Issue: November 2009

Origins of a Mini-Ice Age: Take Two

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by Jesse Richardson-Jones

Thirteen thousand years ago, ancient humans roamed North America, hunting giant animals — called megafauna — with flint-tipped spears. The early hunters briefly flourished, bringing down mammoths and huge saber-toothed cats. But all of this abruptly ended when a series of catastrophic comets or meteors exploded in our skies 12,900 years ago, sparking vicious wild fires that blazed across the American plains and caused mass extinction by suffocation and starvation. The impacting bodies — smaller versions of the one that killed the dinosaurs — wiped out the megafauna, plunged the Earth back into a thousand-year mini-ice age, and nearly decimated our spear-wielding ancestors.

Unless, of course, they didn't.

Scientists studying these ancient times rely on buried evidence to reconstruct historical events, but they don't always agree on the results. The scenario described above, where comets or asteroids killed the giant mammals and triggered climate change, was supported by evidence reported in the Proceedings of the National Academy of Sciences by a group of researchers led by Richard Firestone in 2007. Their findings are chronicled in an earlier Today's Science article, Extraterrestrial Impact: Studying a Mini-Ice Age Mystery. But this wasn't the end of the story.

Top and Bottom Left: Lawrence Berkeley National Laboratory; Bottom Right: University of Wyoming
According to a group of researchers led by Richard Firestone (top left), asteroids fell from the sky and killed many of the large prehistoric mammals that roamed North America. Iron particles were found embedded in the outer layer of mammoth tusks in Alaska (top right and bottom left). Firestone suggests that the event may have doomed one of the first Stone Age cultures and plunged the world into a mini-Ice Age. Now another group of scientists, led by Todd Surovell (bottom right), came up with very different results while trying to replicate Firestone's findings.

Now another group of scientists, led by Todd Surovell of the University of Wyoming, say otherwise. In a report in the same journal, the Surovell team claims that the evidence for the meteor strike does not hold up. So how can we tell who's right?

Unearthing a Mystery

Without a doubt, we know that the landscape of North America 13,000 years ago was very different than it is now. The planet was only just emerging from a deep, 10,000 year ice age, called the Pleistocene epoch. Most of the Northern Great Plains were covered by receding glaciers, but temperatures were starting to warm up.

Suddenly, temperatures dropped again, in a climatic period called the Younger Dryas (after a plant that flourished during that time), and remained icy for another thousand years. At the same time, an early human civilization all but disappeared, and the huge animals they had hunted vanished. But were these three events related, and, if so, what could have caused them?

Unfortunately, there's no written record of events dating back 13,000 years. Finding out what was happening to our planet, our ancestors, and the other life on Earth requires careful study of evidence that's been buried underground for thousands of years. One basic assumption we have to make is that things that happened around the same time will leave their traces together — imagine digging through your laundry basket and finding all the clothes you wore four days ago in the same layer.

Scientists who study ancient events are very careful diggers. And they've found some pretty interesting things by digging in the layers corresponding to the period dating back 13,000 years.

David Nockels/Bridgeman Art Library/Getty Images

Thirteen thousand years ago, ancient humans roamed North America, hunting giant animals — called megafauna — with flint-tipped spears. The Clovis culture existed for less than 200 years, disappearing about the same time as the North American megafauna.

First, they've found a lot of evidence that early humans in North American had highly evolved technology for the time. Sites where these people — called Clovis people — lived are scattered around North America, in areas we now call "Clovis sites." All the so-called Clovis sites contain the high-tech stone tools, like spear points, that the Clovis people developed. [See America: The First Family, July 2005; Monte Verde Breaks the Clovis Barrier, April 1997; Amazon Cave Sheds Light On Early Americans, July 1996]

Most scientists agree that the Clovis people's culture didn't exist for very long (perhaps no longer than 200 years), and that their overlap with North American megafauna was brief. Scientist have used wood and other organic matter found near Clovis artifacts to pinpoint the date that they were buried, using a technique called radiocarbon dating. [See Six Sure-Fire Methods to Get You a Date, August 2001]

They can also determine what happened before and after the Clovis people lived by examining the layers above and below the Clovis artifacts. The bones of megafauna — giant bears, saber-toothed cats, and mammoths — are found within and below the layers containing Clovis artifacts, but not above them; this means that the megafauna disappeared around the same time as the Clovis. Some scientists even believe that the Clovis people, with their high-tech spear points, were responsible for killing off the megafauna — hunting the enormous creatures into extinction.

Adding to the mystery, an intriguing, 4-inch-deep deposit of dark soil is often found just above the Clovis layer, making it slightly less ancient than the Clovis layer. This layer, called a "black mat," is very carbon-rich, like soot. The origin of this sooty layer is the subject of much debate: some scientists think it might represent actual soot from wildfires, while others envision widespread swamps that laid down thick layers of dark, organic material. Could this black mat be evidence of fires that wiped out the Clovis and the North American megafauna? Unraveling the origin of the black layer could help explain the mysterious disappearance of the Clovis and their huge prey.

A Unifying Theory?

In 2007, a group of scientists led by Richard Firestone said they'd found the key to the mystery of the Younger Dryas, the black mat formation, the megafauna extinction, and the disappearance of the Clovis. They reported evidence for the dramatic scenario outlined above: 12,900 years ago, one or more comets or meteors smashed into Earth's atmosphere, creating an explosion so massive that it melted ice sheets and started wildfires across North America. [See Extraterrestrial Impact: Studying a Mini-Ice Age Mystery, October 2007]

The theory holds that the explosions and impacts would have spewed dust into the atmosphere, partially blocking the sun and choking animals and humans alike. At the same time, the wildfires that scorched the land would have further filled the atmosphere with smoke. Darkened skies would block the warmth of the sun, explaining the dip in temperatures in the Younger Dryas cooling period. Animals that didn't die in the fires would have starved in a land devoid of vegetation, with little sunlight for photosynthesis. The Clovis people might have met a similar fate.

PNAS
A 4-inch-deep deposit of carbon-rich, soot-like soil, called a "black mat," is often found just above the Clovis layer.

To support this theory, Firestone's group had carefully studied samples of soil around the black mats at Clovis sites, and had found telltale evidence of an impact and ensuing fires at each of them. They found soot, charcoal, carbon microspherules, and microscopic diamonds (known products of forest fires or large impacts); tiny magnetic particles containing traces of nickel and iridium (metals that likely come from extraterrestrial sources, like comets); and cage-shaped carbon molecules called fullerenes with bits of extraterrestrial helium trapped inside.

Searching for Validation

These findings were compelling, and many people took notice. However, not all researchers agreed with the conclusions that Firestone and his colleagues had drawn. Some people suggested that the extraterrestrial material, like the fullerenes and the iridium, could have come from the normal influx of cosmic dust into Earth's atmosphere, rather than from a comet strike. Others found no evidence of the widespread charcoal across North America that would have resulted from the proposed wildfires. Lots of questions about the original study remained.

One of the principles of a strong scientific theory is that different researchers can test it, independently; if the theory is correct, the scientists should be able to reach a consensus. Recently, the group of researchers led by Surovell tried to replicate the findings of the Firestone paper. So did this compelling theory stand up to independent testing? Well, not exactly.

Surovell and his colleagues reasoned that, if the comet theory were correct, they should be able to find the tiny magnetic microspherules all across North America. They selected a number of dig sites, some of which were the same as in the Firestone study, as well as some new sites. They were very careful to use the exact same methods to collect and analyze their samples as the researchers in the Firestone study had used.

However, when they examined soil layers corresponding to the time of the supposed comet strike — 12,900 years ago, at the beginning of the Younger Dryas — they found hardly any magnetic particles or microspherules at most sites. And even at the sites where they found the most particles, particle levels were about the same as in other layers of the sites corresponding to times when no comets are thought to have hit Earth.

To explain the discrepancy, Surovell and his colleagues suggest that the dating techniques that Firestone used might have been unreliable. For example, many of the dig sites had a history of water flowing through them; flowing water could easily mix up the layers, thus confusing the sequence of events.

However, it's important to point out that the new study hasn't completely discredited the old one. Surovell's study only looked at one of the markers of a possible comet impact that the earlier Firestone study looked at: tiny magnetic particles. In fact, one recent article did find high numbers of nanodiamonds in the layers around 12,900 years ago — just like the Firestone study did. It may be that other researchers may find some of the other markers, like fullerenes.

But the basic fact remains: two different groups of scientists have now come up with completely different answers to the same question.

So what does this mean for the disappearance of the megafauna and the Clovis people? Were they, as many think occurred with the dinosaurs, wiped out by a massive comet or meteor striking the Earth? The answer is that it's still an open question. As it turns out, this kind of disagreement happens in science all the time — and it's actually not a bad thing.

In fact, disagreements like this one prove that science is still working the way it should: theories are being formed, tested, and then retested by different scientists. This process is essential across all fields of science, and helps us ensure that we're ultimately getting closer to the real story. In time, as more research is done and the various findings are carefully sifted, a consensus may emerge in favor of one hypothesis or another, or possibly develop around a new theory that incorporates some elements of each hypothesis.

Discussion Questions

Why is it important for scientific findings to be replicated independently by different scientists? If a finding can't be replicated, does it necessarily mean that it was wrong? Why or why not?

Can you think of reasons other than the possibility of water mixing up the soil layers at the dig sites that the two different groups of researchers might have come up with different results?

Journal Abstracts and Articles

(Researchers' own descriptions of their work, summary or full-text, on scientific journal websites).

"Evidence for an Extraterrestrial Impact 12,900 Years Ago that Contributed to the Megafaunal Extinctions and the Younger Dryas Cooling." www.pnas.org/ content/ 104/ 41/ 16016.abstract.

"An Independent Evaluation of the Younger Dryas Extraterrestrial Impact Hypothesis." www.pnas.org/ content/ 106/ 43/ 18155.abstract.

Bibliography

Kennett, D.J., et al. "Nanodiamonds in the Younger Dryas Boundary Sediment Layer." Science (January 2, 2009) [accessed November 3, 2009]: www.sciencemag.org/ cgi/ content/ abstract/ 323/ 5910/ 94.

Marlon, J.R., et al. "Wildfire Responses to Abrupt Climate Change in North America." Proceedings of the National Academy of Sciences (February 24, 2009) [accessed November 3, 2009]: www.pnas.org/ content/ 106/ 8/ 2519.abstract .

Timmer, John. "Asteroid impact-driven climate change called into question." Ars Technica (October 12, 2009) [accessed October 31, 2009]: arstechnica.com/ science/ news/ 2009/ 10/ irreproducible- results- raise- doubts- about-ice- age- impact.ars? utm_source= rss& utm_medium= rss& utm_campaign= rss.

Keywords

Clovis people, Younger Dryas, Mini�Ice Age, megafauna, Richard Firestone, Todd Surovell