A Guide to the Dinosaurs of Colorado

Dinosaur Extinction
The Giant Meteor Impact Theory

Working with his Nobel-prize winning physicist father, Luis Alvarez, at the Lawrence Livermore Laboratory at the University of California at Berkeley, he prepared the samples by dissolving out the calcium skeletons with acid. This left equal concentrations of nearly pure clay for each sample. The samples were then subjected to neutron-activation bombardment. Analysis of the resulting neutron decay produced the expected iridium decay signature. For samples taken from the chalk above and below the clay layer, the measured iridium concentration was around 0.3 parts per billion: about the expected concentration for cosmic fallout. Iridium concentrations within the clay layer itself, however, measured as high as 10 parts per billion, some 30 times higher than expected.

The Alvarezes then analyzed samples from a famous site in Denmark, known as Stevn's Klint, and found an iridium concentration even higher than the Gubbio samples: 65 ppb., some 200 times higher than expected. Other platinum-group rare earths known to occur in cosmic debris were also found to be similarly enriched. Similar "iridium spikes" have since been identified all over the world, wherever K/T boundary sediments have been identified.

As the signature of rare earths coincided with the composition of known stony meteorites, an extraterrestrial origin for the iridium was postulated. Thus, in 1980, the Alvarez team published in Science magazine: "Extraterrestrial causes for Cretaceous-Tertiary extinctions," wherein they proposed that the impact of a giant meteor or asteroid, on the order of 10 km in diameter, had caused the demise of the dinosaurs.

Additional evidence of a meteor impact was also discovered, in the form of 'microtektites,' small, spherical particles of molten ejecta with a distinctive fracture pattern. Microtektites are normally associated only with the most violent of explosions, such as occur when a giant meteor strikes the Earth. Microtektites have been found at many, but not all, of the boundary clay deposits in various parts of the world.

Several months of darkness would wreak havoc on the photosynthesizing nannoplankton, many of which have only a one month or less life span. Collapse of the ocean's algae communities would similarly devastate the zooplankton, and all the animals that feed on them. This would lead to a complete collapse of the oceanic food chain, leading to the demise of such diverse groups ad the ammonites and the mosasaurs and plesiosaurs.

On land, a protracted period of darkness would halt new production of plant growth, leading to the starvation of large herbivores and the carnivores that fed on them. Smaller animals such as the early mammals could probably hibernate through the dark period. Land plants, however, could regenerate from roots and/or seeds after the dust had cleared and normal daylight was restored.

Aside from these technical objections, several other assumptions of the theory have been challenged. Iridium and the other platinum-group rare earths are rare in the Earth's crust because they are siderophilic, or iron-loving. Siderophiles are thought to be much more common in the Earth's interior, which is composed largely of iron and its associated elements. But material from the Earth's interior is just as likely to be injected into the atmosphere as is extraterrestrial material -- from volcanic eruptions. Iridium in measurably concentrations has been detected from the eruptions of the volcano Kilauea in Hawaii. Now it is known that for about half a million years, spanning the Cretaceous/Tertiary boundary, one of the greatest volcanic eruptions of all time was going on, forming the Deccan Traps of western India. The Deccan Traps are basaltic deposits, as are the eruptions of Kilauea, and it is quite possible that volcanics could be the source of the Iridium Spike.

The Meteor Impact theory also fails to explain the perceived gradual die of of foraminiferans and dinosaurs. It postulates a very sudden dieoff, striking down whole lineages of organisms in their prime. Most paleontologists reject this claim. While most of the scientific community heartily embraces the theory, the majority of paleontologists reject it. Many paleontologists are willing to believe that a meteor impact could have occurred, but don't accept that it caused the extinctions. It could, they maintain, have been the last straw that finished off an already dying breed. Many question that a meteor impact ever occurred at all.

Also, the theory does not address the last condition for a "really good" extinction theory; that it explain ALL mass extinctions, not just the K/T event. Search though they might, no advocate of the meteor impact theory has been able to find an iridium spike associated with any other mass extinction event.

Another objection, not discussed in the previous chapter, concerns climate change. Granted that a meteor impact could cause severe climatic changes for a few months, a few years, or even for decades. But the climate of the Cretaceous was radically different than any climate that has occurred since. For one thing, there were no freezing temperatures -- at the poles, during the winter, anywhere, anytime (except, of course, at high elevations). Ocean bottom water, which now is a uniform 4°C (39°F), ran close to 17°C (63°F), because there were no polar ice caps to replenish the cold bottom water. Many tropical and some temperate plants today are totally intolerant of frost; freezing temperatures, even for only an hour in the middle of the night, can kill them totally. A number of such plants are survivors from the Cretaceous Period, and were found in high paleolatitudes that today would mean instant death. Alligators lived in such a subtropical flora at a paleolatitude of around 70°, roughly where Greenland sits today.

Global warming, caused by an increase in greenhouse gases, could conceivably elevate temperatures enough to produce subtropical climates at 70° latitude, but the same warming would elevate temperatures at the equator to such a temperature that little or no life could endure. Yet essentially the same kinds of flora and fauna thrived at the equator as in polar regions. In other words, the temperature gradient from tropics to poles that we consider an invariant condition of Earth's climate simply did not exist during the Cretaceous Period! It is difficult to imagine how a meteor impact, no matter how giant, could cause a climate change that lasted 65 million years.

This problem will be discussed further in the concluding chapter.

Next Chapter:
Other Dinosaur Extinction Theories