Chicxulub Crater and the Death of the Dinosaurs

Until just twenty years ago or thereabouts, paleontologists and dinosaur hunters had to grapple with a huge, gaping hole in their understanding. Roughly 65 million years ago, there was what is classified as a "mass extinction event". In nature, species come and go in relative monotonous frequency; even the most successful creatures may only persist for a few million years. But mass extinctions are taking natural selection to the extreme; in these situations, massive changes cause corresponding mass dieoffs of species.

Dinosaurs, in fact, were the benefactors of an extinction event. At the end of the Permian period, 251 million years ago, was Earth's largest extinction event-96% of marine species were wiped off the face of the Earth, along with nearly 70% of the species on land (1). The dieoff ended the reign of mammal-like reptiles, and ushed in a new era of geologic time-the Mesozoic. The Mesozoic is divided into three periods: the Triassic, Jurassic, and Cretaceous. In the Triassic nature recovered from the Permian-Triassic extinction and a new breed of reptiles, archosaurs, became the dominant land creatures. Dinosaurs also appeared, and while they were smaller then the archosaurs, another extinction event at the end of the Triassic 205 million years ago [mya] finished off their competitors. The rest of the Mesozoic would be dominated by the dinosaurs, who proliferated into many species of all shapes and sizes. Every child knows about their reign.

But then, in a flash of geologic time, the dinosaurs vanished, ushering in the Tertiary or Paleocene period and a new era, the Cenozoic-dominated not by reptiles, but by mammals (and eventually, mankind.) What could kill off the most successful land animals of all time so thoroughly?

For years, scientists had come up with different theories. One was that shifting plate tectonics caused widespread volcanic activity. For example, the largest volcanic feature on the earth are the Deccan Traps near present-day India. At its height, the feature covered half the square mileage of the subcontinent. Such eruptions could have caused ecological imbalances. Another theory was that dinosaurs migrating over newly-formed land bridges spread deadly plague to each other (2).

But the prevailing theories about dinosaur extinctions would soon give way to a new theory: that the reign of great reptiles was brought to a crushing end by a chance impact of a massive rock from space-an asteroid.

How did this theory come about? It's an interesting story.

In the 1970s, scientist Walter Alvarez was doing research in Italy and came across the K-T boundary-a specific mark in the rock that divides the Cretaceous and Tertiary periods. Alvarez removed a piece from the boundary layer and noticed a strange green clay dividing the periods. He and his father, Nobel-prize winning Luis Walter Alvarez, decided to investigate. In order to figure out how quickly the clay layer was put down (thus, how long the extinction event may have occurred), Alvarez Sr. decided to use iridium as a benchmark. Iridium is rare on planets but highly concentrated in asteroids. Over time, micro-meteors would burn up in Earth's atmosphere, almost dusting the planet with these extraterrestrial iridium particles. Thanks to his connections at the Lawrence Berkeley National Laboratory in California, Alvarez made an astounding discovery-the iridium content in the clay layer (but not the surrounding rock) was enormous, too much to have come from anything save an asteroid (3). Their evidence, published in the early 1980s, sparked enormous debate and some backlash from segments of the scientific community. One valid criticism of the theory was that there was no crater-akin to police not having the murder weapon. For such a massive amount of iridium, there needed to be a correspondingly massive crater of the right size and age. Other scientists, such as Alan Hildebrand, continued research into furthering the Alvarez hypothesis, and found additional evidence (such as "shocked quartz" and other compounds only created through high pressure and found at impact sites) in the K-T boundary. But still, a crater was nowhere to be found.

Rewind time back to 1978, when geophysicist Glen Penfield worked for the Mexican-run oil company Pemex. Penfield used geologic to make informed guesses as to where oil deposits might be buried. Within the geophysical data Penfield made a remarkable discovery-a massive ring that caused gravatic anomalies in its wake. Penfield was not the first scientist to notice this ring, on and off the coast of Mexico's Yucatán Peninsula, but to him the data he discovered suggested a massive cataclysmic event that left a circle 180 kilometers wide. Penfield presented his findings at the 1981 meeting of Society of Exploration Geophysicists (ironically, it was under-attended and the experts in impact features were attending another conference on Earth-meteor impacts.) Part of Penfield's problem was that he had no evidence in the form of actual rocks from the feature. While Pemex had dug exploratory wells in the region in the early 1950s, Penfield was told those samples had been destroyed, and his own expeditions revealed nothing. He wrote up his findings and returned to work looking for oil (2).

In 1990, a reporter informed Hildebrand about Penfield's findings. Hildebrand contacted Penfield, and the two managed to discover that the lost Pemex samples were actually in a building in New Orleans, Louisiana. Penfield sent the rocks to Hildebrand, who confirmed telltale impact elements (such as the shocked quartz and volcanic glass) within the rock (2). Penfield had discovered a massive crater; the actual crater wall is now estimated at over 300 kilometers across (4). Finally, scientists had their smoking gun as proof-the Chicxulub crater, named after the town close to its epicenter.

Why would a meteor impact, even one as large as the Chicxulub feature, be enough to cause the death of the dinosaurs? The impact itself was massive-the energy released was equivalent to 100,000,000 megatons of dynamite (5). The impact would have created a shockwave culminating in a megatsunami thousands of feet high; debris was scattered for hundreds of miles. Superhot material shot into the atmosphere would return to earth as fiery cinder, causing global forest fires and devastation, while the seismic shocks triggered earthquakes and volcanoes.

As bad as the immediate impact would have been (and as deleterious to the living things on the planet as just the immediate aftermath would have been,) the longer-term effects were even worse. Carbon dioxide locked up in the rocks would have been shot into the air on impact, creating a sudden greenhouse or warming effect all over the planet. Over time, the dust and ash ejected into the atmosphere would cover the entire planet. This would block sunlight, in turn causing a dramatic drop in temperature. The photosynthetic cycle would shut down after too long without sunlight; this in term would cause the food chain to break down and the massive dieoffs of organisms. As if that wasn't enough, the material in the air would also create poisonous acid rain (2).

There is no doubt that the Chicxulub impact was a horrific event... but was it the actual cause for the death of the dinosaurs? Some argue that the impact was too early to have been the casual factor-it is hard to measure a few thousands of years difference in geologic time, let alone a few decades. Another problem with the impact theory is what was killed. Acid rain, global cooling, and the poisoning of water and estuaries should have been a death knell for amphibians like frogs (who are now disappearing due to our similar environmental damage to their homes.) So why weren't they driven to extinction (2)?

While the impact hypothesis is now one of the strongest and most widely-accepted theories about the extinction of the dinosaurs, other ideas remain valid, and its highly unlikely the question will be definitively answered in the near future. What is important to note, however, is that the death of the dinosaurs was the direct cause for the rise of mammals. Dinosaurs had their chance, and lost out in the cosmic game of craps to a bunch of small, furry varmints that would grow and evolve into the forms that now have dominion over the Earth. But that doesn't mean we're exempt from the same dangers. Consider this: large (>5km diameter) asteroids collide with Earth on average once every 10 million years-but the most recent recorded was the Chicxulub impact 65 million years ago (6). We're overdue for an impact. So pause for a moment and look up-you never know what's coming.

"Chicxulub Crater and the Death of the Dinosaurs" References
* (1) Baez, John (2006) "Extinction". Accessed 2009-12-05.
* (2) The Dinosaurs! (Parts 1-4). 1992, WHYY-TV (distributed by PBS Home Video).
* (3) Trower, W.P. (1987). Discovering Alvarez: Selected Works of Luis W.Alvarez with Commentary by His Students and Colleagues. (ISBN 0226813045).
* (4) Sharpton VL, Marin LE (1997). "The Cretaceous-Tertiary impact crater and the cosmic projectile that produced it". Annals of the New York Academy of Sciences (n822).
* (5) Bralower, Timothy J.; Charles K. Paull and R. Mark Leckie (April 1998). "The Cretaceous-Tertiary boundary cocktail: Chicxulub impact triggers margin collapse and extensive sediment gravity flows". Geology.
* (6) Chapman, Clark; David Morrison (1994) "Impacts on the Earth by asteroids and comets: assessing the hazard". Nature (367).

For more information and sources, I recommend consulting the materials referenced at the Wikipedia pages for: "Impact Event", "Chicxulub Crater" and "K-T Extinction".