What Really Killed the Dinosaurs?

An illustration of Tyrannosaurus rex trying to escape the fire and heat from a meteorite crash.

(Image credit: Elenarts / Shutterstock.com)

An extraordinary vessel—part ship and part drilling rig — is being equipped in the port of Progreso, Mexico, to drill into Earth's past. This spring and summer it will attempt to recover a thin cylinder of rock, 3 ¼ inches wide by 3,300 feet long, starting in the Eocene world about 50 million years ago, drilling all the way back into rocks created and contorted by an asteroid impact, 66 million years ago, when the dinosaurs disappeared.

Among its many scientific goals, ithe projectmeasure new dates for the Chicxulub impact widely blamed for wiping out the dinosaurs since that theory was first proposed in 1980.

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At the center of this revolution is EARTHTIME, an international collaborative project that has made root-and-branch improvements to the precision and accuracy of radiometric rock dating, spearheaded by Professor Sam Bowring of the Massachusetts Institute of Technology. Radioactive elements (such as uranium) decay into other elements (such lead). The rate they do that is called the "decay constant" and is known very precisely. If you know how much of the parent element has changed into the daughter element, you can use the decay constant to calculate the date when that rock formed.

To find out more, I talked geochronologist Seth Burgess, a former student of Bowring's, now with the United States Geological Survey. "In the last 10 years or 15 years there have been significant advances in accuracy and precision of geochronology," Burgess told me. The distinction between accuracy and precision is important. Think about archry. If an archer fires several arrows and they miss the target but all hit the same spot in a tree, then he was precise, but not accurate. If the archer peppers his arrows a l over the target, he is accurate but not precise. If he gets all the arrows in the bullseye, then he is both accurate and precise.

1,000 years is roughly the time it takes for the world's oceans to mix completely today, but in warming climates it could take longer. Large CO₂ emissions over centuries (like human emissions) are mainly absorbed by the surface layer of oceans before it has a chance to be mixed and diluted into the far larger reservoir of the deep ocean, leading to life-challenging global warming and surface ocean acidification. These geologically "fast" changes tend to be dangerous to life.

As a result, over the last three years or so, a series of landmark papers has used these new high-resolution dates to nail the link between several mass extinction events and an epic class of volcanic eruption called "Large Igneous Provinces" or "LIPs," whose effects were frighteningly evocative of modern climate change. This is the case for the end-Triassic mass extinction 201 million years ago, the end-Permian mass extinction 251.9 million years ago, as well as for the Toarcian extinction in the Jurassic, the Capitanian extinction in the Permian, The Early-Middle Cambrian Extinction, and the more minor Paleocene-Eocene Thermal Maxiumum (PETM).

Burgess used the new technique to establish that the end-Permian mass extinction – Earth's most severe extinction — unfolded in less than 61,000 years, starting at 251.9 million years ago, coinciding with a massive shift in the carbon cycle recorded by carbon isotopes measured in contemporary sediments. He was then able to compare those dates with dates he measured from the Siberian Traps volcanic rocks. They were a precise match.

For years, the end-Cretaceous extinction was thought to have been set up by volcanic eruptions in India, then finished off by the Chicxulub asteroid impact in Mexico (the "Press-Pulse hypothesis"). The Indian Deccan Traps eruptions were considered too slow, and their effects too mild, to cause global species death on their own. But as the new dates for the end-Permian, end-Triassic and other extinctions have now shown, LIP eruptions can indeed cause extinctions without the help of an asteroid. In fact, no asteroid impact has been linked with any other mass extinction since complex animals evolved, despite the fact that there have been several other impacts almost as big as Chicxulub in that time.

But argon-argon dating is a very different technique from uranium-lead. It requires calibration to a reference material, and it involves irradiating samples in a nuclear reactor "There is a difference between argon-argon and uranium-lead [techniques]," says Burgess, :In some cases, it's 0.1-0.2 percent difference — we're talking about 60,000 years at the end of the Cretaceous. A 60,000-year inaccuracy on one of those dates means a hell of a lot for the biosphere. "In order to compare uranium-lead and argon-argon dates, and do it in a robust way, you've got to layer-on uncertainty to account for potential inaccuracy. This allows us to compare apples to oranges, in effect."

Gerta Keller of Princeton University has long argued that the Chicxulub impact occurred 100,000 years or more before the mass extinction, so therefore can't have caused the mass extinction itself, which she and others attribute the Deccan LIP. In favor of that idea, scientists have recorded a number of tracers of volcanic activity straddling the end-cretaceous extinction in sediments around the world, including osmium isotopes, a mineral called "akaganeite," iron oxide loss, and a spike in mercury levels (mercury spikes seem to be a consistent signature of several LIPs).

Geochronologists from UC Berkeley: Paul Renne, Courtney Sprain, and colleagues, recently used argon-argon dates for the Deccan lavas to establish that the most voluminous Deccan eruptions occurred within about 50,000 years of the asteroid impact. These dates are the first firmly to locate the asteroid impact date measured in Wyoming within the Indian Deccan Traps eruption time period using the same argon-argon technique. In other words, it's an apples-to-apples comparison.

These new dates have transformed the impact extinction hypothesis first proposed in 1980 by father and son Luis and Water Alvarez. Walter is a coauthor on a recent paper led by Professor Mark Richards of UC Berkeley, which suggests that the Deccan eruptions were made more severe by the seismic shaking that reverberated through the planet after the Chicxulub impact, putting the "press" and the "pulse" of the Press-Pulse extinction hypothesis at exactly the same time (as far as date precision allows).

But there's a wrinkle. For some years now the date of the extinction has been assumed to be the same as the date for the impact — an assumption criticized as "circular reasoning" by Professor Keller. Within the wide date uncertainties of a few years ago, that assumption seemed reasonable and practical to many scientists. But now, as geochronologists allow us to zoom-in to the end-Cretaceous mass extinction in unprecedented detail, they can begin to tease that assumption apart.