Dinosaur-killing asteroid impact fouled Earth's atmosphere with dust

By Will Dunham

WASHINGTON (Reuters) - It was, to put it mildly, a bad day on Earth when an asteroid smacked Mexico's Yucatan Peninsula 66 million years ago, causing a global calamity that erased three-quarters of the world's species and ended the age of dinosaurs.

The immediate effects included wildfires, quakes, a massive shockwave in the air and huge standing waves in the seas. But the coup de grâce for many species may have been the climate catastrophe that unfolded in the following years as the skies were darkened by clouds of debris and temperatures plunged.

Researchers on Monday revealed the potent role that dust from pulverized rock ejected into the atmosphere from the impact site may have played in driving extinctions, choking the atmosphere and blocking plants from harnessing sunlight for life-sustaining energy in a process called photosynthesis.

The total amount of dust, they calculated, was about 2,000 gigatonnes - exceeding 11 times the weight of Mt. Everest.

The researchers ran paleoclimate simulations based on sediment unearthed at a North Dakota paleontological site called Tanis that preserved evidence of the post-impact conditions, including the prodigious dust fallout.

The simulations showed this fine-grained dust could have blocked photosynthesis for up to two years by rendering the atmosphere opaque to sunlight and remained in the atmosphere for 15 years, said planetary scientist Cem Berk Senel of the Royal Observatory of Belgium and Vrije Universiteit Brussel, lead author of the study published in the journal Nature Geoscience.

While prior research highlighted two other factors - sulfur released after the impact and soot from the wildfires - this study indicated dust played a larger role than previously known.

The dust - silicate particles measuring about 0.8-8.0 micrometers - that formed a global cloud layer were spawned from the granite and gneiss rock pulverized in the violent impact that gouged the Yucatan's Chicxulub (pronounced CHIK-shu-loob) crater, 112 miles (180 km) wide and 12 miles (20 km) deep.

In the aftermath, Earth experienced a drop in surface temperatures of about 27 degrees Fahrenheit (15 degrees Celsius).

"It was cold and dark for years," Vrije Universiteit Brussel planetary scientist and study co-author Philippe Claeys said.

Earth descended into an "impact winter," with global temperatures plummeting and primary productivity - the process land and aquatic plants and other organisms use to make food from inorganic sources - collapsing, causing a chain reaction of extinctions. As plants died, herbivores starved. Carnivores were left without prey and perished. In marine realms, the demise of tiny phytoplankton caused food webs to crash.

"While the sulfur stayed about eight to nine years, soot and silicate dust resided in the atmosphere for about 15 years after the impact. The complete recovery from the impact winter took even longer, with pre-impact temperature conditions returning only after about 20 years," Royal Observatory of Belgium planetary scientist and study co-author Özgür Karatekin said.

The asteroid, estimated at 6-9 miles (10-15 km) wide, brought a cataclysmic end to the Cretaceous Period.

The dinosaurs, aside from their bird descendants, were lost, as were the marine reptiles that dominated the seas and many other groups. The big beneficiary were the mammals, who until then were bit players in the drama of life but were given the opportunity to become the main characters.

"Biotic groups that were not adapted to survive dark, cold and food-deprived conditions for almost two years would have experienced massive extinctions," Karatekin said. "Fauna and flora that could enter a dormant phase - for example, through seeds, cysts or hibernation in burrows - and were able to adapt to a generalistic lifestyle - not dependent on one particular food source - generally survived better, like small mammals."

Absent this disaster, dinosaurs might still dominate today.

"Dinos dominated Earth and were doing just fine when the meteorite hit," Claeys said. "Without the impact, my guess is that mammals - including us - had little chance to become the dominant organisms on this planet."

(Reporting by Will Dunham, Editing by Rosalba O'Brien)