Scientists have reconstructed an almost blow-by-blow account of the catastrophe that overtook the Earth 65 million years ago at the end of the age of dinosaurs.
They believe a marauding asteroid bigger than Mount Everest slammed into Earth at a shallow angle, blasting an inferno of white-hot debris for thousands of miles across the young continent of North America and turning its mountains and valleys into killing fields.
The incinerated plants and animals were the first victims in a wave of global extinction that eventually erased hundreds of thousands of species from the face of Earth, marking the end the Cretaceous Period.
That is the conclusion of two scientists at Brown University and the University of Rhode Island who have studied the riddle of why the global effects of the doomsday rock were so uneven.
"It was a corridor of incineration," Dr. Peter H. Schultz, the scientist from Brown, said in an interview. Schultz, a planetary geologist who specializes in impact studies, said "the kill zone went toward North America" and only later mushroomed outward around the planet to include the global effects, including "the long-term changing of the climate."
For more than 15 years, scientists have debated how the wallop of the speeding rock at the end of the Cretaceous might have touched off wide extinctions of plants and animals. The main mechanism is thought to have been a global pall of dust that blotted out the sun, wreaking environmental havoc.
But the problem with that theory is that the massacre depicted in the global fossil record is turning out to be far from uniform. Some lush parts of the planet were transformed into lifeless zones. Others were merely battered or in some cases virtually spared.
The capricious nature of the catastrophe has fueled debate on whether the asteroid was really responsible for one of the biggest mass extinctions of all time, or whether other factors were at work.
Schultz and his colleague, Dr. Steven D'Hondt, a paleontologist at the University of Rhode Island, say the answer to the riddle is that the rock struck at an oblique angle, producing a colossal blowtorch of death and destruction that blazed out in one direction over thousands of miles and only later widened to create a global pall.
The key evidence, they say, lies in an impact crater found some years ago that in theory marks ground zero for the mass extinction. It is deep beneath the Gulf of Mexico and the northern tip of the Yucatan Peninsula of Mexico and is known as Chicxlub (pronounced cheek-soo-LOOB) after a nearby town. Its diameter is estimated at 185 miles.
Writing in the November issue of Geology magazine, the two scientists say that Chicxlub's shape, as revealed by magnetic and gravity readings, is plainly characteristic of an oblique angle of approach. Moreover, they say, the global fossil and mineral record is thick with evidence of the impact's highly directional aftermath.
The mountain of rock zoomed in from the southeast, the team says, and its initial blaze of fire raced to the northwest over North America.
Reaction to the new study is positive, with scientists saying it will probably prompt new studies and new fossil hunts, if not necessarily winning over all skeptics to the doomsday theory.
"It's very interesting," said Dr. Leo J. Hickey, a paleobotanist at Yale University who has long studied the extinction riddle and was once a doomsday skeptic. "It explains a lot of disparate facts."
Dr. J. John Sepkoski Jr., a paleontologist at the University of Chicago, said the new work "is consistent with two observations that have been problematic."
The first, he said, is that the extinction of fossil plants 65 million years ago was much higher in North America than in the rest of the world. The other is that grains of quartz rock that the fireball hurled around the globe are bigger in North America than elsewhere, implying that the firestorm was directional.
"It's a very interesting idea that is based on very thorough knowledge of cratering dynamics," Sepkoski said of study. But like any new idea, he added, "it has to be examined by many experts" before it becomes widely accepted.
Scientists say the study may prompt new examinations of the 150 or so impact craters that mar Earth's face, which tend to be smaller than Chicxlub. The scars might harbor signs of oblique impacts and the surrounding areas might hide fossils that show evidence of directional killing and extinction.
"People have tended to focus on global interpretations" of what happens when speeding rocks from space struck Earth, said Dr. D'Hondt, the Rhode Island paleontologist and co-author of the report.
"But to me," he said, "one of the cool things about this study is that now we have to think about regional effects" and the possibility of tongues of destructive fire.
The idea that a doomsday rock did in the dinosaurs and many other forms of life was first proposed in 1980 by a team led by Dr. Walter Alvarez, a geologist at the University of California at Berkeley. He and his colleagues had found unusually large amounts of the rare metal iridium in sediments laid down about the time the dinosaurs died out, at the end of the Cretaceous Period. They proposed that the iridium came from a cosmic catastrophe when an asteroid some six miles wide struck Earth.
A weak link in the theory was the lack of a candidate crater formed at the right time and big enough to touch off mass extinctions by blotting out the Sun. The theory nonetheless won wide attention and was hotly debated, leading to scores of new studies.
Evidence soon began to accumulate that the catastrophe was temporally and geographically variable, leading some scientists to dismiss altogether the idea of a doomsday rock and a global pall of dust.
In 1981, Hickey of Yale University published a paper in Nature, the weekly British scientific journal, saying that evidence from land plants was compatible with gradual, not catastrophic, change at the end of the Cretaceous. In a finding only later recognized as important, he noted that extinctions late in the age were far greater in North America than elsewhere.
More clues of directionality came to light in 1990 when Dr. Bruce F. Bohor, a geologist at the United States Geological Survey in Denver, presented an exhaustive review of grains of quartz rock that the hypothetical fireball had hurled around the globe, showing their sizes. Only later was it noticed that the grains were much bigger in North America than elsewhere, implying a skewed firestorm.
Also in 1990, scientists announced that they had at last found the candidate crater, Chicxlub, a huge irregular ring on the northern edge of the Yucatan. Geological processes like erosion and sedimentation had long ago buried all visible signs of the vast scar, which lies a mile or so beneath Earth's surface.
Like hundreds of other scientists around the world, Schultz of Brown was fascinated by this find and began studying the crater's intricacies. An expert on extraterrestrial cratering, he knew well from close study of the Moon and Mars that many cosmic scars are oblong, often with long plumes of debris trailing off in one direction.
And he suspected that an intruder striking Earth at an oblique angle might well account for a doomsday event, since a shallow angle would have vaporized more of Earth's crust and rock and would have lofted more debris into the air than one that hit vertically. "It can have a profound impact on the biosphere," Schultz said, "because it transfers most of the energy into the atmosphere rather than into the Earth."
Soon, he found signs that Chicxlub was indeed elongate, based on an analysis of magnetic and gravity readings. He presented this analysis at a scientific meeting in 1994, catching the eye of Dr. D'Hondt, of the University of Rhode Island. The two decided to team up, with Dr. Schultz tying together the physical evidence, and Dr. D'Hondt the biological.
"The thing that struck me almost immediately," Dr. D'Hondt recalled, "is that the idea provided a very nice explanation of why the plant record shows such a strong response to the impact in North America."
Plant extinctions are considered some of the best indicators of global upheaval since plants are among the most abundant and sturdiest forms of life, their seeds and spores remaining alive for long periods and traveling over long distances on wind, water and animals.
In their paper, the two scientists pull together all the clues for doomsday obliquity, including evidence from extraterrestrial craters, from laboratory experiments in the development of directional fireballs and from the Chicxlub crater itself. The gravity map of the Yucatan shows a horseshoe-shaped crater, its southeastern side deep and its northwestern side increasingly shallow.
The two scientists say the intruder slammed into Earth at an angle 20 degrees to 30 degrees above the horizontal, making the southeastern side of the crater deeper than the northwestern side. In contrast, an intruder that zoomed in from straight overhead would have an angle of 90 degrees and would leave a crater that was more circular.
As for ejecta, the paper cites not only Bohor's 1990 paper on the global distribution of quartz-rock sizes but notes that only North America has two distinct layers of ejecta, which laboratory experiments suggest is a fallout signature of an oblique impact.
Finally, the paper notes the geographic diversity of extinctions, especially among plants. They were lost at very high rates in North America and much lower rates in North Africa and Antarctica.
The trend is "strikingly illustrated," the paper says, by the araucaria, a genus of primitive cone-bearing trees with flat, scalelike needles now native to the Southern Hemisphere and grown as ornamentals in other areas. These conifers include the Norfolk Island pine from the South Pacific and the Chilean monkey puzzle. Such trees once flourished across North America but vanished from such areas right at the end of the Cretaceous.
The idea of doomsday obliquity can and should be tested, the team wrote. First, additional crater studies at Chicxlub should reveal shallow faulting to the northwest, and deep faulting to the southeast. Second, studies should continue to show asymmetrical patterns of fallout ejecta. And finally, excavations of fossils from 65 million years ago should continue to suggest that North America was hardest hit.
"What we're saying is that you can have good or bad impacts from the point of view of extinctions," Schultz said, adding, "The Chicxlub kind of angle or lower should occur in about one out of every five impacts."
And that makes for a relative low frequency of global cataclysm, Schultz said. "It means we don't have them very often."
Original file name: CNI - Ancient Asteroid of Death
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