K-T Study a Mass Murder Mystery

Theories Evolving Again

Enjoy a good mystery story?

The geologic record provides five of the best.

Mass extinction events brought much of the world's existing life to an untimely end in the late Ordovician, Devonian, Permian, Triassic and Cretaceous periods.

Now science sleuths are taking a new look at global extinctions — with findings that could change the way we see life on Earth today.

Kenneth G. MacLeod is an assistant professor of geology at the University of Missouri-Columbia, and co-editor of the Geological Society of America's special paper volume, Catastrophic Events and Mass Extinctions: Impacts and Beyond.

He understands the allure of extinction events for paleontology puzzle-solvers.

"The geologic column is based on these things," MacLeod noted. "The nice thing about mass extinctions is that they're a huge signal. They really stand out."

There's no mystery why geologists appreciate all this detective work.

Research related to mass extinctions pays a huge bonus in advancing stratigraphy and improving knowledge of geochronology.

On Second Thought …

Investigations into the sudden, widespread disappearance of species took a new turn in the early 1990s.

Based on the theories of Walter and Luis Alvarez, and the fieldwork and research of Alan Hildebrand, the Chicxulub impact crater in Mexico's Yucatan Peninsula became linked to the mass extinction at the Cretaceous-Tertiary (K-T) boundary.

As evidence for this link accumulated through the decade, more and more scientists accepted that a major impact by an asteroid or comet had produced a global K-T catastrophe.

The impact theory caught the public imagination, since it implied a fiery and dramatic end for the last of the world's dinosaurs.

For scientists, the Chicxulub strike offered a novel but viable explanation for sudden extinction.

"Catastrophic explanations aren't a last resort now. They are a legitimate alternative hypothesis," MacLeod said.

Researchers began searching in earnest for impacts coeval with other extinctions, although they could still dip into a grab-bag of potential culprits. Those include:

Global cooling, global warming, carbon dioxide, sulfuric acid, atmospheric changes, eutrophication, euxinic conditions, anoxia, eustacy, flood basalt, volcanoes, earthquakes, orogenies and other major tectonic shifts.

To name a few.

As work progressed, evidence seemed to point away from impact-driven catastrophic events outside the K-T extinctions.

True believers insisted that the tell-tale craters hadn't been discovered yet.

Other researchers looked to new theories.

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Enjoy a good mystery story?

The geologic record provides five of the best.

Mass extinction events brought much of the world's existing life to an untimely end in the late Ordovician, Devonian, Permian, Triassic and Cretaceous periods.

Now science sleuths are taking a new look at global extinctions — with findings that could change the way we see life on Earth today.

Kenneth G. MacLeod is an assistant professor of geology at the University of Missouri-Columbia, and co-editor of the Geological Society of America's special paper volume, Catastrophic Events and Mass Extinctions: Impacts and Beyond.

He understands the allure of extinction events for paleontology puzzle-solvers.

"The geologic column is based on these things," MacLeod noted. "The nice thing about mass extinctions is that they're a huge signal. They really stand out."

There's no mystery why geologists appreciate all this detective work.

Research related to mass extinctions pays a huge bonus in advancing stratigraphy and improving knowledge of geochronology.

On Second Thought …

Investigations into the sudden, widespread disappearance of species took a new turn in the early 1990s.

Based on the theories of Walter and Luis Alvarez, and the fieldwork and research of Alan Hildebrand, the Chicxulub impact crater in Mexico's Yucatan Peninsula became linked to the mass extinction at the Cretaceous-Tertiary (K-T) boundary.

As evidence for this link accumulated through the decade, more and more scientists accepted that a major impact by an asteroid or comet had produced a global K-T catastrophe.

The impact theory caught the public imagination, since it implied a fiery and dramatic end for the last of the world's dinosaurs.

For scientists, the Chicxulub strike offered a novel but viable explanation for sudden extinction.

"Catastrophic explanations aren't a last resort now. They are a legitimate alternative hypothesis," MacLeod said.

Researchers began searching in earnest for impacts coeval with other extinctions, although they could still dip into a grab-bag of potential culprits. Those include:

Global cooling, global warming, carbon dioxide, sulfuric acid, atmospheric changes, eutrophication, euxinic conditions, anoxia, eustacy, flood basalt, volcanoes, earthquakes, orogenies and other major tectonic shifts.

To name a few.

As work progressed, evidence seemed to point away from impact-driven catastrophic events outside the K-T extinctions.

True believers insisted that the tell-tale craters hadn't been discovered yet.

Other researchers looked to new theories.

And with so much talk going on about global warming, some began to focus on ancient atmospheric and climatic changes.

Read, "greenhouse effect."

Today, that approach "has replaced asteroid impacts as the sexiest explanation invoked" for mass extinctions, MacLeod said.

Hot Stuff

Peter Ward spent years in South Africa looking into possible extinction causes, his latest work involving the massive die-out at the end of the Permian.

The Permian catastrophe brought the Paleozoic era to a screeching halt and may have wiped out more than 90 percent of then-existing life, the largest extinction known.

"I went there fully expecting to find evidence of an impact," he said. "But it didn't look that way."

Ward, professor of biology and earth and space science at the University of Washington, has written extensively about mass extinctions.

His latest book, published in January, is Gorgon: Paleontology, Obsession and the Greatest Catastrophe in Earth's History.

It combines a memoir of Ward's experiences with an account of his research into the Permian extinction.

That event's victims included the Gorgon, a fearsome, mammal-like, lizard predator.

Ward said growing evidence for the K-T impact intrigued him in the 1990s. Like many other researchers, though, he began to doubt that the impact explanation could be applied universally.

"We've had a paradigm change. About 2000, things sort of turned around again," he recalled.

For other extinctions, theories began to shift from instantaneous catastrophe to sudden but slower change, from meteor strikes to climate spikes.

"K-T may be unique," he said. "As we get better precision going back in time, we realize there can be really big changes in temperature."

In particular, Ward found evidence of barren, desert-like conditions, with braided streams in a desolate landscape.

"Once you strip away the plants, you get enormous changes in sedimentation," he said. "If you removed the plants now, you'd have a major mass extinction in the oceans."

The eruptions of the Siberian Traps flood basalt occurred at this same time, and could have played a major role, Ward noted.

He also thinks atmospheric change came from more than a release of carbon dioxide.

"The one word that's been used more and more during the past five years is 'methane,'" he said.

"It looks like sudden global warming where, when the planet got hot, it got a lot hotter," Ward explained.

"It's not sudden a-day-to-a-month, and it's not over millions of years. It's a 10,000-year type of thing," he added.

If ocean levels fell and hydrocarbon gas from clathrates began to enter the atmosphere, the Permian would have ended in "a positive feedback loop causing ever-increasing temperatures," according to Ward.

"The Permian was a runaway greenhouse effect," he said. "We didn't become Venus, but we got real hot."

War of the Words

As researchers moved beyond meteor strikes in explaining other extinctions, a new debate evolved over the Chicxulub impact.

Gerta Keller, a professor of geosciences at Princeton University, led a group of researchers who came to challenge the accepted theory.

Those researchers included Wolfgang Stinnesbeck and Doris Stuben of Karlsruhe University, Germany, and Thierry Adatte of the University of Neuchatel, Switzerland.

"Let's start with where we agree," Keller said. "We agree there was a major impact and it coincides with major extinctions at the K-T boundary. Where we don't agree is that this major impact was Chicxulub."

In fact, Keller places the Chicxulub impact about 300,000 years before the final K-T extinction.

She sees evidence for several large impacts during the end of the Cretaceous, building her extinction case on multiple meteor strikes, Deccan volcanism and a K-T greenhouse effect.

"The greenhouse warming that occurred between about 200,000-400,000 years before the K-T boundary had a profound effect on marine plankton and other life forms, and resulted in severe biotic stress that caused species dwarfing, reduced populations and reduced species diversity," she said.

Keller served as lead author for the paper "Multiple Impacts Across the Cretaceous-Tertiary Boundary," published last September in Earth-Science Reviews.

The authors followed with a paper on K-T spherule deposits, published in the Journal of the Geological Society, and an article, "The Non-Smoking Gun," in Geoscientist magazine.

This led to an online debate of comments, queries and ripostes on the Geological Society's Web site, a Chicxulub discussion that extended from November 2003 through January 2004.

Keller's case rests largely on examination of deposits between the Chicxulub ejecta layer and the identified K-T boundary layer.

She and her associates claim to find planktic foraminifera in these lithological units indicative of the last 300,000 years of the Maastrichtian, as well as fossil burrows.

They also claim to identify multiple glass spherule layers, as well as zeolite-enriched zones associated with smectite.

Special attention was given to greenish layers that might have been glass altered to clay, according to Keller.

But an independent expert "found that, lo and behold, it was not (altered) glass. It was glauconite — glauconitic clay with burrows in it," she said.

"We found more evidence of bioturbation, which indicates there was repeated colonization of the ocean floor by invertebrates in the interval after the impact ejecta was deposited and prior to the K-T boundary.

"Whether in northeastern Mexico or the Chicxulub crater, the bioturbated and glauconitic sediments therefore indicate the impact occurred well before the K-T boundary and was followed by a long period of normal marine sedimentation," she added.

Keller's revisionist bait drew few nibbles in the online discussion.

The primary counter-argument came from Dutch paleontologist Jan Smit of Free University in Amsterdam, who cited tsunami-wave deposits or tsunami-triggered deposition, altered by slumping.

"A lot of people are surprised that we have not heard from the very large support community that Chicxulub has. I expected to be inundated with attacks," Keller said.

Researchers now are studying core samples obtained at the crater site in early 2002, providing an opportunity for new interpretations of the event.

The final word on Chicxulub remains to be written.

A study recently published in Geology found little charcoal in K-T layers, casting doubt on a global firestorm theory.

A Fine Line

New expertise is pushing stratigraphic resolution to finer levels than anyone imagined, according to MacLeod.

"It has opened the door to time scales that were not considered before," he said. "People are looking at the boundaries with a new sense of detail."

In the future, extinction research may benefit from techniques that provide even better resolution.

MacLeod talked about analyzing stratigraphy on less than a 1,000-year timescale.

"Is the stratigraphic record well enough preserved so you can pull that out? It's really pretty amazing that we all think we can," he commented.

Future research also will draw on better integration among fields, with "a whole range of experts brought to bear," MacLeod predicted.

And he foresees a focus on boundaries that have gotten less attention, like the Ordovician and Frasnian-Fammenian extinction events.

Greater and lesser extinctions pepper geologic history. Researchers tend to concentrate on one or two of them, leaving plenty of room for other investigators to join the game.

"There aren't many people who work up and down the column on mass extinctions," MacLeod noted.

Given the possibilities, the potential and the publicity, research into extinction events — with attending controversies — seems certain to continue undiminished.

"We are always fascinated by death," Ward said. "Look at how interested we are in serial killers. Death writ large is even more intriguing."

Natural cataclysms have threatened the existence of life before, and "it's our own, deep-seated nightmare that we'll do the same thing," he observed.

For Keller, extinction research opens a window into the world's past.

"I'm interested in seeing what happened in the history of the Earth," she said. "I'm basically a historian."

Then there's the drive to sort out an ancient puzzle, to play detective on a grand scale, to read the final chapter in one of Earth's great murder mysteries.

"Probably," MacLeod said, "most people are driven by curiosity more than anything else."

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