Death of the Giant Reptiles

LL THE magnificent animals that I have been writing about became extinct at the end of the Cretaceous period, 65 million years ago. The dinosaurs died out, leaving the birds (which seem to have evolved from them) as their only descendants. The pterosaurs, ichthyosaurs, mosasaurs, and plesiosaurs died out leaving no descendants. Many invertebrate groups became extinct including the ammonites, marine mollusks with coiled shells which are extremely common fossils in Mesozoic rocks. The extinctions were devastating, but there were also a lot of survivals. The mammals (which had all been small in the time of the dinosaurs) survived well, and so did the land plants. Most of the main groups of lizards (other than mosasaurs), snakes, turtles and crocodiles also survived. What killed the dinosaurs and left the crocodiles?

There have been a lot of suggestions about why the dinosaurs went extinct, some of them distinctly far-fetched. Two hypotheses are strongly supported at present, by different groups of scientists. One says that the earth was hit by a collossal meteorite (a lump of material from outer space) and the other says that there was a period of violent volcanic activity.

The meteorite hypothesis started with a strange observation. The latest Cretaceous rocks in central Italy and the earliest rocks of the next period (the Tertiary) are both limestones. Each contains fossil shells of foraminiferans (microscopic marine animals) typical of its period. Between these limestone layers is a layer of clay, two centimeters thick, with no fossils in it. A team of scientists led by Drs. Luis and Walter Alvarez (father and son) analyzed this clay using neutron activation analysis, a technique that can measure tiny quantities of rare elements. They found remarkably high concentrations of iridium, one of the platinum group of metals.

When I say high, I mean 9 parts per billion (9 parts in 10y: I am using the American billion, not the larger British one). That may seem too little to get excited about, but it is 30 times higher than in the limestone immediately below or a short distance above (figure 10.1). Iridium concentrations are generally exceedingly low in the earth's crust but much higher in meteorites, typically 500 parts per billion. Could the iridium in the clay have come from a meteorite?

After the Italian rocks had been analyzed, samples of rock were taken from other places, scattered around the world, where Cretaceous and Tertiary deposits meet with no apparent interruption. These were analyzed in the same way, and high iridium concentrations were found in them all. The iridium layer seemed to be everywhere.

A meteorite hitting the earth might explode, scattering iridium-rich

Graphs Iridium

Iridium content (parts per billion)

FIGURE 10.1. A graph showing iridium concentrations in the clay at the Cretaceous-Tertiary boundary at Gubbio, Italy, and in the limestones immediately above and below it. The calcium carbonate was dissolved out with acid before the analyses were carried out, and the concentrations refer to the acid-insoluble residue. The data are from L. W. Alvarez et al., Science (1 980), 2 0 8:1 095 - 1 1 08.

Iridium content (parts per billion)

FIGURE 10.1. A graph showing iridium concentrations in the clay at the Cretaceous-Tertiary boundary at Gubbio, Italy, and in the limestones immediately above and below it. The calcium carbonate was dissolved out with acid before the analyses were carried out, and the concentrations refer to the acid-insoluble residue. The data are from L. W. Alvarez et al., Science (1 980), 2 0 8:1 095 - 1 1 08.

dust, but any ordinary explosion would scatter material only over a restricted area. Here we have material scattered all over the world. The Alvarez team suggested that a huge meteorite might have disintegrated in a collossal explosion, throwing dust many kilometers up into the atmosphere. If this dust were fine enough it would be slow to settle, and might get scattered all over the earth.

It is quite easy to calculate how big the meteorite would have had to be, to scatter so much iridium. First we need to know how much extra iridium there is, above what would be expected in the same thickness of ordinary rock. Analyses from twenty-one widely scattered places give an average of 0.6 milligrams of extra iridium per square meter of the earth's surface. The area of the earth is 5 x 1014 (500 million million] square meters, so the total amount of iridium can be estimated as 0.6 x 5 x 1014 = 3 x 1014 milligrams or 300,000 tonnes. The meteorite would probably have contained about 500 parts per billion of iridium (1 part in 2 million), so we must multiply the 300,000 tonnes by 2 million to get an estimate for the total mass of the meteorite, 600 billion tonnes.

Typical meteorites have densities of about 2.2 tonnes per cubic meter (which is rather lower than most other rocks) so a 600-billion-tonne one would have a volume of 270 billion cubic meters. A sphere of that volume would have a diameter of 9 kilometers. It would be similar in size to Manhattan Island (which is about 20 kilometers long and 4 kilometers wide).

It may seem far-fetched to imagine such a thing hitting the earth, but it is not too improbable. As well as planets orbiting the sun, there are a lot of smaller bodies called asteroids, a few kilometers in diameter. The meteorites that have been observed landing on earth seem to be fragments from collisions between asteroids, but there is a constant danger of whole asteroids hitting us. It has been estimated from telescope observations that there are about a thousand, with diameters of a kilometer or more, whose orbits take them inside the earth's orbit at times and outside it at others. None of these asteroids have collided with the earth in historic times (so far we have been lucky), but eventually some will. A few craters have been found that are believed to have been made in the distant past by small asteroids, and it has been calculated that the earth is likely to be hit by an asteroid of 10 kilometers or more diameter about once every 100 million years. That is very seldom, but the event we are trying to explain happened just once, 65 million years ago.

The earth, traveling its orbit round the sun, is hurtling through space at 30 kilometers per second (one hundred times the speed of sound in air). Asteroids travel round the sun in the same direction, so there will be no head-on collisions. Figure 10.2 shows how the earth, traveling its near-circular orbit, might be hit by an asteroid with a more elliptical orbit. Astronomers tell us that the speed of an approaching asteroid, relative to the earth, would probably be something like 20 kilometers per second.

We will calculate the energy of an impact at this speed. This is the kinetic energy of the asteroid, due to its movement relative to the earth. Kinetic energy is j (mass) x (speed)2, but in using the formula we must be careful about units. Six hundred billion tonnes is 600 million million kilograms. Twenty kilometers per second is 20,000 meters per second. If we put into the formula the mass in kilograms and the speed in meters per second we get the energy in joules; it is about 101' (100,000 million million million) joules. This is enormous—equivalent to the explosion of 60 million megatonnes of TNT. The atomic bombs dropped in Japan had energies of only 0.02 megatonnes each. The biggest explosion of modern times, the explosion of the island volcano Krakatoa in 1883, had about 200 megatonnes energy.

My calculation is very rough because the speed that I used in it could be badly wrong, but it seems clear that the landing of the asteroid would have been incomparably more devastating than anything people have ever experienced. The energy would have been amply sufficient to vaporize the asteroid. Twenty million joules are needed to vaporize a kilogram of rock so 600 million million kilograms could be vaporized by

FIGURE 10.2. Collision between the earth and an asteroid. Not to scale.

12,000 million million million joules (1.2 x 1022 joules). This is only one-eighth of the estimated kinetic energy.

If the asteroid had hit dry land it would have made an enormous hole, but no crater that could have been made by it has been found. This need not worry us: it would be more likely to land in the oceans that cover 70 percent of the earth's surface. If it did fall in an ocean it would make less of a crater, and even a big crater in the ocean floor would be hard to find.

A lot of the energy of the asteroid would be transmitted to the ocean water, turning some of it to high-pressure steam, but there would be enough left to vaporize most of the meteorite. There would be a terrific explosion that would blast a column of steam and rock vapour high into the atmosphere where they would condense out as tiny ice crystals and dust particles. The dust would sink down again onto the earth but the rate of settling would depend on the size of the particles: it would probably take a few months.

Now we will think about how the catastrophe could have affected dinosaurs and other animals. First, the dust in the atmosphere would have blotted out the sun. Sunlight all over the world is dimmed by dust after major volcanic eruptions, and would be dimmed far more by the catastrophe we are imagining. It has been estimated that after the Krakatoa eruption, sunlight was dimmed by 3 percent to 0.97 of its normal intensity. Twice as much dust would dim it to (0.97)2 times the usual intensity, three times as much to (0.97)' and so on. If the asteroid threw up 200 times as much dust as Krakatoa (and the ratio of energies suggests it would have thrown up far more than that) sunlight would be dimmed to (0.97)2"" - 0.002 times its normal intensity: the world would have been plunged in darkness.

Calculations of the size of the dust particles and of the rate at which they would settle suggest that the darkness would have lasted for several months. Plants would have suffered because they depend on the energy of sunlight to make foodstuffs by the process of photosynthesis. Many land plants would probably have survived a few months darkness (remember that deciduous trees lose their leaves each fall and have to survive a few months without photosynthesis), and other land plants would probably have survived as seeds. However, the microscopic plants that float as plankton in lakes and seas would probably have suffered badly, because they are too small to have substantial food reserves. Almost all the animals in lakes and seas get their energy ultimately from these microscopic plants: tiny crustaceans and other animal plankton eat the plants and are in turn eaten by fish and other larger animals. Life in lakes and seas would suffer very badly.

A second effect would result from the blotting out of sunlight: the earth's surface would get cold. The effect on the oceans would not be great because of their enormous heat capacity, but there would be severe frosts on land that might be lethal to many plants and animals.

A third possible effect would be acid rain. The high temperatures of the explosion would make some of the nitrogen in the atmosphere combine with oxygen to form nitrogen oxides. These would react with water and more oxygen to form nitric acid, which would fall from the atmosphere in rain.

Acid rain due to very different causes is a serious modern problem. Nitrogen oxides are formed in the engines of motor vehicles and released into the atmosphere with the exhaust. Sulphur dioxide is emitted by coal-burning power stations. The nitrogen and sulphur oxides react with oxygen and water to form nitric and sulphuric acids, which fall in rain. The acid rain falling on trees makes leaves yellow and fall off. When it drains into lakes it makes them acid, sometimes too acid for fish to survive. Many forests in industrial countries are in poor health and many lakes have lost their stocks of fish. The acid rain after the asteroid explosion would have had similar effects.

The asteroid explosion would have had disastrous effects on many kinds of animals and plants. Nevertheless, the asteroid hypothesis for the extinctions at the end of the Cretaceous has several problems. One is that the high iridium concentrations are not limited to very thin layers of rock as would be expected if they had been formed by dust settling in the few months after the explosion. Instead, they extend through thicknesses of 30 to 100 centimeters, that probably took several tens of thousands of years to form. Indeed, some American samples show several iridium-rich layers sandwiched between iridium-poor ones. Another problem is that the extinctions do not seem to have happened all at once. The numbers of ammonite and dinosaur species seem to have declined gradually during the last few million years of the Cretaceous, and the last North American dinosaurs are in rocks above the iridium-rich layer, formed 40,000 years after it.

These observations seem to favor the volcanic hypothesis, which postulates a few tens of thousands of years of intense volcanic activity. Volcanoes throw up material from deep inside the earth, where iridium concentrations are much higher than in surface rocks, though lower than in meteorites. The iridium-rich layers may have come from volcanoes.

Major eruptions throw enough dust into the upper atmosphere to dim sunlight perceptibly, but eruptions big enough to black out the sun and stop photosynthesis seem unlikely. Volcanoes seem more likely to cause extinctions by way of acid rain. They emit sulphur dioxide and other gases as well as molten rock. The sulphur dioxide (like the sulphur dioxide from power stations) eventually falls in rain as sulphuric acid. Scientists have analyzed emissions from a Hawaiian volcano, measuring the quantities of sulphur dioxide and iridium. Their measurements suggest that if the 300,000 tonnes of iridium in the irid-ium-rich layers came from volcanoes, they would have been accompanied by about 10 million million tonnes of sulphur dioxide. If this was emitted over a period of 10,000 to 100,000 years, the average rate of sulphur dioxide emission would have been between 100 million and 1 billion tonnes per year. The rate would probably vary, with peak rates far above average.

The surphur dioxide, released annually in the United States and Europe by burning coal and oil, totals 80 million tonnes, and the nitrogen oxide emissions are less. I have already described the damage that these emissions are causing, through acid rain. The acid rain in the worst parts of the supposed period of volcanic activity would have been far worse.

It would have been so much worse that the oceans would have been seriously affected, as well as lakes. At present the oceans are slightly alkaline with a pH value of 8.2, but the acid from 10 million million tonnes of sulphur dioxide would reduce the pH to about 7.4. This is still very slightly on the alkaline side of neutrality (pH7 is neutral and anything less is acid), but it is not alkaline enough for foraminiferans. It is also near the limit for another group of microscopic plankton, the coccoliths.

Foraminiferans have calcium carbonate shells and coccoliths have plates of calcium carbonate on their outer surfaces, and calcium carbonate dissolves in acid. Foraminiferans need a pH of 7.6 or more and coccoliths need at least 7.0-7.3. Both groups suffered many extinctions at the end of the Cretaceous but dinoflagellates and other groups of plankton, without calcium carbonate skeletons, survived better.

Ozone is a form of oxygen that is rare at ground level but relatively more plentiful in a layer high in the atmosphere. It absorbs much of the ultraviolet radiation from the sun, protecting living things from these harmful rays. The ozone layer is depleted after major eruptions because volcanoes inject a little hydrochloric acid into the atmosphere, as well as the much larger quantity of sulphuric acid. The hydrochloric acid reacts with the ozone in the ozone layer to form chlorine, water, and ordinary oxygen. It has been calculated that 8 percent of the ozone was destroyed after the Krakatoa eruption. The many eruptions that are supposed to have happened at the end of the Cretaceous would have nearly destroyed the ozone layer, leaving animals and plants exposed to abnormally high doses of ultraviolet radiation. This might have been fatal for many of them, but the mammals of the time were small and would have survived if they spent their days in burrows and were active mainly at night.

The volcanic hypothesis seems quite attractive but there is at least one observation that it seems unable to explain. Damaged sand grains have been found wherever they have been looked for, in samples from the iridium-rich layer from various places, scattered around the world. Sand grains are small quartz crystals, made of neatly stacked layers of atoms. The damaged ones look cracked (when examined under the microscope) because some of their layers of atoms have been thrown into disarray. Similar damage is found in sand grains from meteorite craters and is believed to have been caused by shock waves. The volcanic hypothesis seems unable to explain shocked quartz being scattered widely round the world.

Both hypotheses seem reasonably plausible. Collision of an asteroid with the earth, and a prolonged period of fierce volcanic activity, would each have had dire consequences, and would probably have caused widespread extinctions. The volcanic hypothesis is possibly the better of the two, in explaining why the extinctions were so selective: acid rain would kill foraminiferans but not dinoflagellates, and ultraviolet radiation would be more damaging for diurnal dinosaurs than for nocturnal mammals. However, the asteroid hypothesis seems better able to explain the shocked quartz. The supporters of the two hypotheses are still arguing fiercely, and it is quite possible that neither is right. Indeed, it has recently been suggested that the extinctions were due to another cause, a shower of comets hitting the earth. The effects of comet impacts would be much like those of asteroid impacts: comets are massive bodies travelling at exceedingly high speeds, and contain more iridium than the surface rocks of the earth. A series of comet impacts could explain extinctions spaced out in time, and there are theoretical reasons for expecting comets to come in showers lasting about a million years. The implications of the comet shower hypothesis have not yet been worked out in as much detail as those of the asteroid and volcano hypotheses. It remains to be seen which (if any) of the three hypotheses triumphs.

Principal Sources

The meteorite hypotheses was put forward by Alvarez ct al. (19801 and discussed in much more detail by Silver and Schultz (1982). The volcano hypothesis has been presented by Officer et al. (1987). Sloan et al. (1986) published evidence that the dinosaurs declined gradually. Bohor, Modreski, and Foord (1987) presented the shocked quartz evidence. Hut and others (1987) presented the comet shower hypothesis.

Alvarez, L. W. W. Alvarez, F. Asaro, and H. V. Michel. 1980. Extraterrestrial cause for the Cretaceous-Tertiary extinction. Science 208:1095-1 108.

Bohor, B. F., P. ). Modreski, and E. E. Foord. 1987. Shocked quartz in the Cretaceous-Tertiary boundary clays: Evidence for a global distribution. Science 236:705-709.

Hut, P. and others (1987). Comet showers as a cause of mass extinctions. Nature 329:118-126.

Officer, C. B., A. Hallam, C. L. Drake, and J. D. Devinc. 1987. Late Cretaceous and paroxysmal Cretaceous/Tertiary extinctions. Nature 326:143-149.

Silver, L. T. and P. H. Schultz. eds. 1982. Geological implications of impacts of large asteroids and comets on the earth. Geological Society of America Special Paper 190:1-528.

Sloan, R. E., J. K. Rigby, L. M. Van Valen, and D. Gabriel. 1986. Gradual dinosaur extinction and simultaneous ungulate radiation in the Hell Creek Formation. Science 232:629-633.

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  • ALEX
    How could volcanoes create an iridium rich layer?
    8 years ago

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