Around 13,000 years ago, a chilly Earth was thawing. It was the end of the last great ice age. Vast frozen sheets had covered much of North America, Europe and Asia for thousands of years. Now they were retreating. Giant mammals like woolly mammoths and saber-toothed cats roamed the tundra and grasslands. An ancient people called the Clovis roamed North America, hunting mammoth with distinctive spears.
Then, about 12,800 years ago, something strange happened. Earth abruptly plunged back into a deep chill. Temperatures in parts of the Northern Hemisphere fell to as much as 8 degrees Celsius (almost 14.5 degrees Fahrenheit) colder than today. Then, just as abruptly, Earth once again began to warm.
The cold snap lasted only about 1,200 years. That’s a mere blip in the history of our planet. But afterward, many of the giant mammals were dying out. And signs of the Clovis people were now gone.
Geologists call this cold snap the Younger Dryas. Its cause is a mystery. Most researchers think a large pulse of freshwater from melting ice flooded into the ocean. This might have interfered briefly with ocean currents that carry heat around the world. However, geologists have not yet found firm evidence of how and where this happened. For example, they haven’t found traces of the path that this ancient flood took to the sea.
But for more than a decade, one group of researchers has made waves by suggesting a more controversial story. They blame a space rock for the sudden deep freeze. About 12,800 years ago, these researchers say, a comet — or pieces of a comet — hit or exploded over the Laurentide Ice Sheet. This ice sheet once covered much of North America.
Pieces of the comet most likely exploded in Earth’s atmosphere, the researchers say. This kind of aboveground explosion is called an airburst. And this triggered wildfires across North America. Those fires gave off enough soot and other compounds to block out the sun and cool the planet.
Most scientists think a similar explosion happened in 1908 over Siberia. That event produced as much energy as 1,000 Hiroshima atomic bombs. The researchers behind the impact hypothesis think the Younger Dryas started with an even larger explosion. That would neatly solve several prehistoric puzzles, they note. For instance, it could explain the extinction of large animals as well as the disappearance of the Clovis people.
Many scientists question the space rock theory
For more than a decade, researchers have argued in scientific journals about this hypothesis. Experts from a wide range of scientific fields think the impact hypothesis is wrong. They say there’s little to no solid evidence for many of its key points.
“Over and over and over, there are these things that are claimed to be [indirect evidence of] an impact,” says Vance Holliday. He’s an archaeologist and geologist at the University of Arizona in Tucson. But these bits of so-called evidence are “all debatable,” he says — “every single one.”
Allen West disagrees. He’s long been a lead supporter of the impact hypothesis. West is a retired geophysicist who owned GeoScience Consulting in Dewey, Ariz. He admits that many types of scientists have attacked the hypothesis. “We have different battles with different disciplines,” he says.
He compares these battles to the fights in the 1980s over whether an asteroid struck Earth 66 million years ago, killing off all dinosaurs except the birds.
“There were just vicious, nasty attacks for nearly a decade on that,” West says. “People said it just couldn’t have happened.” But then the crater turned up. Today, the asteroid-based extinction is widely accepted. Hard-to-refute evidence, like that telltale crater, he says, is “probably what it would take with us, too.”
Indeed, no craters have been found from the Younger Dryas. And the landscape of North America — the likely site of such an impact — has been pretty thoroughly checked out. Without a crater or other direct evidence of an impact, West and his colleagues have turned to indirect evidence. They’ve published a steady stream of findings that they say are signs of an impact some 12,800 years ago.
The latest came out in March. West and more than two dozen other researchers published a pair of papers in the Journal of Geology. These papers included data from ice cores and sediment cores — tube-shaped samples drilled from land or under the sea. The cores held signatures of giant wildfires that may have happened during widespread burning about 12,800 years ago, West notes.
Some opponents of the impact hypothesis, however, just feel fed up by the new papers. For 10 years, “they keep building on their past record, ignoring the critiques,” Holliday says. “It just drives me crazy.”
Birth of a hypothesis
The world first heard about the Younger Dryas impact hypothesis in 2007.
Four researchers, including West, sat in front of reporters at the American Geophysical Union’s spring meeting. It was held in Acapulco, Mexico. The researchers had taken a close look at about two dozen sites across North America. Underground, the sites all showed a “boundary layer” — a stripe in the sediment dating to the start of the Younger Dryas. Half a dozen of the sites also had thin layers called “black mats” right above the boundary layer. Several of those half-dozen sites also showed signs that the Clovis people lived there.
The black layers were made of soft soil with a lot of nutrients. They apparently marked when the Clovis people stopped living at these sites. For example, a black mat at Murray Springs, in Arizona, sits above a trove of Clovis artifacts. These include a fire pit and the almost complete skeleton of a mammoth. But no such artifacts exist above the mat.
In fact, the stone spear tips made by the Clovis culture abruptly disappear from soils above that time period at Murray Springs. The same is true at other sites. This has made researchers think the Clovis people suddenly and mysteriously vanished.
Back in 2007, West and his colleagues noted that those Younger Dryas boundary layers held many intriguing items. These included tiny round magnetized grains called microspherules (My-kroh-sfeer-yools). Other magnetized sediment grains showed up there, too. There were even tiny carbon balls (hollow round carbon molecules called fullerenes) and super-tiny diamonds. Unusually high levels of the elements iridium (Ih-RID-ee-um) and nickel also marked these layers, as did signs of burning: charcoal and soot.
None of these clues, on its own, could prove there had been an impact from space. Microspherules, for instance, form when a material heats up and then rapidly cools. A strike from a space rock could create them. So could a volcanic eruption or industrial pollution.
But taken together, the authors charge, these clues add up to only one thing: Something hit Earth or exploded high in its atmosphere about 12,800 years ago. The soot and charcoal hint that the impact triggered widespread fires. And smoke from those fires could have blocked out the sun, causing some 1,000 years of frigid temperatures throughout the Northern Hemisphere.
Because no impact crater from this time has turned up, the researchers say the space rock was probably already in pieces when it entered Earth’s atmosphere. Smaller fragments would have done plenty of damage as they exploded over the shrinking Laurentide Ice Sheet. But they wouldn’t have left much of a smoking hole in the ground.
When West and his colleagues first shared their theory, the news made a splash. Other scientists were intrigued. The idea that clues to an ancient impact were buried across the continent was definitely worth probing further. Mark Boslough says that at first he accepted the data. Boslough is a physicist at the University of New Mexico in Albuquerque. “I thought, ‘they’re onto something interesting,’” he recalls.
Then Boslough and other scientists began to analyze the data on their own. And they turned up questions.
Some researchers claimed that they couldn’t find strong evidence of impact clues such as those microspherules and nanodiamonds. Others questioned how precisely the researchers knew the ages of many Younger Dryas boundary layers. If the ages weren’t exact, it would call into question the idea that one event affected all of the sites at the same time.
Boslough says he also had a problem with the physics of the impact hypothesis. The researchers had suggested different scenarios. These range from a single object striking the ice sheet to multiple pieces exploding high in the air. But none of the scenarios made sense, Boslough wrote in 2012. Either the pieces of a broken-up comet would have been too small to do much, or they would have been too large not to leave craters.
Holliday raises another issue. He says that the apparent disappearance of the Clovis culture isn’t really a big mystery that needs to be solved. Hunter-gatherers like the Clovis people didn’t stay at one site for long. So it’s no surprise, he says, that they would have left their artifacts behind and moved on.
Immediately after the Clovis period, he adds, a different style of spear points appeared. These artifacts are called Folsom points. Ancient peoples probably just changed their spear technology as they shifted from hunting mammoths and mastodons to bison, Holliday says.
Those large Ice Age animals such as mammoths, he adds, didn’t suddenly disappear either. Their numbers were already dropping. “All these animals running around and then, boom, at 12,800 years ago they just go away? That’s just not the case,” Holliday says. “These extinctions were global and happened at different times around the world.”
The latest papers from the supporters of the impact hypothesis focus mainly on wildfires. These fires were part of the original hypothesis.
Ammonium (Ah-MOH-nee-um) is an electrically charged molecule derived from ammonia. Ice cores from Greenland show that there was more ammonium in the ice at the start of the Younger Dryas. The researchers say this suggests large-scale burning of biomass — plants and other living things. Adrian Melott and his colleagues first shared these data in 2010. Melott is an astrophysicist at the University of Kansas in Lawrence.
The researchers said that the best explanation for ammonium ions in those ice cores was an impact from space. Scientists have found a similar spike in those same ice cores that dates to 1908 — the year of the airburst over Siberia. The new papers also describe finding charcoal peaks that date to the start of the cold snap.
The papers combine all of these possible signs of an impact and try to compare when they happened, says Melott. He’s also one of the authors on the new impact papers. Those markers include evidence of microspherules, iridium and platinum dust that researchers have previously described. All of these markers could have been caused by the same event, he says.
Jennifer Marlon has taken her own look at sediments in North America that were deposited between 15,000 and 10,000 years ago. Marlon studies ancient ecology and climate at Yale University in New Haven, Conn. She’s also an expert on biomass burning. She sees no evidence that the beginning of the Younger Dryas, specifically, was a time of continent-wide fires.
“I’ve studied charcoal records for many years now,” Marlon says. In 2009, she and colleagues reported data on sediments from the bottoms of North American lakes. They studied charcoal and pollen in those sediments. Importantly, her study covered not only the years of the Younger Dryas cold snap, but also a few thousand years before and after.
Her team turned up signs of multiple small wildfires. But none were near the beginning of the Younger Dryas. “Forests burn in North America all the time,” she says. “You can’t find a cubic centimeter of sediment in any lake on this continent that doesn’t have charcoal in it.”
Such fires could have been triggered by rapid climate change. As ecosystems quickly shift, more dead fuel might become available. “That can cause major vegetation changes and fires,” Marlon says. “We don’t need to [blame] a comet.”
The problem with the data in the recent papers, Marlon says, is that the researchers look only at a narrow time period. That makes it hard to know how large or unusual the signals really were. From her data, there may have been more burning toward the end of the Younger Dryas, when the planet began to again warm abruptly.
That’s her biggest problem with the part of the new papers about biomass burning. “I don’t understand why they’re zooming in [on that],” she says. “It’s what makes me skeptical.”
Holliday has the same concern. “Most of the time they sample only around this time interval,” he says. He thinks it would be better to zoom out and look for more fires in sediment cores that span 15,000 to 20,000 years. “If this is a unique event, then we shouldn’t see anything like it in the last 15,000 years.”
West says that isn’t necessary, and that the team was just focusing on one big fire from around 12,800 years ago. The new papers also suggest that Marlon and her colleagues didn’t correctly interpret the ages of their samples, he says.
Marlon disagrees. She says that she analyzed and compared her sites in several different ways. Still, she found no unusual peak at 12,800 years. In fact, she says, many of the sites show no signs of burning at that time.
A complicated story
West says he and the other supporters of the impact hypothesis haven’t ignored their critics. “We have directly rebutted those criticisms multiple times,” he says. He and others are working on another new paper that will describe in detail their arguments to counter their critics’ charges.
Those critics say too many questions remain unanswered. Holliday says he and others are preparing a response to the Journal of Geology papers. It will outline many points on which they disagree. These include the proper dating of rock layers and soils. They also include data that seem to contradict how and why animals went extinct and human artifacts disappeared.
“We all love a good debate,” Marlon says, “but I know there’s a lot of frustration in the community” that this hypothesis is still around. Indeed, the data presented in the new papers have done nothing to change her mind about the comet strike. “It didn’t happen,” she says.
Yet Marlon understands why the other scientists are sticking to their comet story.
“I have had pet theories, too,” she says. “We are pattern-seekers. We tend to see things that look like a signal and so many times they’re not. A comet is simpler and more visually compelling — more appealing, in a way — than trying to sort out what Earthbound trigger might have caused such an abrupt climate change.
“I wish the evidence were stronger for [an impact],” she says. “It’s not as much fun when it turns out to be a more complicated, nuanced story.”
airburst The term for an explosion that occurs high in the atmosphere. It usually refers to a large explosion, such as one made by a nuclear weapon or the breakup of a meteor.
ammonia A colorless gas with a nasty smell. Ammonia is a compound made from the elements nitrogen and hydrogen. It is used to make food and applied to farm fields as a fertilizer. Secreted by the kidneys, ammonia gives urine its characteristic odor. The chemical also occurs in the atmosphere and throughout the universe.
artifact An object made by people.
asteroid A rocky object in orbit around the sun. Most asteroids orbit in a region that falls between the orbits of Mars and Jupiter. Astronomers refer to this region as the asteroid belt.
astrophysics An area of astronomy that deals with understanding the physical nature of stars and other objects in space. People who work in this field are known as astrophysicists.
atmosphere The envelope of gases surrounding Earth or another planet.
atomic Having to do with atoms, the smallest possible unit that makes up a chemical element.
biomass Matter that contains carbon and can be used as a fuel, especially in a power station for the generation of electricity. Plants are a kind of biomass.
birds Warm-blooded animals with wings that first showed up during the time of the dinosaurs. Birds are jacketed in feathers and produce young from the eggs they deposit in some sort of nest. Most birds fly, but throughout history there have been the occasional species that don’t.
carbon The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.
climate The weather conditions that typically exist in one area, in general, or over a long period.
climate change Long-term, significant change in the climate of Earth. It can happen naturally or in response to human activities, including the burning of fossil fuels and clearing of forests.
Clovis people Prehistoric humans who inhabited much of North America between about 13,000 and 12,600 years ago. They are known primarily by the cultural artifacts they left behind, especially a type of stone point used on hunting spears. It’s called the Clovis point. It was named after Clovis, New Mexico, where someone first found this type of stone tool.
colleague Someone who works with another; a co-worker or team member.
comet A celestial object consisting of a nucleus of ice and dust. When a comet passes near the sun, gas and dust vaporize off the comet’s surface, creating its trailing “tail.”
compound (often used as a synonym for chemical) A compound is a substance formed when two or more chemical elements unite (bond) in fixed proportions. For example, water is a compound made of two hydrogen atoms bonded to one oxygen atom. Its chemical symbol is H2O.
continent (in geology) The huge land masses that sit upon tectonic plates. In modern times, there are six established geologic continents: North America, South America, Eurasia, Africa, Australia and Antarctica. In 2017, scientists also made the case for yet another: Zealandia.
core (in geology) A long, tube-like sample drilled down into ice, soil or rock. Cores allow scientists to examine layers of sediment, dissolved chemicals, rock and fossils to see how the environment at one location changed through hundreds to thousands of years or more.
crater A large, bowl-shaped cavity in the ground or on the surface of a planet or the moon. They are typically caused by an explosion or the impact of a meteorite or other celestial body. Such an impact is sometimes referred to as a cratering event.
culture (n. in social science) The sum total of typical behaviors and social practices of a related group of people (such as a tribe or nation). Their culture includes their beliefs, values and the symbols that they accept and/or use. Culture is passed on from generation to generation through learning. Scientists once thought culture to be exclusive to humans. Now they recognize some other animals show signs of culture as well, including dolphins and primates.
diamond One of the hardest known substances and rarest gems on Earth. Diamonds form deep within the planet when carbon is compressed under incredibly strong pressure.
dinosaur A term that means terrible lizard. These ancient reptiles lived from about 250 million years ago to roughly 65 million years ago. All descended from egg-laying reptiles known as archosaurs.
ecology A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.
ecosystem A group of interacting living organisms — including microorganisms, plants and animals — and their physical environment within a particular climate. Examples include tropical reefs, rainforests, alpine meadows and polar tundra. The term can also be applied to elements that make up some an artificial environment, such as a company, classroom or the internet.
element A building block of some larger structure. (in chemistry) Each of more than one hundred substances for which the smallest unit of each is a single atom. Examples include hydrogen, oxygen, carbon, lithium and uranium.(in chemistry) Each of more than one hundred substances for which the smallest unit of each is a single atom. Examples include hydrogen, oxygen, carbon, lithium and uranium.
eruption (in geoscience) The sudden bursting or spraying of hot material from deep inside a planet or moon and out through its surface. Volcanic eruptions on Earth usually send hot lava, hot gases or ash into the air and across surrounding land. In colder parts of the solar system, eruptions often involve liquid water spraying out through cracks in an icy crust. This happens on Enceladus, a moon of Saturn that is covered in ice.
extinct (n. extinction) An adjective that describes a species for which there are no living members.
freshwater A noun or adjective that describes bodies of water with very low concentrations of salt. It’s the type of water used for drinking and making up most inland lakes, ponds, rivers and streams, as well as groundwater.
fullerenes Molecules of carbon that resemble tiny, soccer ball–like cages when the chemical bonds between all of the carbon atoms are drawn. Fullerenes, which chemists first created in 1985, are nicknamed “buckyballs” after Buckminster Fuller, the famous architect and engineer who designed dome-shaped structures that resemble fullerene molecules.
geology The study of Earth’s physical structure and substance, its history and the processes that act on it. People who work in this field are known as geologists. Planetary geology is the science of studying the same things about other planets.
geoscience Any of a number of sciences, like geology or atmospheric science, concerned with better understanding Earth. People who work in this field are known as geoscientists.
Greenland The world’s largest island, Greenland sits between the Arctic Ocean and North Atlantic. Although it is technically part of North America (sitting just east of Northern Canada), Greenland has been linked more politically to Europe. Ice covers roughly 80 percent of Greenland. Indeed, the Greenland ice sheet is the world’s largest. If its frozen water were to melt, it could raise sea levels around the world by 6 meters (about 20 feet). Although this is the 12th biggest nation (based on surface area), Greenland averages the fewest people per square kilometer of its surface area.
hunter-gatherer A cultural group that feeds itself through hunting, fishing and gathering wild produce (such as nuts, seeds, fruits, leaves, roots and other edible plant parts). They can be somewhat nomadic and do not rely on agriculture for their foods.
hypothesis (v. hypothesize) A proposed explanation for a phenomenon. In science, a hypothesis is an idea that must be rigorously tested before it is accepted or rejected.
ice age Earth has experienced at least five major ice ages, which are prolonged periods of unusually cold weather experienced by much of the planet. During that time, which can last hundreds to thousands of years, glaciers and ice sheets expand in size and depth. The most recent ice age peaked 21,500 years ago, but continued until about 13,000 years ago.
ice sheet A broad blanket of ice, often kilometers deep. Ice sheets currently cover most of Antarctica. An ice sheet also blankets most of Greenland. During the last glaciation, ice sheets also covered much of North America and Europe.
ion (adj. ionized) An atom or molecule with an electric charge due to the loss or gain of one or more electrons. An ionized gas, or plasma, is where all of the electrons have been separated from their parent atoms.
iridium Discovered in 1803, its name comes from the Latin for rainbow. It’s a hard, brittle and corrosion-resistant metal in the platinum family. Slightly yellowish, the principle use for this element is as a hardener for platinum. Indeed, its melting point is more than 2,400° Celsius (4,350° Fahrenheit). The element’s atomic number is 77.
journal (in science) A publication in which scientists share their research findings with experts (and sometimes even the public). Some journals publish papers from all fields of science, technology, engineering and math, while others are specific to a single subject. The best journals are peer-reviewed: They send all submitted articles to outside experts to be read and critiqued. The goal, here, is to prevent the publication of mistakes, fraud or sloppy work.
lead A toxic heavy metal (abbreviated as Pb) that in the body moves to where calcium wants to go (such as bones and teeth). The metal is particularly toxic to the brain. In a child’s developing brain, it can permanently impair IQ, even at relatively low levels.
mammal A warm-blooded animal distinguished by the possession of hair or fur, the secretion of milk by females for feeding their young, and (typically) the bearing of live young.
marker (in biomedicine) The presence of some substance that usually can only be present because it signals some disease, pollutant or event. As such, this substance will serve as a sign — or marker — of that related thing.
molecule An electrically neutral group of atoms that represents the smallest possible amount of a chemical compound. Molecules can be made of single types of atoms or of different types. For example, the oxygen in the air is made of two oxygen atoms (O2), but water is made of two hydrogen atoms and one oxygen atom (H2O).
nickel Number 28 on the periodic table of elements, this hard, silvery element resists oxidation and corrosion. That makes it a good coating for many other elements or for use in multi-metal alloys.
nuance A term for some the small (subtle) degree by which some feature or situation differs from another.
nutrient A vitamin, mineral, fat, carbohydrate or protein that a plant, animal or other organism requires as part of its food in order to survive.
physics The scientific study of the nature and properties of matter and energy. Classical physics is an explanation of the nature and properties of matter and energy that relies on descriptions such as Newton’s laws of motion. Quantum physics, a field of study that emerged later, is a more accurate way of explaining the motions and behavior of matter. A scientist who works in such areas is known as a physicist.
planet A celestial object that orbits a star, is big enough for gravity to have squashed it into a roundish ball and has cleared other objects out of the way in its orbital neighborhood.
platinum A naturally occurring silver-white metallic element that remains stable (does not corrode) in air. It is used in jewelry, electronics, chemical processing and some dental crowns.
pollen Powdery grains released by the male parts of flowers that can fertilize the female tissue in other flowers. Pollinating insects, such as bees, often pick up pollen that will later be eaten.
prehistoric An adjective for something that happened tens of thousands to millions of years ago, periods before people began deliberately recording events.
range The full extent or distribution of something. For instance, a plant or animal’s range is the area over which it naturally exists.
saber-toothed cat Once popularly referred to as a saber-toothed tiger, this cat (Smilodon fatalis) is not closely related to tigers at all. Adults of this bobtailed species were about 30.5 centimeters (roughly one foot) shorter than today’s lions but would have weighed twice as much. Unlike lions and other big cats of Africa, the saber-toothed cat probably did not chase down its prey, but instead ambushed it from a hiding place. The species died out roughly 10,000 years ago.
scenario A possible (or likely) sequence of events and how they might play out.
sea An ocean (or region that is part of an ocean). Unlike lakes and streams, seawater — or ocean water — is salty.
sediment Material (such as stones and sand) deposited by water, wind or glaciers.
Siberia A region in northern Asia, almost all of which falls within Russia. This land takes its name from the language of the Tatar people, where Siber means sleeping land. This region is vast. It has become famous for its long, harsh winters, where temperatures can fall to −68° Celsius (−90° Fahrenheit).
skeptical Not easily convinced; having doubts or reservations.
soot Also known as black carbon, it's the sometimes oily residues of incompletely burned materials, from plastics, leaves and wood to coal, oil and other fossil fuels. Soot particles can be quite small — nanometers in diameter. If inhaled, they can end up deep within the lung.
spherule Something (usually small) that has a round shape.
theory (in science) A description of some aspect of the natural world based on extensive observations, tests and reason. A theory can also be a way of organizing a broad body of knowledge that applies in a broad range of circumstances to explain what will happen. Unlike the common definition of theory, a theory in science is not just a hunch. Ideas or conclusions that are based on a theory — and not yet on firm data or observations — are referred to as theoretical. Scientists who use mathematics and/or existing data to project what might happen in new situations are known as theorists.
trove A collection of valuable things.
tundra A cold, usually lowland area in far northern regions. The subsoil is permanently frozen. In summer, a tundra's top layer of soil thaws and can support low-growing mosses, lichens, grasses, shrubs and trees (some only a few centimeters high).
unique Something that is unlike anything else; the only one of its kind.
vegetation Leafy, green plants. The term refers to the collective community of plants in some area. Typically these do not include tall trees, but instead plants that are shrub height or shorter.
woolly mammoth A type of extinct mammal that resembled a hairy elephant.
Journal: W.S. Wolbach et al. Extraordinary biomass-burning episode and impact winter triggered by the Younger Dryas cosmic impact ~12,800 years ago. 2. Lake, Marine, and Terrestrial Sediments. The Journal of Geology. Vol. 126, March 2018, p. 185. doi: 10.1086/695703.
Journal: W.S. Wolbach et al. Extraordinary biomass-burning episode and impact winter triggered by the Younger Dryas cosmic impact ~12,800 years ago. 1. Ice Cores and Glaciers. The Journal of Geology. Vol. 126, March 2018, p. 165. doi: 10.1086/695703.
Journal: V.T. Holliday et al. The Younger Dryas impact hypothesis: a cosmic catastrophe. Journal of Quaternary Science. Vol. 29, August 8, 2014, p. 515. doi: 10.1002/jqs.2724.
Journal: M. Boslough. Inconsistent impact hypotheses for the Younger Dryas. Proceedings of the National Academy of Sciences. Vol 109, August 21, 2012, p. E2241. doi: 10.1073/pnas.1110614109.
Journal: T.A. Surovell et al. An independent evaluation of the Younger Dryas extraterrestrial impact hypothesis. Proceedings of the National Academy of Sciences. Vol. 106, October 27, 2009, p. 18155. doi: 10.1073/pnas.0907857106.
Journal: R.B. Firestone et al. Evidence for an extraterrestrial impact 12,900 years ago that contributed to the megafaunal extinctions and the Younger Dryas cooling. Proceedings of the National Academy of Sciences. Vol. 104, October 9, 2007, p. 16016. doi: 10.1073/pnas.0706977104.
Journal: J.R. Marlon et al. Wildfire responses to abrupt climate change in North America. Proceedings of the National Academy of Sciences. Vol. 106, February 24, 2009, p. 2519. doi: 10.1073/pnas.0808212106.