How are the Mighty Fallen
Contents Updated: Monday, September 13, 1999
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.
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.
Recently a crater has been found in the Yucatan peninsula and is considered a prime candidate.
A Universal Explanation
The Alverez team used the idea of a cosmic collision to encompass four previous explanations:
- suppression of photosynthesis—the global darkness of three to six months curtailed photosynthesis and led particularly to extinctions in the oceans;
- 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;
- an ice age—exclusion of the sun's radiation for many months would lead to a global cooling which would kill off many species;
- 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.
Some scientists were not convinced that the meteorite impact could trigger mass extinctions any more than a large volcanic eruption could. Would the debris thrown up stay aloft long enough to cause any lasting damage? After all many genera and individual species did survive showing that, despite darkness, dust and poison gases, conditions could not have been bad for too long. A fairly short period of darkness could explain the excessive extinction of water dwelling species relative to land types because the food reserves of the plankton in the sea is only sufficient to last for between ten and a hundred days, but land plants can suffer the absence of light for longer.
A period of darkness of three years, as the Alverezes supposed, would have destroyed most genera, perhaps all higher ones. Many species were not seriously affected—and many others, we know, had been suffering decline before the hypothetical collision. According to E.G.Kauffman of Colorado University, 75 per cent of marine organisms were in decline at the end of the Cretaceous period. They had been on the wane for two to five million years and few species seem to have died off at exactly the same time.
Ammonites had fluctuated in population previously. Some ammonite genera declined and expanded periodically while others seemed relatively steady. When extinctions had occurred before, the steady species had tended to survive while the fluctuating species had died off. In the upturn the steady species had radiated into available niches including those suiting the fluctuating species and the cycle continued. At the end of the Cretaceous the ammonites were in such decline that there were no reliable steady species to bring them through. What was different? What had killed off the steady species of ammonites?
Doubts
Dinosaurs were similarly on the wane and by the end of the Cretaceous survived in numbers only in the West of North America, having died out in South America and possibly Europe. Even in North America the decline was severe. Half of the 36 genera of dinosaurs alive about ten million years before the end of the Cretaceous had died out by the time the final million years was entered.
Robert Bakker is among those who do not support the catastrophe theories: he claims there is no doubt that dinosaurs did not die out in a geological instant but petered out over thousands if not millions of years. Plainly life was under stress. What was its cause?
Leigh Van Valen and Robert Sloane attributed the extinctions to climatic changes over the last five to ten million years of the Cretaceous. At the beginning of the period vegetation was prolific and typically tropical or sub-tropical: towards the end of the period the climate had become typically temperate with cool woodlands. Dinosaurs thrived in the warmer climate but in the cooler one mammals had the advantage. The proposed reason for the change in climate was that the ocean floor had lifted with renewed mid-ocean spreading and sea levels had risen. Shallow seas divided North America and also divided Europe from Asia. Ocean currents and wind patterns may therefore have altered.
On the other hand, Bakker argues a fall in sea level draining the shallow continental seas could have triggered the mass extinctions. This accounts for the loss of a lot of marine species, those preferring the continental margins and intra-continental seas obviously, but the loss of light warm water draining from them on to the ocean surfaces would also lead to the demise of many open sea species that could not adapt to the colder surface conditions. What though of the land vertebrates? Surely they would have had more lebensraum and should have multiplied. No. The linking of previously isolated continents by land bridges created conditions similar to those in the great Permian extinction—hypercompetition between species and the unchecked spread of disease and parasites in populations not adapted to be immune from them. Large active animals like the dinosaurs could migrate faster, therefore they experienced more competition and disease, and suffered most. Smaller creatures like the mammals could migrate, but more slowly, having more time to adapt and freshwater species, which seemed least affected, could not migrate, though they were affected to a lesser extent by pests or diseases carried in by the migrants.
Late in the Cretaceous some Asian genera of dinosaurs appeared in North America having crossed the Bering Straits (or whatever the dinosaurs might have called them) showing the two continents had linked. A more recent example was the land bridge which formed between North and South America, about 30 million years later than the dinosaurs, when many South American species failed to survive competition from invaders from the North.
Bakker puts particular emphasis on the spread of diseases unrestricted by hereditary immunity. Warm-blood is at the ideal temperature for bacteria and viruses to multiply. The warm-blooded dinosaurs would therefore be susceptible to the new pathogens being introduced to all the continents. He quotes examples like the spread of the Black Death and the carrying of rinderpest from India to Africa with devastating effects on the antelope herds. V.D. and smallpox devastated the Amerindians. Virulent strains of myxomitosis were deliberately developed by CSIRO in Australia and introduced into the wild to control rabbits. All have depended upon the intelligent mammal with its capacity for travel and his insensitivity towards other species.
Volcanic Activity?
Bakker contradicts his advocacy of the evolutionary resilience of the dinosaurs. The dinosaurs, he persuades us, are great competitors and the forming of land bridges, while leading to mass extinctions, also provides lots of empty niches for enterprising species to adapt to. If the theory is true at all, and it might be partly true, we need to know why the genetic variability of the dinosaurs had been reduced to such an extent that they could not cope with the new challenge as they had always done before.
Those doubtful of the Alverez's theory thought the iridium anomaly could be explained by differential sedimentation rates or volcanic activity. Further careful testing of the iridium layer showed that the iridium concentration seemed to build up slowly during the last few thousand years prior to the supposed cataclysm. Volcanic activity occurring over an extended period, they argue, would be more likely to match such a pattern. India had broken from Africa and raced (in geological terms) across the Indian Ocean to hit Asia. The collision pushed up the Himalayas and created such friction that lava spewed forth for centuries to form the Deccan Peninsula. This was the extended vulcanism they sought.
Calder noted that 65 million years ago in Western North America plants were dusted with exotic elements. Besides iridium, volcanoes emit other metals present in the K-T sediments not commonly present in meteors, like arsenic and antimony. For Charles Officer and Charles Drake of Dartmouth College this proved the boundary layer and the extinctions were related to the break up of the old continents. The level of the seas fell to their lowest for 200 million years. The warmth which gave subtropical conditions to northerly climes subsided. Widespread volcanic activity over a long period (but short on a geological timescale) led to pollution, climatic changes and ecological damage which destroyed species.
You will, by now, have noticed that some experts have postulated high sea levels and others low sea levels as reasons for the Cretaceous extinctions. You may well ask: Don't they know where the sea level was? Wasn't it where it usually is? Sea level provides a riddle of its own. Geologists, notably those working for oil companies have built up a detailed knowledge of changes in sea level over the ages. In the last 200 million years it reached its highest consistent level from about 85 to 67 million years ago when about twice the area of continental shelf presently inundated was flooded. In the preceding 100 million years sea levels had steadily risen due to the activity of the mid-oceanic ridges and the sea floor spreading associated with continental drift. The welling up of magma under the mid-oceanic ridges displaces the water of the oceans causing higher sea levels.
Sudden unexplained falls in sea level occur periodically but are rarely linked with meteorite falls. The sudden onset of an ice age freezing large amounts of water in extensive ice sheets might explain some recent sea level changes but not those in the Cretaceous. They also do not normally coincide with mass exterminations, throwing doubt on an idea like Bakker's. The sea level did fall rapidly about 67 million years ago, the event identifiable with Bakker's theory, but there was a greater fall in sea level 95 million years ago which is associated with only a minor turnover of species compared with the extinctions terminating the Cretaceous. Why did this earlier event not have the impact of the later one if Bakker's idea is correct?
Any convincing explanation of the extinctions has to account for all the genera that became extinct not just those that are representative of the dinosaurs. It also has to be sudden, at least on a geological timescale. The dinosaurs had shown that they were well able to adapt over 140 million years and were still evolving in the Cretaceous. A gradual change of conditions was unlikely therefore to overwhelm them—they would have adapted into the new conditions.
Though doubt is being cast on the asteroid impact, many of its anticipated effects like adverse climatic disturbance and pollution of the environment remain persuasive—but were these the shadow of an asteroid or did they come from closer to home? Today we see similar effects created by the intelligent mammal. Volcanoes and asteroid impacts do not have to be invoked to explain the environmental problems we are experiencing, or the mass extinction of species currently taking place. Similar things are happening today to events at the end of the Cretaceous.