciences, a team of scientists from the Netherlands say they have found the first hard evidence of the hypothesized impact winter, buried deep in the geological record.

To take the temperature of the Earth 66 million years ago, the researchers looked at lipids produced by an ocean-dwelling microorganism called Thaumarchaeota, preserved in sediment rocks near the Brazos River in Texas.

Thaumarchaeota adjust the composition of the lipids in their cell membranes to the temperature of the sea water. When the organism dies, it sinks to the sea floor, and the lipids in its membrane are preserved in sandy ocean sediments.

Because the impact winter didn’t last long, it was difficult for the researchers to find a place where there was a thick enough sediment layer to look for the tell-tale lipid composition that would imply a short but severe cool spell.

But at the Brazos River site they got lucky. Back in the Cretaceous period this site was covered by a warm sea, said Johan Vellekoop of Utrecht University in the Netherlands and the lead author of the paper. When the giant asteroid hit, a tsunami rolled over the site and covered it with a series of sandy layers. On top of that, the researchers found a thin layer of sediment that is more fine at the top than at the bottom, and it is in this layer that they found lipid evidence of a major cooling.

According to Vellekoop, the story embedded in the rocks goes like this: Initially, the lipids tell us there is a warm climate. Then, the asteroid hits, and a wash of sandy layers from a resulting tsunami arrives. Next, storms and hurricanes churn the ocean waters stirring up sediments in the ocean. Finally, the storms subside. As the seas settle, bigger sediments fall to the seabed first, and the finest sediments fall last. Embedded in this layer are the lipid evidence of cooler temperatures.

In other words, it seems the computer models were right — and evidence of the impact winter has been found at last.

Flickr via Rupert Taylor-Price

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