Dinosaurs may have already been in decline on Earth some 65 million years ago, but a team of NASA scientists now believes it was the sulfur-rich atmosphere created in the aftermath of an immense asteroid collision with Earth that brought about a global freeze and the demise of these giant Mesozoic creatures.
The impact of this large asteroid -- perhaps the largest since life evolved -- hit a geologically unique, sulfur-rich region of the Yucatan Peninsula in Mexico, according to planetary geologist Adriana C. Ocampo and atmospheric scientist Dr. Kevin H. Baines, both of the Jet Propulsion Laboratory's Earth and Space Sciences Division. They estimate the impact was between 10,000 to 50,000 times more powerful than the comet Shoemaker-Levy 9 impact on Jupiter last July and kicked up billions of tons of sulfur and other materials.
The researchers and colleagues Dr. Kevin O. Pope of Geo Eco Arc Research in La Canada, Calif. and Dr. Boris A. Ivanov of the Russian Academy of Sciences in Moscow have co-authored a paper detailing the global atmospheric impact of this asteroid collision at Chicxulub, Mexico in the latest issue of Earth and Planetary Science Letters.
"We estimate that this asteroid was between 10 kilometers to 20 kilometers (6 miles to 12 miles) in diameter and its collision on Earth brought about total darkness around the world for about half a year," Ocampo said. "But more importantly, persistent clouds generated by the impact on this geologically distinct region of sulfur-rich materials caused temperatures to plunge globally to nearly freezing."
"These environmental changes lasted for a decade and subjected organisms all over the world to long-term stresses to which they could not adapt in such a brief time span," Pope added. "Half of the species on Earth became extinct as a result."
The researchers based their work on computer models of the impact and atmospheric effects, studies of the crater geology and extensive fieldwork at a rock quarry located 360 kilometers (223 miles) south of Chicxulub at Albion Island in Belize, where fragments bearing the unique characteristics of the impact were found.
In studying the sites and modeling the resulting changes in the biosphere, the scientists discovered that it was the specific geological location of the impact in a region that is rich in sulfur materials that created catastrophic climate changes and led to the downfall of the dinosaurs.
"If this asteroid had struck almost any other place on Earth, it wouldn't have generated the tremendous amount of sulfur that was spewed into the atmosphere to create such a devastating, worldwide climate change," Baines said. "In fact, we human beings owe our existence to the uniqueness of this impact region."
On impact, the asteroid hurled some 35 billion to 770 billion tons of sulfur high into the atmosphere, along with other materials. The NASA team, in collaboration with Dr. Alfred Fischer of the University of Southern California, recently discovered rocks --some the size of a Volkswagon bug -- that were blown out of the crater and landed 360 kilometers (223 miles) south of the Chicxulub site in Belize.
The boulder deposit in Belize also contained fragments of glass that were created by the melting of rock when the asteroid crashed into Earth, Ocampo said. And spherical fragments, known as "tektites," were scattered about and formed as the molten glass flew through the air and cooled. The tektites have been found in other regions near the crater, such as Haiti, Mexico, Texas and Alabama, but never in association with large boulders.
As the researchers continued to excavate, they found spherical pieces of calcium carbonate, some of which have an unusual radial structure. The formation of these "spherules" remains a mystery, Ocampo said, but the scientists speculate that they could have formed from the residue of vaporized sulfur-rich rocks.
Another important find at the Belize rock quarry was limestone with fossils dating to the early part of the Cretaceous.
"Fossils of this age don't belong in northern Belize," Ocampo said. "Early Cretaceous fossils have been found deep below the surface near the crater during drilling by the Mexican Petroleum Company. We think the limestone found in Belize was excavated by the impact, which probably blew a hole more than 15 kilometers (9 miles) deep in the Yucatan Peninsula."
Since 1980, when University of California-Berkeley geology professor Walter Alvarez and his colleagues first proposed the theory, researchers have been searching for impact sites that would explain the sudden disappearance of the dinosaurs. The main evidence to support the theory came from finding a substance called iridium in a layer of clay in Italy. The concentration of iridium, an element found on Earth in very small quantities, was quite large. However, a high concentration of the element is found in asteroids and comets.
It took another decade for researchers to find an actual impact site. In 1989, Pope and Charles Duller of NASA's Ames Research Center in Mountain View, Calif. discovered a semi-circle of sinkholes at Chicxulub. Ocampo studied gravity and magnetic data from the crater and correlated them with the sinkholes. She concluded that the area had the classic characteristics of an impact crater, indicating that Chicxulub was, in fact, the place where a colossal asteroid had smashed into Earth millions of years ago. Current estimates of the crater size range from 180 kilometers to 300 kilometers (112 miles to 186 miles) in diameter, making it one of the largest craters known on Earth.
The researchers used sophisticated atmospheric models of the sulfur-rich atmosphere of Venus to paint their doomsday scenario.
"Initially, thick sulfur clouds, combined with soot and dust generated by this impact, would have spread worldwide and blocked out the sun," Baines and Pope said. "Night-like conditions probably existed all over Earth for at least six months and wiped out many species of plants because the blackout essentially brought photosynthesis to a halt. Unlike the aftermath of typical impacts, the skies remained murky for at least a decade, due to chemically generated clouds of sulfuric acid high in the stratosphere."
The reflection of sunlight back into space from these high-altitude clouds caused surface temperatures to drop to nearly freezing for many years all over the planet, even over normally balmy islands in once-tropical seas.
These atmospheric conditions occur in Venus's perpetually cloudy atmosphere, Baines said, where ultraviolet sunlight and water in the high atmosphere can convert sulfur dioxide into sulfuric acid clouds. Sulfuric acid clouds like those that cover Venus may well have continued to blanket the Earth for more than a decade after the initial impact of the asteroid, causing a secondary and more long-lasting effect -- the coup-de-grace or final knockout blow -- which killed much of life on Earth.
"The entire ecosystem of Earth, including plants and animals, was subjected to extreme environmental conditions, which a large number of well-established species, such as the dinosaurs, simply could not cope with," Baines said.
Six months of total darkness and 10 years of global freezing ultimately destroyed the dinosaurs and many other organisms, Pope added. Miraculously, many species survived the catastrophe and evolution took a new turn, ushering in the era of mammals and, eventually, humankind.
Results of the Belize research are scheduled to appear with other works in an upcoming Special Paper of the Geological Society of America, which will feature recent research on major catastrophes in Earth's history.
The research was sponsored by the Exobiology Program in NASA's Solar System Exploration Division. Fieldwork in Belize was supported in part by the Planetary Society in Pasadena, Calif.
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