Last Extinction

PBS Airdate: March 31, 2009
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NARRATOR: Before the last Ice Age, great mammals ruled the plains of North America: woolly mammoths, saber-tooth cats and other extraordinary animals.

GARY HAYNES (Archeologist): You can't see anything like it today.

NARRATOR: But then, suddenly, they disappeared, and nobody really knows why.

PAUL MAYEWSKI (Glaciologist): It happened so fast.

NARRATOR: Did humans kill them off?

JAMES KING (Geologist): Man probably came to North America as a super-predator.

NARRATOR: Or was their fate more like the one we face today with our climate in crisis?

JAMES KENNETT (Geologist): We need to know how our planet responds to such events.

NARRATOR: Or was it something from outer space?

Now, a new theory has blown the question wide open.

ALLEN WEST (Geologist): It would have been hell on Earth...been a very, very bad day.

NARRATOR: And scientists are fighting over the meaning of startling, new discoveries.

MARK BOSLOUGH (Physicist): You need extraordinary evidence. And I haven't seen that yet.

NARRATOR: NOVA wades into the thick of this controversy in hopes of finding the answer.

JAMES KENNETT: This is art. This, to me, is a Mona Lisa image.

NARRATOR: This is science in action; the stakes are high, the outcome uncertain. Right now, on NOVA, go inside the investigation to solve the mystery of this Last Extinction.

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NARRATOR: Thirteen-thousand years ago, the Earth's climate was not unlike ours today. But then, suddenly, it changed radically. It was mysteriously thrown back into the Ice Age, and some of the greatest animals that have ever lived vanished: enormous creatures including the woolly mammoth, like a modern elephant, with thick fur and huge tusks; and the saber-toothed cat, a vicious predator, built more like a bear than a lion.

These magnificent animals dominated the Earth for more than 100,000 years, then, like the dinosaurs some 65 million years earlier, disappeared across North America in what is a geologic instant. But unlike the dinosaurs, nobody really knows why.

GARY HAYNES: There was an incredible diversity of animal life, an incredible diversity. You can't see anything like it today.

NARRATOR: Gary Haynes, an archeologist at the University of Nevada, Reno, studies the remains of these animals, and their modern relatives, to try and figure out what happened to this incredible diversity of animal life.

GARY HAYNES: We had several kinds of mammoths, mastodons...we had big predators. We had a lion that was even bigger than the one in Africa today...very much looked like it. We had saber-toothed cats, probably the most memorable animal of all. It looks like a huge cat but it's got these funny fangs. Some of these teeth are big arcing teeth that look just like the saber or dagger blades.

NARRATOR: In all there were between 15 and 35 types of large mammals that went extinct suddenly, including giant sloths, which could be as tall as a giraffe with the girth of an elephant. There were some wonderfully odd creatures, too, like the glyptodont.

GARY HAYNES: Imagine an enormous armadillo with a big armored cover. These are well-defended against predators. You don't see anything like that today; sometimes they probably reached the size of a Volkswagen beetle.

JAMES KENNETT: There was such a rich assemblage of animals. I mean, we go to Africa on safari, well, that is what it was like in North America. All these animals were wandering around, a great diversity of large animals, beautiful animals, and they were lost, in an instant it seems.

NARRATOR: Geologist James Kennett is also fascinated by what killed off these animals, and several of the discoveries throughout his career help illuminate the mystery. It is such a long-standing passion that he, in fact, wrote about it in his very first scientific textbook, at age 11.

JAMES KENNETT: I was going to be a geologist, when I was just 11 years old, and I wrote some books. One's on paleontology. There's a picture of a saber-tooth cat. Here's my rendition of the woolly mammoth. Obviously I must be interested in what knocked these animals off because, having explained what this megatherium is, this giant ground sloth of South America, it says, "Having no protection, the caveman killed it off because of its stupidness."

NARRATOR: But there is nothing stupid, Kennett says, in trying to figure out why these large mammals disappeared, especially today when humans worry about changes in our environment.

JAMES KENNETT: So much happened and so quickly. It was a sad day that these were lost. These were lost and lost so suddenly.

NARRATOR: The extinction of these animals has perplexed scientists for centuries, but today there is a controversial new theory that might just explain it.

Allen West is digging for the evidence. A former geophysicist for the mining and oil industry, West is now using his expertise to investigate this mystery.

ALLEN WEST: Millions and millions and millions of mammoths—mastodons, saber-toothed cats, an American camel, American horse—they all were happily grazing across North America, and they disappeared right at this boundary. They vanished exactly at this point.

NARRATOR: This extinction boundary, plainly visible in this riverbed in southern Arizona near the Mexican border, shows up right under an ominous dark layer of earth, formed by decayed plants and algae. It is called the "black mat."

ALLEN WEST: What I've done here is remove some of the sediment that's above the black mat. And then I'm removing the centimeter or so, the half inch, that's right below the mat, and this is where the mammoths would have walked. So this is the action zone, right there.

The beauty of this mat is this is an extraordinary snapshot into the past. It's so rare in any kind of geologic record to be able to point to a spot and say, "Here's where something happened very suddenly."

NARRATOR: In the past three years, West has dug into his retirement savings, shipping boxes of dirt to colleagues around the world, trying to solve the mystery. West has become his very own FedEx hub of Ice Age dirt.

In a tantalizing early discovery, the team found traces of iridium, one of the rarest elements in the Earth's crust. That discovery meant that they could be onto something really big.

In the late 1970s, a Nobel prize-winner named Luis Alvarez and his son, Walter, also found iridium in geologic layers from 65-million years ago, the time when the dinosaurs got wiped out. It was a stunning find.

ALLEN WEST: While there are amounts of iridium found on the surface of the Earth, they're in tiny quantities. They are, however, in high quantities in meteorites and other cosmic material. So if you find a peak in iridium in a particular layer, then the first suspect is, was there a cosmic event?

NARRATOR: As the evidence mounted, the verdict seemed inescapable. An asteroid from outer space wiped out the dinosaurs. The era of giant beasts that had ruled the Earth for millions of years ended with a bang.

So, do elevated levels of iridium just below the black mat mean that the giant land animals of the Ice Age shared a similar fate?

NARRATOR: Allen West and James Kennett are part of a growing team of scientists that are analyzing this new evidence, and now propose that 12,900 years ago, a comet slammed into the Earth. This cosmic catastrophe would have devastated the great land animals.

It's hard to comprehend the disaster. One of the closest things we have to picturing it is the impact of a nuclear bomb or perhaps several of them.

JAMES KENNETT: If you imagine multiple nuclear explosions occurring over wide areas, generating major pressure waves, flash heat waves, knocking down forests. And this led to wildfires over wide areas, with major destruction of the vegetation. The burning over broad areas of the continent would have destroyed the food resources for many of these animals. And, we suggest, that is why the larger animals, preferentially became extinct.

ALLEN WEST: The larger the animal, the more devastating the effects of this would have been. It would have been hell on Earth...been a very, very bad day.

NARRATOR: Surely such a bad day would leave behind obvious evidence, like this impact crater from 50,000 years ago. But, so far, no crater has been found, and that means the impact team needs to find other compelling proof. And they are now going to the ends of the earth to do so, including here at the Greenland ice sheet.

Paul Mayewski, from the University of Maine, has joined the team and is particularly well-equipped for the job. He is one of the world's foremost glacier experts and has traversed more of Antarctica than anyone else. If there is any evidence sealed away in this magnificent glacier, of a comet that killed off the great animals, Mayewski wants to find it.

The reason he might be able to is that this ancient ice is not only beautiful, but it is also an exquisite frozen library of information about the Earth's history.

PAUL MAYEWSKI: These icebergs have captured within them a remarkable story. Trapped within every single year's worth of snow is an amazing archive of what the environment was like at that time.

NARRATOR: These glaciers are like giant computer hard drives filled with icy data, including ancient microscopic dust.

PAUL MAYEWSKI: The snow itself pulls all of the little dust particles: chemistry that comes out of volcanoes, from forest fires, meteorite impacts.

NARRATOR: Somewhere in the enormous Greenland ice sheet is the very snow and dust that fell when mammoths walked the Earth, and, possibly, the chemical trace evidence of a cosmic impact that wiped them out. But how can you possibly find that specific layer of ice?

During the brief Greenland summer, NOVA brought Paul Mayewski here to take up the challenge. Mayewski brought with him his colleague, Andrei Kurbatov, an expert in analyzing glaciers for microscopic particles.

Mayewski and Kurbatov will be looking for evidence of the impact sealed away in the ice, particularly extra-terrestrial materials like iridium, the clue that helped solve the dinosaur extinction.

The first step, though, is the daunting problem of finding the exact layer of ice from 12,900 years ago in the vast, ancient Greenland ice sheet.

J.P. STEFFENSEN (Glaciologist): We will not move those two dates because we have an eclipse happening.

NARRATOR: And that is why Mayewski is working with J.P. Steffensen, a Danish scientist who knows this glacier as well as anyone in the world.

J.P. STEFFENSEN: If you look here, you can actually see there's this white line between the two black ones. And that is the last layer of this Ice Age ice.

NARRATOR: The layer of ice they are looking for is located between two distinct features caused by Ice Age conditions. Glaciers preserve snow in annual layers, which can be clearly visible, much like tree rings. At the edges of the Greenland ice sheet, these layers get exposed. It's similar to the way layers of Earth are exposed at the black mat.

And there is another parallel. Just as the black mat, in Arizona, appears right where the animals went extinct, here in Greenland, there is a visual marker of this time, as well. Ice from the last Ice Age is gray in color because the arid climate during an ice age erodes the soil, puts a lot of dust into the atmosphere that gets trapped in the ice.

These bands of color, alternating between gray and white, can be clearly seen in this aerial photograph. The ice from the time the mammoths died off should be right between those bands of gray. And each year, during the short Greenland summer, when the glacier is not covered in snow, these color differences are visible on the surface of the ice.

J.P. STEFFENSEN: This dark ice is what we call velvet ice because it has such a velvety, rolling, soft way of running. And then you see in the background...you have much more bumpy and whitish ice comes up. You can see, almost, the transition up there to the hill.

NARRATOR: Steffensen has analyzed this area and found evidence of ice from this mysterious period. Using these color differences, he zeros in on a slope with both colors of ice.

J.P. STEFFENSEN: This is a promising spot. But then again, as you know, Paul, in science, you have to try and hope you are right. But that would be my best guess right now.

NARRATOR: This is science in action and also a very long way to come on a guess, even an educated one.

PAUL MAYEWSKI: I want to make sure to sample the upper part.

NARRATOR: But confirming if this is indeed the right spot will take a sophisticated molecular analysis, which can only be done back in the lab. For now, the job is to collect the samples and hope they're in the right spot.

ANDREI KURBATOV: Just try to use chisel.

NARRATOR: Mayewski and Kurbatov cut a 17-meter trench down the hillside into these exposed layers, from the more recent ice, to older ice, deep into the Ice Age. Samples are taken in 15-centimeter sections. That way any change in the levels of extraterrestrial materials, like iridium, will show up. Many different samples are needed. Specially cleaned bottles are used to collect ice to analyze for the presence of iridium.

PAUL MAYEWSKI: That should do it.

NARRATOR: Other samples are collected to test for oxygen isotopes that can help determine the exact age of the ice. And finally, chunks of ice are taken to see if there are other trapped clues that could be the fingerprint of a cosmic catastrophe.

The stakes are high. Only when these samples are analyzed will Mayewski know if this trip was a waste of time or if it could yield the smoking-gun proof that Earth was slammed by an object from outer space only 12,900 years ago, an object big enough to blast up to 35 kinds of animals into extinction...truly an extraordinary claim.

MARK BOSLOUGH: It's not impossible that you could get an event like that, that recently; it's just extraordinarily improbable. And when you are making an extraordinarily improbable claim, you need extraordinary evidence. And I haven't seen the extraordinary evidence.

NARRATOR: Physicist Mark Boslough analyzes impacts of comets and asteroids. He says one way to evaluate if there was a devastating impact is to calculate the statistical probabilities of it happening. And the odds don't look good.

MARK BOSLOUGH: You have improbability stacked upon improbability. We're talking about something that may happen only once in the entire age of the Earth, and I have a hard time accepting something like that happened that recently.

NARRATOR: Boslough uses some of the Department of Energy's most powerful supercomputers to model the physics of various cosmic impacts and mid-air explosions...

MARK BOSLOUGH: In this simulation I bring in an asteroid—a small asteroid—about 30,000 miles an hour, and it explodes three miles above the surface.

NARRATOR: ...and is able to show clearly their devastating power.

MARK BOSLOUGH: At ground-zero...temperatures as hot as the surface of the Sun, and this vortex is moving at supersonic speeds. So it's like an ultra-white-hot tornado. And this is one of the most extreme events, I think, that could ever happen on the surface of the Earth.

NARRATOR: Boslough says that while an event like this would certainly devastate the environment and the great animals, he thinks there are far simpler explanations.

MARK BOSLOUGH: You know, Ockham's razor? The simplest explanation is probably the best one. And there are other hypothesis that, to me, don't seem as extraordinary.

GARY HAYNES: It seems a little far-fetched. There's a lot of species that did not become extinct, which would be hard to explain. Why weren't they affected if all these other large mammals were? Why weren't the deer and elk and bear affected? Why did they survive?

NARRATOR: Could there be a more down to earth explanation for these mysterious extinctions? Possibly, and to understand them, it is important to investigate what the Earth looked like at that time. And that is the work that Vance Haynes, a legendary archeologist from the University of Arizona, has done.

One day in the 1960s, while walking through this riverbed, Haynes saw bones lodged into the canyon wall. They turned out to be from a mammoth. His ensuing investigation of this site has given us one of the best pictures of life here some 13,000 years ago.

That picture is hard to imagine looking at Arizona today. Instead of sprawling cities, there were valleys full of animals. And instead of the retirees and golfers who flock here, there were other immigrants, the first well-documented humans in North America, the Clovis people.

The Clovis people appear to have arrived in this area just before the black mat is formed. The little information we have about them comes from their beautifully engineered stone tools and spear tips, called Clovis points.

VANCE HAYNES (Archeologist): They are, essentially, functional art. This can be used both as a knife as well as a projectile point. But the basic function of this is for killing game, but yet they took the trouble to get very nice symmetry.

NARRATOR: These sleek, stone weapons were a major advance in hunting technology.

VANCE HAYNES: I think, from seeing their stone technology, they were very sophisticated people. These were probably on a spear that was used for either thrusting or for throwing with a spear thrower. So the idea was, once they penetrate an animal, this would come off, this would stay with the animal, but they would have another one, sort of reload and be ready to go again.

NARRATOR: Haynes' excavations at the black mat proved that the Clovis people hunted the mammoth and other animals here, right up until they disappeared.

VANCE HAYNES: And this is where these animals just came to a sudden end. We've repeatedly dated this level right here at 12,900 calendar years ago. My bottom line is something happened 12,900 years ago that we don't understand.

NARRATOR: Efforts to explain this mystery have led to some very peculiar clues.

The value of this evidence, held under lock and key in the basement archives of the Arizona State Museum, is no B.S. But the evidence itself is exactly that: an ancient piece of poop, giant sloth poop. Yes, you heard that right, giant sloth poop.

JAMES KING: It's not classy science. Working with this stuff in the lab, it smells like you know what.

NARRATOR: Jim King, the former director of the Carnegie Museum of Natural History, says the sloth dung is a goldmine of evidence about how these animals lived and possibly died.

JAMES KING: Fossilized dung is fabulous because in my hand is the evidence of what these animals ate. You see all these fibers, little bits of twigs. Much of this will be Joshua-tree fiber.

NARRATOR: The manure, discovered in a cave, piled five feet high, is high in carbon and therefore makes for perfect radio carbon dating. The last manure in this cave

dates to around 12,900 years ago, the time the black mat appears and not long after the stone artifacts show the arrival of the Clovis people.

Scientists used that coincidence to support a theory that these early hunters were responsible for killing off the great land animals.

JAMES KING: Man probably came to North America at that point as a super-predator with a naïve fauna that had no idea what they were facing. The super-predator had communication, it had weapons, it hunted in groups, it had coordination, it had all the things you would do if you and I were going out to try and hunt big animals.

NARRATOR: More recent history supports this theory. In Mauritius, the arrival of the Dutch doomed the dodo, and in New Zealand, the first settlers killed off the moa. But could this also have happened to these great animals all across North America?

JAMES KENNETT: The data just doesn't support this. It's inconceivable to me.

NARRATOR: Kennett says the idea that primitive humans killed off these powerful animals is absurd, and while it might happen in small island environments, it is impossible to imagine they could wreak such havoc on a continent as vast as North America.

JAMES KENNETT: They didn't have the technology that modern humans have. They didn't have helicopters and machine guns and satellite navigation and so forth. It's always puzzled me. How could they track down that last horse or that last mammoth or that last camel? It just perplexed me. It just didn't make sense.

NARRATOR: If the human overkill theory, as it's called, cannot provide the whole answer, what else can?

NARRATOR: This is not the first expedition that Paul Mayewski has made to these glaciers in search of evidence. In the 1990s Mayewski drilled into the center of the Greenland ice sheet and extracted ice cores going back more than 200,000 years to understand how the world's climate changed. At the time, nobody thought to test for impact markers like iridium, but what they did find was remarkable.

The ice cores proved that the Earth's climate can change extremely rapidly. Sophisticated molecular analysis of the ice can reveal the temperature at the time it formed. The ice from this time, when the animals went extinct, shows a very unusual change: temperatures plummeted.

PAUL MAYEWSKI: It happened so fast. When you think about it, it's unbelievable. It's a complete change in the state of the climate system. It would be the equivalent of going from maybe two or three months of winter in northern New England, let's say, to 11 or 12 months of winter throughout the year.

NARRATOR: In possibly less than two years, the annual temperatures in North America dropped up to 18 degrees Fahrenheit, which may not sound like a lot, but the last time it got that cold in what is present-day Chicago, it was buried under a mile of ice.

Throughout much of the world, climates changed. Some places got colder others dried out, causing more fires. Was this sudden climate change, not the Clovis hunters, what killed off the great land mammals?

ALLEN WEST: There's periods in geologic history where things really go haywire and this was one of those times. We were going through a major climate change. Well, some animals just don't make that change.

NARRATOR: Gary Haynes says that's true, but there were other similar periods of climate change, and these animals survived. Why would this one kill them off? He believes that while climate may have played a role, man was ultimately responsible.

GARY HAYNES: We don't actually like to think of how destructive we are. Even a small amount of killing may have been a decisive factor with some species that were already stressed by climate change, leading to eventual extinction. Humans have always had an impact on their environment.

NARRATOR: The disappearance of the great land mammals is one of the most dramatic and recent extinction events in Earth's history, the last extinction. It also happens to be the time when the Clovis artifacts disappear from the record.

The only history we have is written in the chemistry and archeology that remain behind. And so far these clues have not adequately solved the mystery. Now the new theory of a cosmic impact has blown the question wide open.

The initial finding of iridium at the black mat was a startling discovery. But the levels were much lower than those found at the time of the dinosaur extinction. Because cosmic dust accumulates naturally on the Earth's surface, the low-level iridium findings are not a smoking gun, but were a good tip-off about where to look for more clues. Including the black mat in Arizona, there are at least 31 sites in North America, and six in Europe, where the impact team has collected dirt from the time of the extinctions. That means there is an awful lot of dirt to analyze.

James Kennett and his archeologist son, Douglas, are literally up to their elbows in it.

ALLEN WEST: Look at this, Ted. Here's one, two, three.

NARRATOR: So are Allen West, Jim Wittke, a geologist, and Ted Bunch, who is retired from NASA.

And new technologies, developed since the hunt for the dinosaur killer, have opened up the nano-scale natural world that is embedded in these samples of Earth. What West and team are finding in this ancient dirt is extraordinary.

JIM KING: What in the world is that stuff?

ALLEN WEST: Now, this is a split-open carbon spherule. And we know from the chemistry that these are formed by burning pine trees, burning spruce seeds. This is tree sap, in effect, that's been scorched, burned. But the thing that makes this extremely unusual, in fact the thing that ties it to a cosmic impact, is embedded in the ribs here that you see around.

NARRATOR: One of the most intriguing discoveries, so far, is buried in the spherules. Inside the carbon structures, the impact team has found something that was as thrilling to a scientist as it would be to a young bride, diamonds, nano-scale diamonds to be exact.

ALLEN WEST: All of the rims of each one of these little holes in here is absolutely laced with diamonds. In some cases, 30 percent of that carbon spherule is made up of tiny diamonds.

NARRATOR: These nanodiamonds can be so small that one million of them could be squeezed inside a single grain of sand.

ALLEN WEST: You know, if they could get them out, there's about a trillion dollars worth of these diamonds spread across the United States, so...but it might cost you a trillion to extract them, so...

NARRATOR: All geology jokes aside, it is an important discovery.

TED BUNCH (Retired NASA Scientist): It's like looking at a Hubble image of all the galaxies out in the universe; there are an incredible number of these things.

NARRATOR: Exactly how these nanodiamonds are formed is poorly understood. The team has discovered several different kinds.

Most diamonds found on Earth are cubic, and have a highly symmetrical structure. But one type they are finding, the hexagonal diamond, is not at all common. The only known way to make these is through a high-pressure blast, such as an impact.

TED BUNCH: We see a little hexagonal outline. Woo! Okay, that's cool! These are hexagonal diamonds. Hexagonal diamonds are not found in the Earth's mantel. They do not occur terrestrially. They only in occur in craters, in meteorites, interplanetary dust, et cetera. They are all E.T.

JAMES KENNETT: It's very hard to explain the presence of such numbers of diamonds over such broad areas of the planet, other than from the production from an extraterrestrial impact.

NARRATOR: But is this necessarily so?

MARK BOSLOUGH: I'm still skeptical because they really are invoking a large comet, maybe a couple of miles in diameter. Something that big has enough mass to carry it all the way to the ground, and it would generate a big crater. It would make a big hole in the ground.

NARRATOR: Mark Boslough says that we don't know enough about the formation of nanodiamonds, but if they are the indicator of an impact, where is the crater?

MARK BOSLOUGH: Thirteen-thousand years ago is not very long ago in terms of geologic time. That would be a very young crater. It would be very obvious if something like that existed in North America.

NARRATOR: When the dinosaur-killing asteroid theory was proposed, no one could find a crater either. But then, more than a decade later they did, mostly underwater, and it confirmed the early iridium findings.

NARRATOR: So far, no one has found a crater from the extinction of the large land animals.

Will a crater eventually be found related to this event, or could there be another explanation?

That is the question that Peter Schultz is trying to answer. And he can simulate impacts—on a much safer scale—here, with the hypervelocity gun at the NASA Ames Research Center outside of San Francisco.

PETER SCHULTZ: Okay.

NARRATOR: A small glass pellet is used as the bullet in the gun, and it simulates the cosmic projectile.

The pellet is loaded into this 40-foot long gun and fired into a vacuum chamber to model different possible impact scenarios.

PETER SCHULTZ: This part kills me.

NARRATOR: In this test, Schultz wants to know what would happen if the object from outer space, or the glass bullet, broke into thousands of pieces before hitting the Earth, an idea inspired by the 1994 comet Shoemaker Levy that broke apart before impacting into Jupiter.

PETER SCHULTZ: This could be interesting, especially with the new cameras. I hope we get to see the spray pattern.

Oh, that is so sweet. Let's go see the high-speed cameras.

NARRATOR: High-speed cameras capture the impact and stop the action at up to an astonishing one million frames per second.

PETER SCHULTZ: We have thousands, millions of small fragments slamming into the surface simultaneously. Okay, now, that bright stuff in here, that's the projectile, that's carrying any iridium, any signature of the impact. And after it hit, it would have been blown all over the place and redistributed across the Earth. That is where the gold is. That is the stuff that tells us there was an impact.

NARRATOR: This is the massive crater a single impact would form: pretty obvious. So, what size crater would show up if the same size comet broke up into thousands of smaller objects?

PETER SCHULTZ: We can actually see the individual small craters. And then wind came in and destroyed it. The Earth would recover very easily. Just a little bit of rain and little bit of weather, and you'd lose the evidence.

NARRATOR: A second scenario is that the comet, or at least a major part of it, would have slammed into the Ice Age glacier that covered a large part of North America at that time. That can be simulated here with the impact gun, using a large piece of ice.

PETER SCHULTZ: You have this really intense temperature. The crater just simply forms in the ice. The ice around it cracks, then it simply disappears. Again, you lose the evidence. The ice acted as this flak jacket.

NARRATOR: An impact into the glacier could be another way to explain the lack of a crater, but the burden of proof still lies with the comet theory team, and their strongest evidence, the nanodiamonds, is under attack.

Mark Boslough believes there may be other, far more likely, explanations for the nanodiamonds in the black mat.

For instance, tiny micrometeorites are raining down through our atmosphere at low levels all the time. Was there some way, during all the environmental changes happening at this time, that they became concentrated right under the black mat?

MARK BOSLOUGH: We have this constant rain of micrometeorites. Those contain diamonds. So the question is can you concentrate those diamonds by some mechanism that changes when you have abrupt climate change?

NARRATOR: So now a troubling new question has emerged: when did these nanodiamonds appear? Did they rain in slowly, or appear all at once? Fortunately that is exactly what the glaciers, and their amazing library of information, can answer.

PAUL MAYEWSKI: This is a classic example of how the surface of a glacier can capture an environmental record. You see dust and debris being blown onto the surface of the glacier. More snow comes in, and dust, and effectively gets trapped and then held there for decades, centuries, thousands, and tens of thousands of years, so we have an exact archive of what happened at that time.

NARRATOR: A modern example of this will be smoke from the recent California wildfires. This satellite picture shows how it rises into the atmosphere. Eventually that smoke will make its way to Greenland and be captured in the top layer in the ice sheet. This glacier is, in effect, a safety deposit box with a date recorded of when it was sealed off.

The key question is, can Mayewski find the nanodiamonds or iridium within one narrow layer—equivalent to the layer beneath the black mat from 12,900 years ago—and not above or below? That would prove that this evidence did not rain down gradually over a long period of time.

NARRATOR: Remember how samples were taken across thousands of layers of ice history to see if there is a spike in iridium, carbon spherules or nanodiamonds? Now, back at Mayewski's Climate Change Institute at the University of Maine, the field samples are run through different tests.

The first batch of samples is analyzed to answer the key question of whether or not they succeeded in finding that narrow layer of ice from the time these animals went extinct.

This machine can do that by analyzing the atomic structure of oxygen in the ice, which can reveal the exact temperature when the ice formed.

Mayewski and team have already done this analysis on more than 100,000 years of ice from the Greenland ice cores. So the results from the new sample are compared to this well-established record. It's like comparing temperature fingerprints, and, remarkably, the lab results from this period show a suspected match.

PAUL MAYEWSKI: The likelihood that we would find it was very, very small; it's worse than looking for a needle in a haystack. And suddenly to see on the screen in front of you, it's pretty amazing.

NARRATOR: It looks like Mayewski, Steffenson and Kurbatov found the ice from when these large mammals vanished.

In the lab, they test for the rare element iridium, the finding that helped solve the dinosaur extinction. The results show a spike right at the time when the animals disappear but not a very big one.

PAUL MAYEWSKI: It's not that they're catastrophically higher—they are about three times higher than background—but to have a peak of that level preserved is pretty fascinating stuff.

NARRATOR: This iridium is a tantalizing clue, but, just like at the black mat, there's no smoking gun, because the levels are low. Now it comes down to the nanodiamonds, which are also the hardest to process. Searching through samples from a 17-meter trench for evidence that is a million times smaller than a grain of sand is a very, very painstaking process. That job fell to Allen West.

ALLEN WEST: Pretty hard to work with a 10-micron spherule.

NARRATOR: When the meticulous work is finally done, and West has managed to prepare samples for the transmission electron microscope, NOVA asked Paul Mayewski to join James Kennett and materials scientist Chris Mercer, at U.C. Santa Barbara to have a first look at the evidence.

PAUL MAYEWSKI: That's remarkable—just tons of them all over.

JAMES KENNETT: It is clearly and immediately remarkable.

NARRATOR: The first sample they look at is from the suspected time of the extinctions.

JAMES KENNETT: It consists of a large number of particles.

PAUL MAYEWSKI: ...clearly rained out of the atmosphere.

JAMES KENNETT: This has huge implications. There's no way you are going to get these kinds of particles in the ice sheet unless they are raining out of the atmosphere.

PAUL MAYEWSKI: It's amazing.

JAMES KENNETT: If these indeed are diamonds, then this is a remarkable breakthrough.

NARRATOR: Mercer conducts a test to see if these particles are diamonds. A beam of electrons is shot into them, which casts a series of rings. Each crystal structure scatters the beam in a unique way, forming an identifiable pattern. Immediately, a familiar pattern shows up.

JAMES KENNETT: This is art. This, to me, is a Mona Lisa image, as far as Earth sciences goes.

NARRATOR: These black specks are nanodiamonds in an extraordinary concentration in this one slide.

The test reveals that there are different kinds of diamonds, including hexagonal diamonds, the clearest indicator of an extraterrestrial impact: a diamond believed to have had its atomic structure shaped by massive force.

JAMES KENNETT: I've never seen anything like this before. This is exciting to me, very exciting. I'm looking for cubics on the list and here's hexagonals coming. I'm saying, "Hexagonals in Greenland?" And yet here it is, alive and true in the Greenland ice sheet.

PAUL MAYEWSKI: It's particularly phenomenal because the Greenland ice sheet contains 450,000-plus years of ice, we're trying to find a little layer and bang, the very first slide that we look at has billions of nanodiamonds, the thing that is probably the absolute key to demonstrating that there was a swarm of comets at this time.

JAMES KENNETT: Looking at this kind of material is dramatic.

MAN: How so?

JAMES KENNETT: Well...

NARRATOR: For James Kennett this moment of discovery becomes overwhelming.

JAMES KENNETT: It's, it's...excuse me.

MAN: It's very exciting?

JAMES KENNETT: Exciting is really not the word. It's an experience you usually don't have much in your scientific career.

Moments of intense discovery are very emotional for scientists. When scientists make discoveries that they think are really important breakthroughs, if you like, "eureka moments," there's an elation. There's an elation, an emotion. These were emotional moments. The hypothesis predicts that the diamonds should have been there and there they were.

NARRATOR: So far, the nanodiamonds have been found only in the layers of ice around the time of the extinctions, a clear indication that they did not rain out of the atmosphere over a long period of time.

PAUL MAYEWSKI: Every single time we find another piece of evidence from the ice sheet, you think to yourself, "My goodness, what else could be possibly be in there?" They capture everything that was going on in the atmosphere. And in this particular case, the evidence is very, very dramatic.

NARRATOR: Each new finding bolsters the theory that a comet wiped out the great land mammals 12,900 years ago.

It is still early in the investigation, and no doubt other explanations will be suggested. Extinctions are very messy things, and many stressors—both climate change and human overkill—were altering the environment. But clearly, this new evidence for a massive impact is changing our understanding of the disappearance of the Ice Age animals of North America.

JAMES KENNETT: It was a sad day for North America, the loss of these animals. The whole landscape would have been so different. In fact, even the human cultural development would've been dramatically different in the last 13,000 years if these animals had, in fact survived.

NARRATOR: On NOVA's Last Extinction Web site, watch video extras, examine a collection of beautiful Clovis artifacts and try your hand at identifying ancient stone tools. Find it on PBS.org.

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