What Really Killed the Dinosaurs?

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.

The new dates, using the latest generation of high-precision rock dating techniques, are needed because a quiet revolution in Earth Science has transformed our understanding of mass extinctions Earth’s past, including the end-Cretaceous mass extinction.

EARTHTIME

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.

In geochronology, you need both, and, a lot of the improvements in these crucial parameters, Burgess says, "has fallen out of the EARTHTIME initiative, so there's better agreement between labs. It has been this big inflection in data quality."

Coincidence and pace

In order to understand what happened during the big changes in Earth's distant past there are two basic time problems to solve: coincidence and pace.

Scientists must establish a coincidence between a proposed cause and its effect. There is a vast amount of time in the geological timescale, so it's no good blaming, say, a volcanic eruption if it occurred after a mass extinction event, or if it occurred so long before the extinction that there is no plausible mechanism to link the two.

Scientists must also establish how fast the change was.

How fast is "fast?"

Simply put, there are two timeframes to consider: 1,000 and 10,000 years.

When it comes to mass extinctions of ocean life, ocean acidification has been implicated as one of the kill mechanisms (the spread of oxygen-starved dead zones called "anoxia" is another, which itself is triggered by a cascade of biological consequences from ocean acidification and global warming).

Changes that take place over timescales longer than 10,000 years tend to be neutralized by compensating chemical responses in the oceans and on land, so a plausible driver of ocean acidification has to overload the oceans in under about 10,000 years.

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.

Better dates

Until recently rock date uncertainties were typically several million years for rocks more than about 100 million years old. That's more than 100 times worse than needed to answer these questions of coincidence and pace in mass extinctions.

But scientists have now drastically reduced those uncertainties, achieving date precisions that are plus or minus about 13,000 years for dates in the Cretaceous, or about 50,000 years for dates in the Permian period.

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.

"Over half of the entire volume of Siberian Traps lavas erupted prior to [251.9 million years ago], within uncertainty of the onset of the mass extinction, and the cessation of the mass extinction for that matter. So it is a really, really quick situation."

That's enough to cover the entire Unites States in lava 900 feet deep, all erupted at precisely the right time, and fast enough, to declare this a "smoking gun," settling the link between the eruptions and the mass extinction.

Dating the dinosaurs' demise

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.

In January 2015, geochronologist Blair Schoene of Princeton University and colleagues measured dates for the Deccan eruptions that showed they were at precisely the right time and duration to have triggered the end-Cretaceous extinction, in a pattern remarkably similar to that observed for the end-Permian and end-Triassic.

But the eruptions also appeared to coincide with the date for the Chicxulub impact.

Comparing Apples and Oranges

Blair's study used uranium-lead dating on a mineral called zircon, but the accepted date for the Chicxulub impact uses a different technique: argon-argon dating.

Argon-argon dating is a more common technique for dating volcanic rocks because it can be measured from feldspar minerals that are common in LIP lavas and volcanic ash deposits, whereas zircons are rare in LIP rocks. Argon-argon dating has undergone its own precision revolution, now achieving precisions almost as good as the new uranium-lead dating using zircon.

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."

To underline the point, in 2010 and 2011 the date for a widely-used reference material, the "Fish Canyon sanidine," was revised from 28.02 million years old to 28.3 million years old, which had the effect of changing dates in the Cretaceous by about half a million years (which is why the end-Cretaceous event was revised from 65 to 66 million years ago).

This difference adds fuel to the debate surrounding the principal cause and order of events surrounding thismass extinction.

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).

To make matters worse, correlating end-Cretaceous rocks around the world involves the measurement of ancient magnetic field reversals frozen into rock, but there is considerable disagreement between different dating techniques on the duration of these reversals near the end of the Cretaceous (about 740,000 years vs about 400,000 years).

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.

So there's now a 3-way coincidence between the Impact, the Deccan Traps eruptions, and the mass extinction. Even the high mercury levels linked to the Deccan eruptions straddle the traces of the asteroid impact indicated by a spike in iridium levels, strengthening the idea that the impact and eruptions happened very close together in time.

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.

Clearly, being able to compare precise dates for the impact itself (rather than distant traces of it), the extinction itself (rather than the impact as its assumed proxy), and the eruptions, is going to be critical.

Drilling the impact

That's where the new Chicxulub drilling project, led jointly by the University of Texas and Imperial College London, comes in.

Among other scientific goals, scientists plan to date the rocks melted by the Chicxulub impact as well as rocks from a smaller Cretaceous asteroid impact in Ukraine, and rocks from the Deccan Traps. Project scientists have confirmed they intend to use both high-precision argon-argon and uranium-lead dating, using EARTHTIME calibrated tracers.

Provided they recover suitable samples, this Chicxulub drilling project could have an impact all of its own by resolving a scientific controversy that has persisted since the 1980s.

The views expressed are those of the author(s) and are not necessarily those of Scientific American.

This article was first published at ScientificAmerican.com. © ScientificAmerican.com. All rights reserved.

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