Coffin for the Dinosaurs
from the book, "Petrified Lightning"
Coffin for the Dinosaurs
The demise of the dinosaurs, so sudden from a geologic point of view, has been a subject of scientific debate for decades. Not only universities but B movies, comic strips, and dime novels have accepted the death of the dinosaurs as a sudden, mysterious event. Recent evidence has been unearthed that appears to provide convincing explanations.
The dinosaurs lived on earth during the 140 million years of the Mesozoic Era, the age of reptiles, divided into three periods: Triassic, Jurassic, and Cretaceous. A very successful and diverse family of animals, they ruled supreme, occupying every major niche on the land, with related species controlling the sea and air. The dinosaurs died out at the end of the Cretaceous period, 65 million years ago, and the age of reptiles was over.
The succeeding era, the age of mammals, began with the Tertiary Period. The boundary separating it from the earlier period is identified as the K—T boundary because it separates the Cretaceous (K) from the Tertiary (T). After 140 million years, the family of giants that ruled during the Mesozoic Era was suddenly gone.
What caused the extinction of such a successful group of animals? One of the many theories was overspecialization. Until two decades ago, textbooks proclaimed that dinosaurs became so specialized that they were unable to adapt to changing climates and fell victim to environmental modifications they couldn't handle. But the most persistent theories and opinions, even among scientists, were that the dinosaurs were too stupid to survive. This writer, supported by many authorities, once taught that if a student were to walk up to a brontosaurus and kick it in the tail, a full two minutes would pass before the animal, with its dull wits and even duller senses, would be aware of it.
In the 1980s evidence began to demonstrate that dinosaurs were not sluggish, but were far more agile and better adapted to their world than had been imagined. Some may even have been warm—blooded. Rather than the stupid brutes depicted earlier, many had quick reflexes and highly developed senses. At the end of the Cretaceous period, some 75 percent of the known species died out very suddenly on the land, sea, and air. Problem solvers turned to physical causes, such as a drastic fall in sea level, global cooling, and atomic radiation from space brought in by the explosion of a nearby star. Some scientists still prefer this last theory even though no hard evidence supports it.
The Cretaceous extinction was especially severe among sea creatures; in fact, they were more vulnerable than animals on land. Ammonites, so well known to fossil collectors as shelled, jet—propelled relatives of the octopus and squid, vanished from the sea along with sea monsters such as the pleisiosaurs, the mosasaurs, and the ichthyosaurs. The simultaneous termination of a wide variety of life on land and sea devalues any explanations peculiar to the dinosaurs. Even if the newly evolved caterpillars destroyed vegetation as thoroughly as do locusts, or some ratlike mammals developed a hearty appetite for dinosaur eggs, or a drastic disease struck, as several theories suggest, none would affect life beyond the beaches. Another recent theory about the dinosaur's demise targets the rise of the modern flowering plants. This involuntary change in diet deprived the dinosaurs of the laxative oils present in the older conifers, ferns, and cycads, so they all perished of constipation—no small matter for a dinosaur.
Investigators have found that teeming microspecimens of plankton, constituting the pastures of the sea, disappeared in a half—inch of sediment just at the end of the Cretaceous period; the event that ended the Cretaceous evidently was quick and catastrophic. This dismisses all hypotheses proposing a gradual change in climate, ecology, or even dinosaurian bowel habits.
During the late 1970s, Walter Alvarez, a geologist from the University of California, Berkeley, was conducting field research on late—Mesozoic rock layers exposed near the town of Gubbio, Italy. He and his associates were concerned with periodic changes in the earth's magnetic poles. Such paleomagnetism was recorded in the rocks from which he was collecting samples for laboratory research. Out of a brown—green clay that seemed to be advertising that it had something to tell them, Walter cut a sample and took it back to Berkeley to show to his father, Luis Alvarez, a physicist at Berkeley. Luis Alvarez suggested measuring the amount of iridium in the clay to find out whether the clay had been deposited over a short period or had accumulated gradually.
Iridium is an extremely rare element in the earth's outer crust. What little iridium could be measured in the clay layer would probably have come from meteorites, which along with comets are rich in iridium. The amount of iridium in any layer provides an estimate of how quickly the layer accumulated.
The result came as a shock: the amount of iridium in the Gubbio clay was 30 times the amount in the adjacent limestone. This clearly indicated that the earth was suddenly deluged with iridium. Even more startling results came from the same clay layer in other locales. One Danish specimen showed the amount of iridium increasing 160—fold, and a Belgian specimen reached levels 460 times normal.
After puzzling over these extraordinary results for several weeks, Luis Alvarez and his colleagues reached a most startling conclusion: the iridium might have come from a single, very large meteorite striking the earth. They estimated that an asteroid or a comet would have to be 6 to 12 miles in diameter to supply the almost 200,000 tons of iridium found in the clay layer.
Scientists know that the speed of an approaching asteroid would be about 150,000 miles per hour. As it ripped through the atmosphere and impacted the earth, it created tremendous pressures and temperatures, as much as 1 million times atmospheric pressure and several thousand degrees. The extreme violence of the collision completely vaporized the asteroid and about 10 times its mass of target rock, and the material sprayed outward from the expanding crater at 50 times the speed of sound. The blast, which released a thousand times the energy of the world's nuclear arsenals, punched a broad hole several miles deep in the crust and mantle. The fireball created by the impact expanded upward, carrying some 400 cubic miles of debris high above the atmosphere. (The 1980 eruption of Mount St. Helens ejected less than a third of a cubic mile of ash into the atmosphere.) Much of the finer material from the asteroid impact drifted in the stratosphere for years before it eventually settled to the ground, coating the entire globe with a thin layer of iridium—rich dust. The quantity of dust from an asteroid 10 miles in diameter would have blotted out the sun completely, and for a number of years there would have been unending night.
Without sunlight, plants would stop growing, and the ensuing famine would explain the disappearance among the fossil species. Microscopic plants in the sea died out almost completely and consigned to oblivion about 80 percent of marine life. The survivors were presumably those that could scavenge in the mud for the remains of plants of former years. The putrefying remains of plants and animals floating down the streams must have sustained some freshwater animal life, including the sole survivor of the great reptiles, the crocodile.
Blundering about in pitch darkness, the plant—eating dinosaurs stripped every last leaf from bushes and trees. The meat eaters made a regal banquet of their herbivorous cousins and then, enraged by hunger, turned cannibal. A few small animals, such as worms, insects, and birds, survived the long night by feeding on decaying vegetation, seeds, nuts, and one another. Among them were some of the diminutive mammals that lived mostly underground. When the sunlight finally broke through again, bathing the land with brilliant energy, these small mammals founded the new dynasties in the noncompetitive, depopulated world.
The Berkeley team published its hypothesis in 1980 but stressed that it was unproved by hard evidence. Many scientists opposed the impact catastrophe theory, probably because changes in the earth are expected to happen slowly. The first severe test for the idea was soon fulfilled successfully. If dust was scattered all around the world by the impact, then the extraterrestrial iridium should show up on the other side of the globe, as far away as possible from Europe and North Africa. In a short time samples were obtained from a site near Canterbury, New Zealand. To date well over 150 sites have been shown to contain the extraterrestrial—impact clay layer that marks the K—T boundary.
Evidence began to pile up, and by the early 1990s a collision of an enormous extraterrestrial body with the earth 65 million years ago was hard to refute. The location of the impact crater is now known, and the sequence of events set in motion by the colossal collision has become established.
What evidence supports the theory of an extraterrestrial collision? Some has been described in detail: the pencil—thin layer of meteoric iridium, found worldwide. One argument, that the excess iridium could have come from unusually massive volcanic outbursts, was quickly discarded. Further studies of the clay layer turned up more evidence, a form of mineral called "shocked quartz." Quartz crystals occasionally display a series of parallel lines on the face of the mineral as evidence of microscopic fractures. This occurs only when quartz is subjected to sudden extremely high pressures from an enormous explosion. This shocked quartz is found in debris of all major meteorite impact craters but not in volcanic debris.
Additional evidence common in the clay layer was tiny glassy spherules called "tektites," which are formed when molten droplets of melted rock shoot out from an explosion and resolidify into glassy beads. The abundance of shocked quartz and tektites occurring together in the iridium layers confirms that the K—T boundary included debris from a mighty explosion, consistent with an asteroid impact.
In the mid—1980s, scientists discovered that one of the components found in the layer all over the world was soot. To generate the amount of carbon in this soot layer, more forests would have to burn down than exist in the world today! Scientists conclude that most of the world's forests burned at the end of the Cretaceous, strong evidence of an event much different from assorted volcanic eruptions.
Some paleontologists believed that the dinosaurs had peaked and were on their way to extinction before the impact. In a 1989—91 field study of the Hell Creek Formation in the Upper Great Plains, an exceptional yield of late Cretaceous fossils showed that dinosaur extinction was synchronized with the impact. The study of 4,100 bones collected in the area indicated that ecological diversity had remained constant, with no evidence of decline in numbers or species.
Several groups of scientists studied the possible effects of a tsunami. They calculated that, if the impact had happened in an ocean, it would have created a splash more than three miles high near the impact site. Coastal areas adjacent to the site would have been inundated by a massive wall of water hundreds of feet high. In some regions the K—T boundary contained chaotic tumbled rocks, telltale deposits of a tsunami. The discovery of rocks of both continental and deep sea origin in the tsunami deposits provided evidence that the asteroid hit near a continental margin. Evidence of a tsunami was found on the perimeter of the Gulf of Mexico, following the shoreline that existed 65 million years ago. The absence of tsunami deposits elsewhere in the world helped to confirm a Central American impact.
The most awesome effect was proposed by an expert on cratering. Using computer models he calculated that much debris was thrown out of the atmosphere into space, only to fall back again. This created a concentrated rain of meteors streaking through the atmosphere for a few hours. It must have seemed that the sky was falling. The sky was ablaze with meteors, and the radiant energy level on the ground was like that inside an oven. Many above—ground animals must have been broiled to death where they stood. This apparently happened worldwide, and would explain why forests everywhere burst into flame.
Spectacular as the coastal devastation and fires may have been, each was a short—term effect. Surviving plants and animals encountered two longer—range problems. First, the shock of energy from the impact acted to convert large amounts of the atmosphere into nitrogen oxides, thus bringing on acid rain. In addition to other discomforts, survivors had to contend with the sort of pollution that has damaged forests in industrial North America and Europe, but at much higher concentrations. Many scientists believe that acid rain may have been a major agent in the extinctions.
The second long—term effect may have been more visually spectacular. Scientific teams have calculated that the enormous amount of debris ejected into the atmosphere included a high—altitude dust pall that fell back into the atmosphere within hours, dense clouds of dust carried away from the impact site by winds, and a global pall of smoke from the fires. Scientists estimate that about two pounds of dust was initially suspended above every square foot of Earth's surface. Within a week, Earth was densely clouded by dust and smoke; day turned to night.
The dust pall's most astonishing effect was to block sunlight. For one or two months, it was too dark to see; for two or three months, it was as dark as full moonlight; and for as much as a year, it was too dark for most plants to perform photosynthesis. The period of darkness destroyed the light—dependent plankton that float in the surface layers of the ocean, which would have had a rapid, drastic effect on marine life in general. The fossil record confirms that most life in the shallow seas was wiped out abruptly.
But where exactly was the smoking gun, the impact crater left by the deadly missile from outer space as it struck the earth? In 1978, Glen Penfield, a geologist with the Mexican national oil company, discovered a subterranean feature that had the characteristics of an impact crater. He pieced together material from magnetic surveys and gravity maps of the Yucatán peninsula and found a magnificent 100—mile bull's—eye hidden beneath the peninsula, with the town of Chicxulub, Mexico, at its center.
Dr. Alan Hildebrand of the University of Arizona, who was actively engaged in the search for the K—T impact crater, became aware of Penfield's find and joined forces with him. His search for more evidence led him to exposures of the K—T boundary 600 miles away, in southern Haiti. Green—brown clay impregnated with excess iridium, shocked quartz, and tektites was a signature of a giant asteroid, and coarse, jumbled rock fragments and debris scoured from one spot and dropped elsewhere were evidence of seismic sea waves hundreds of feet high.
News of Chicxulub spread to the Alvarez team of scientists, who searched in northeastern Mexico for more evidence of the crash. They discovered layers of undisturbed rock covered with deposits that were laid down violently, suggesting a monster wave. The impact scene was capped by beds of fine sediments of dust particles that were returning to earth, laced with iridium, after being lofted into the air by the impact. This was a recorded history of the area before, during, and after the giant asteroid had struck.
Radiometric dating of outcrops in Haiti and Mexico showed the age of 65 million years, the same as the well cores of Chicxulub; now enough evidence was gathered to re—create the impact. Within seconds after a space rock smashed to earth at 150,000 miles per hour, the crater, a hole 186 miles in diameter (the size of Connecticut), was blasted out to a depth of six miles. The energy of the impact created instant temperatures of over 20,000 degrees Fahrenheit, sent gigantic tidal waves roaring across the ocean, and triggered magnitude—12 earthquakes hundreds of miles away. Within hours dust began to form in the atmosphere, thickening into a cloud cover that completely enveloped the planet. Day became perennial night, and from space the veiled earth would have resembled the planet Venus.
So many ways to die! The life—forms of earth had many choices, all of them fatal. Scientists have recently added a new piece of evidence that spelled doom for the dinosaurs. The great agent of destruction was a global haze of sulfuric acid that blocked sunlight and plunged the planet into a dark, killing chill that lasted for decades. The new theory stems from recent geologic studies of the buried crater Chicxulub, in Mexico, which is believed to be the impact site. Finding the rock in the crater to be unusually rich in sulfur, scientists concluded that if the sulfur had not been added to the atmosphere, the dinosaurs might well have survived the impact that changed the course of evolution.
Scientists further estimated that the collision of an asteroid 6 to 12 miles in diameter would have vaporized much of the sulfur and spewed more than 100 billion tons into the air. This would have filled the air with sulfur dioxide, casting a pall that reeked of the devil's own brimstone, and a sulfuric acid haze in the upper atmosphere as the result of an interaction between sulfur and ultraviolet radiation. The release of sulfur caused by the impact would have been enough to plunge the planet into a perennial night lasting for decades and to shroud it in light—inhibiting sulfuric acid clouds.
The dust and soot from most of the debris would have drifted back to earth within six months, presumably too short a time for any global darkness to have caused mass extinctions. But the sulfur, being lighter, stayed aloft and created a dense and durable haze that covered the entire planet for several decades. According to most scientists, droplets of sulfuric acid would have filtered out sunlight by as much as 20 percent, producing a nuclear winter state 40 to 100 years long. Such a dramatic change in climate persisting over decades subjected organisms all over the world to long—term stresses. Few earth inhabitants could adapt in so short a period of time. The demise of the dinosaurs was probably complete in less than 100 years.
Such an enormous concentration of sulfur was present in less than 5 percent of the earth's crust. Had the asteroid struck almost any other place on the planet, it would not have generated the tremendous amount of sulfur spewed into the atmosphere.
Beneath that layer of iridium—laced clay the dinosaurs thrived; above it there were none. One can look on the clay layer as the lid on the "coffin for the dinosaurs."