How are the Mighty Fallen? 4

Contents Updated: Monday, September 13, 1999

• Asteroid
• Analysis of the K-T Clay Layer
• A Universal Explanation
• Possible Culprits
• Chicxulub and Manson
• Jeff Poling on Asteroid Impact

Asteroid

One explanation, it is claimed, can take in all the feasible theories so far reviewed—an asteroid of exceptional size hit the earth. Edmund Halley, of Halley’s comet fame, suggested two and a half centuries ago that a comet had collided with the Earth gouging out the Caspian Sea and causing the Biblical flood.

Billy P. Glass and Bruce C.Heezen revived the possibility of the earth having catastrophic collisions with cosmic bodies in 1967. They linked the fall of large meteorites with geomagnetic reversals, the extinction of species and the distribution of tektites (curious glassy droplets varying in size from several inches to microscopic, formed from molten rock projected through the air, and widely distributed over the earth).

Lately the idea of a collision with an interplanetary body has been strengthened.

The Cretaceous-Tertiary boundary is marked by the virtual disappearance of the foraminifera, tiny creatures which live in the sea and whose shells of calcium carbonate, sinking to the bottom of the sea over millennia, form limestone. In the Gubbio district in Italy only a single species survived. Vast beds of Cretaceous limestone composed of foraminifera gave way to half an inch of reddish-grey clay which contained no fossils. Then another layer of limestone began. There was no sign of a reversal of the earth’s magnetic field.

This anomaly in the limestone rocks was found by Walter Alverez. His father, the late Luis Alvarez, a Nobel Prize winner, determined to estimate the age of the boundary layer. Micrometeorites shower the earth daily at a constant rate. Knowing that rate and analyzing the sediments for extraterrestrial material would show how quickly the sediments had deposited. Slowly deposited sediments should contain more interplanetary matter because it had been falling on them longer. He chose to look at iridium, a dense metal similar to platinum, rare on the surface of the earth (being dense, it had settled into the earth’s interior when the planet was still molten, as did most other heavy elements) but more common in meteors.

Analysis of the K-T Clay Layer

Luis Alvarez was a specialist in nuclear activation analysis. A sample is bombarded with neutrons in a nuclear reactor until some of the elements present become radioactive. They can then be identified by their differing modes of radioactive decay. Alvarez’s analysis of the clay gave odd results. The amount of iridium was 30 times higher than in the deposits above and below. The quantities were only a few parts per million but that is unusually large for rare elements like iridium, osmium and platinum. Tests on sites in different parts of the world gave similarly high concentrations. Other elements found in meteorites were also detected in comparatively high concentrations and tektites were present.

The Alvarezes’ believed tektites and meteoric materials confirmed that a massive meteorite, the size of an asteroid, had shaken the earth.

Scars on the surfaces of the Moon, Mercury, Mars, Venus and the moons of Saturn and Jupiter show they have been bombarded by meteorites throughout time. The earth also has traces of large meteor impacts in the form of craters, crater lakes or, in older weathered rocks, crater impressions. Only in 1908 some object from space (probably a fragment of the comet Encke) hit Tungusku in Siberia devastating a large area of forest.

A Universal Explanation

The Alverez team used the idea of a cosmic collision to encompass four previous explanations:

1. suppression of photosynthesis—the global darkness of three to six months curtailed photosynthesis and led particularly to extinctions in the oceans;
2. the greenhouse effect, especially of an impact into the ocean—the temperature rise caused by a blanket of water vapor would have killed many land animals;
3. an ice age—exclusion of the sun’s radiation for many months would lead to a global cooling which would kill off many species;
4. pollution and poisoning—the impact heated the air to such high temperatures that large amounts of nitrogen oxides were created by chemical reaction between the normally inert nitrogen and the oxygen in the air, and the acid rain, which subsequently fell, devastated life for a long period afterwards.

R Grieve of the Canadian Department of Energy, Mines and Resources says that 5000 asteroids with diameters of more than 3000 feet (1 km) have struck the earth in the past 600 million years. Meteors 1000 feet in diameter have hit the earth every 10,000 years on average—corresponding roughly with the cycle of ice ages. G W Weatherill writing in Icarus in 1979 and E W Shomaker at the Snowbird Conference on large body impacts in 1981 have given the frequency of cometary impacts as—one km wide every 250,000 years, five km wide every 20 million years and 15 km wide every 100 million years.

A 1000 feet wide asteroid would throw up one or two cubic miles of debris depending upon its entry speed. A 3000 feet wide meteorite would crash with a force equal to that of 10,000 ten megaton hydrogen bombs. It might be expected to leave a crater 12 miles across and displace 25 to 50 cubic miles of debris, more than sufficient to disrupt weather patterns and possibly enough to trigger an ice age in today’s conditions.

The Alvarezes postulated an asteroid six miles across hurtling into the earth at 45,000 miles per hour, gouging out a crater over 100 miles across and shooting debris amounting to 60 times the asteroid’s volume (8000 cubic miles) into the atmosphere. Sunlight would be blotted out for a long period, there would be prolonged cooling, photosynthesis would stop, the base of the food chain would die, animals higher up the food chain would starve. The asteroid must have approached more or less vertically. If it had approached obliquely spending more time in the atmosphere prior to impact, Allaby and Lovelock maintain it would have destroyed all life. Friction would have heated the air to such a temperature that nitrogen and oxygen would have reacted forming nitric acid, sterilizing the earth with its corrosive and oxidizing action.

Possible Culprits

Astronomers have observed suitable objects. The Apollo class of asteroids are prime candidates. They include planetoids of the right size and they cross the earth’s orbit making it likely that they would collide with the earth from time to time. But where is the crater?

Only one large impact crater has an age of 65 million years, Manson Crater, recently found underneath sediments in Idaho, but it is too small to have caused destruction on the scale envisaged. It is only 20 miles across not the 100 miles that would be needed. Conceivably the asteroid broke into fragments on entering the atmosphere and the Manson Crater is the scar of just one of the fragments. Some scientists argued that the asteroid was most likely to have fallen into the sea, vaporizing huge quantities of water, causing torrential rain for months on end, and a temporary greenhouse effect. No crater would then be obvious. If the asteroid disintegrated before impact some fragments might have landed in the sea and some on land, adding to the complexity of the climatic effects.

Others said that the meteor would have made a crater even though it fell into the ocean, but this has now been subducted under continents by the action of plate tectonics. About half of the ocean floors have disappeared under the edges of continents and reformed at the mid-ocean ridges since the end of the Cretaceous period.

But there is a crater, said others or a scar of one at any rate. Fred Whipple, the originator of the dirty snowball theory of comets, largely confirmed by Giotto, claimed that the impact was so energetic that the earth’s crust shattered allowing vast amounts of magma to well up filling and destroying the crater but leaving a massive scar still volcanically active—it is Iceland! Iceland has no rocks more than 65 million years old.

The volcanic hot-spot under the Hawaiian chain of islands in the Pacific was suggested as an alternative point of impact but that is unlikely. It seems to have been active for too long, at least 80 million years.

The catastrophe that signalled the end of the Cretaceous period 65 million years ago may not have been a single event, say geologists in the US. Instead, the Earth may have been bombarded by comets or asteroids for hundreds of thousands of years. The dinosaurs and other species would then have been wiped out in steps rather than all at once.

Chicxulub and Manson Crater

A crater has been found in the Yucatan peninsula and is considered a prime candidate.

Evidence is mounting that the principal event was the impact of an asteroid on the Yucatan coast, creating the Chicxulub crater. However, new studies indicate that the smaller Manson crater in Iowa was created at nearly the same time. The structure, 35 kilometres across and buried under 60 metres of soil, was caused by a body too small to have caused mass extinctions at the end of the Cretaceous.

Mick Kunk of the US Geological Survey in Reston, Virginia, used argon-argon isotope dating to clarify the picture. He found that the Manson crater was 65.4 million years old, give or take 0.4 million years, whereas impact debris in Haitian rocks, near to Clicxulub, was 65.0 million years, give or take 0.2 million years. Although the dates are close, Kunk says they are not accurate enough to prove that Manson was created at the same time as the end of the Cretaceous.

However, magnetic fields "frozen" in Manson rocks fom the period do not match those in other rocks at the end of the Cretaceous. According to Maureen Steiner of the University of Wyoming, most Manson rocks have normal polarity, whereas the polarity of the Earth’s feild is known to have reversed in the half-million year period at the end of the Cretaceous. Four samples fom Chicxulub all have reverse polarity.

Steiner stresses that her results are preliminary. "Right now, we don’t know exactly what the data means," she says. The Manson impact could have occurred either 200,000 to 300,000 years before or after Chicxulub—that is, during the preceding or the following period of normal polarity. Alternatively, Manson could have been created during a brief and previously unknown interval of normal polarity during the reversed-polarity period. This would mean that it was formed closer in time to Chicxulub, though not at precisely the same time, backing up the idea that a series of impacts wiped out the dinosaurs.

One mystery is that some Manson samples do have reversed polarity, and that even those with normal polarity show signs of additional reverse polarity. This makes some observers sceptical of their meaning.

Other studies link Chicxulub with impact debris around the Caribbean that marks the end of the Cretaceous. Wayne Premo and Glenn Izett of the US Geological Survey in Denver estimated that zircon crystals from an impact layer in North America were 33 to 55 million years old. Kunk says this is too young for Manson. However, it meshes with ages for Chicxulub estimated by Buck Sharpton of the Lunar and Planetary Institute in Houston from the tectonic history of the Yucatan.

Manson-size impacts should occur every few million years, so it is suspicious that the Manson crater is so close to Chicxulub. Researchers also suspect that two Russian craters of about the same size as Manson—Kamensk (about 300 kilometres west of Volgograd) and Kara (on the Kara peninsula)—may he roughly the same age, although neither has been dated. If there was a barrage of impacts, it could explain the stepped pattern of extinctions reported by some palaeontologists.

In related work, Sharpton has analysed gravitational data and found that the Chicxulub crater has multiple rings and may be 300 kilometres across—much larger than earlier estimates of 180 kilometres.

Jeff Plescia of the Jet Propulsion Laboratory suggests it maybe the edge of a shattered zone which collapsed inward after the impact. Multi-ringed craters are known on the Moon: the inner ring corresponds to the central peak in smaller craters, while the intermediate 2l0 kilometre ring may be the edge of the original hole.

Jeff Poling on Asteroid Impact

Over the last decade, many Earth scientists have accepted the theory that an asteroid, comet or bolide impact wiped out the non-avian dinosaurs and many other species 65 million years ago. It’s now widely believed that an asteroid of six to 12 miles in diameter smashed to Earth at thousands of miles an hour. It instantly gouged a crater 150 to 180 miles wide.

That energy release was more powerful than if all of the nuclear weapons ever made were set off at once. Billions of tons of soil, sulphur and rock vapor were lifted into the atmosphere, blotting out the sun. The impact may have triggered a global climate change that destroyed plant and animal life around the world.

Up to 70 percent of all species, including the non-avian dinosaurs, perished.

In 1989, scientists found evidence of a huge impact crater north of Chicxulub, on Mexico’s Yucatan Peninsula. Later studies found evidence of debris washed out of the Gulf by waves that went inland as far as what is now Arkansas. Scientists drilling for core samples in the Atlantic Ocean announced Sunday, February 16, 1997 that they have found what they believe to be the strongest evidence yet, indeed proof positive, that an asteroid impact caused the K-T extinction 65 million years ago.

The expedition recovered three core samples that have the unmistakable signature of an asteroid impact approximately 65 million years ago. The cores include a thin brownish layer that the scientists called the "fireball layer" because it is thought to contain remains of the actual asteroid itself.

The scientists, working on the drill ship Joides Resolution, spent five weeks off the east coast of Florida collecting cores from the ocean floor in about 8,500 feet of water. The team penetrated up to 300 feet beneath the sea bed, drilling past sediments laid down at the end of the Creataceous.

The deepest layers contain fossil remains of many animals and came from a healthy "happy-go-lucky ocean" just before the impact layers. Next youngest is a layer with small, green glass pebbles, thought to be ocean bottom material melted by the massive energy release of the impact. Then comes a rusty brown layer which is thought to be the "vaporized remains of the asteroid itself."

The heat of the impact would have been so intense that the stony asteroid would have instantly been reduced to vapor and thrown high into the sky, possibly ejecting some of the matter back into space. It then snowed down, like a fine powder, all over the globe. Brown deposits like that in the core sample have been found elsewhere and they have a high content of iridium, a chemical signature of asteroids.

Following the brown layer, is a two inches of gray clay with strong evidence of a nearly dead world. It was not a completely dead ocean, but most of the species that are seen before [in earlier layers in the core sample] are gone. There are just some very minute fossils. These were the survivors in the ocean.

This layer represents about 5,000 years, and then the core samples show evidence of renewed life. It is amazing how quickly the new species appeared.

Although the impact occurred in the southern Gulf of Mexico, the violence of the impact, followed by tsunamis (giant waves), affected the Gulf of Mexico so much that clear core samples are unlikely to be found there. The waves from the impact would have washed completely across Florida, depositing debris in the Atlantic which is where the samples were taken.

However, a mystery emerged as more and more data was gathered supporting the theory. The fossil record showed that the impact event caused an unusually high number of extinctions in North America. Why?

A possible solution to the mystery appears in an article in the November 1996 issue of Geology by planetary geoscientist Peter H. Schultz of Brown University in Providence, Rhode Island, and paleocoeanographer Steven D’Hondt of the University of Rhode Island.

In the article they cite geological evidence that the asteroid or other object didn’t strike the Yucatan head-on; rather, it approached from the southeast at a shallow angle and hit at perhaps 20 to 30 degrees from the horizontal.

The impact forced the searing debris toward the northwest into a parabola-shaped kill zone over western and central North America, a "parabola of death" that incinerated much of North America. This "corridor of incineration," as Schultz and D’Hondt called it, may have ranged beyond the Pacific shore and the Appalachian Mountains, and possibly all the way to Siberia.

Schultz simulated the asteroid impact in lab experiments at NASA-Ames. He used a hypervelocity gas gun that fires projectiles such as quarter-inch metallic spheres at 4 miles per second at low angles toward targets resembling the Yucatan surface, "literally dirt or carbonates." The collisions generated extremely hot plasmas of ionized gas, "as hot as the sun’s surface, up to 10,000 degrees Fahrenheit."

Any creatures living at the time of the impact 65 million years ago and not immediately killed by the impact might have seen:

1. A brilliant flash in the southeast as the asteroid rammed into what is now Mexico’s Yucatan Peninsula, gouging out a crater about 120 miles wide and vaporizing the upper crust.
2. A brilliant, hot plume of vapor and incandescent sun-bright debris arcing across the sky at about 7 to 10 miles per second, and falling on to North America.
3. A wave of scalding heat killing countless land-dwelling plants and animals.
4. A slower but still rapid cloud of dust, debris and molten material sweeping over North America.
5. Finally, an hour or more after the impact, more dust falling from the sky, and continuing for days, as material was dispersed around the globe.

A few fish and other aquatic creatures may have survived in cool rivers, lakes and coastal waters.

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