The
Younger Dryas impact event or
Clovis comet hypothesis refers to the
hypothesizedA hypothesis is a proposed explanation for an observable phenomenon. The term derives from the Greek, ὑποτιθέναι - hypotithenai meaning "to put under" or "to suppose." For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...
large
air burstAn air burst is the detonation of an explosive device such as an anti-personnel artillery shell or a nuclear weapon in the air instead of on contact with the ground or target or a delayed armor piercing explosion...
or earth
impactAn impact event is the collision of a large meteorite, asteroid, comet, or other celestial object with the Earth or another planet. Impact events have been a plot and background element in science fiction since knowledge of real impacts became established in the scientific mainstream.-Sizes and...
of an object or objects from
outer spaceOuter space comprises the relatively empty regions of the universe outside the atmospheres of celestial bodies. Outer space is used to distinguish it from airspace and terrestrial locations....
that initiated the
Younger DryasThe Younger Dryas stadial, named after the alpine/tundra wildflower Dryas octopetala, and also referred to as the Big Freeze, was a geologically brief cold climate period following the Bølling/Allerød interstadial at the end of the Pleistocene between approximately 12,800 to 11,500 years ago, and...
cold spell about 12,900 BP calibrated (10,900 BP uncalibrated).
One scenario proposes that an air burst and/or earth impact with a rare swarm of carbonaceous chondrites or
cometA comet is a Small Solar System Body that has coma and is bigger than a meteoroid. When close enough to the Sun, a comet exhibits a visible coma , and sometimes a tail, both because of the effects of solar radiation upon the comet's nucleus...
s set vast areas of the
North AmericanNorth American generally refers to an entity, people, group, or attribute of North America, especially of the United States and Canada together.-Culture:*North American English, a collective term used to describe American English and Canadian English...
continent on fire, causing the extinction of most of the large animals in
North AmericaNorth America is the northern continent of the Americas, situated in the Earth's northern hemisphere and in the western hemisphere. It is bordered on the north by the Arctic Ocean, on the east by the North Atlantic Ocean, on the southeast by the Caribbean Sea, and on the west by the North Pacific...
and the demise of the North American
Clovis cultureThe Clovis culture is a prehistoric Paleoindian culture that first appears in the archaeological record of North America around 11,500 rcbp radiocarbon years ago, at the end of the last glacial period, characterized by a particular tool kit adapted to the hunting of large mammals...
at the end of the last glacial period.
The
Younger Dryas impact event or
Clovis comet hypothesis refers to the
hypothesizedA hypothesis is a proposed explanation for an observable phenomenon. The term derives from the Greek, ὑποτιθέναι - hypotithenai meaning "to put under" or "to suppose." For a hypothesis to be put forward as a scientific hypothesis, the scientific method requires that one can test it...
large
air burstAn air burst is the detonation of an explosive device such as an anti-personnel artillery shell or a nuclear weapon in the air instead of on contact with the ground or target or a delayed armor piercing explosion...
or earth
impactAn impact event is the collision of a large meteorite, asteroid, comet, or other celestial object with the Earth or another planet. Impact events have been a plot and background element in science fiction since knowledge of real impacts became established in the scientific mainstream.-Sizes and...
of an object or objects from
outer spaceOuter space comprises the relatively empty regions of the universe outside the atmospheres of celestial bodies. Outer space is used to distinguish it from airspace and terrestrial locations....
that initiated the
Younger DryasThe Younger Dryas stadial, named after the alpine/tundra wildflower Dryas octopetala, and also referred to as the Big Freeze, was a geologically brief cold climate period following the Bølling/Allerød interstadial at the end of the Pleistocene between approximately 12,800 to 11,500 years ago, and...
cold spell about 12,900 BP calibrated (10,900 BP uncalibrated).
One scenario proposes that an air burst and/or earth impact with a rare swarm of carbonaceous chondrites or
cometA comet is a Small Solar System Body that has coma and is bigger than a meteoroid. When close enough to the Sun, a comet exhibits a visible coma , and sometimes a tail, both because of the effects of solar radiation upon the comet's nucleus...
s set vast areas of the
North AmericanNorth American generally refers to an entity, people, group, or attribute of North America, especially of the United States and Canada together.-Culture:*North American English, a collective term used to describe American English and Canadian English...
continent on fire, causing the extinction of most of the large animals in
North AmericaNorth America is the northern continent of the Americas, situated in the Earth's northern hemisphere and in the western hemisphere. It is bordered on the north by the Arctic Ocean, on the east by the North Atlantic Ocean, on the southeast by the Caribbean Sea, and on the west by the North Pacific...
and the demise of the North American
Clovis cultureThe Clovis culture is a prehistoric Paleoindian culture that first appears in the archaeological record of North America around 11,500 rcbp radiocarbon years ago, at the end of the last glacial period, characterized by a particular tool kit adapted to the hunting of large mammals...
at the end of the last glacial period. This swarm would have exploded above or even into the
Laurentide Ice SheetThe Laurentide Ice Sheet was a massive sheet of ice that covered hundreds of thousands of square miles, including most of Canada and a large portion of the northern United States, between c. 95,000 and c. 20,000 years before the present day...
north of the
Great LakesThe Great Lakes are a collection of freshwater lakes located in eastern North America, on the Canada – United States border. Consisting of Lakes Superior, Michigan, Huron, Erie, and Ontario, they form the largest group of freshwater lakes on Earth. They are sometimes referred to as the "Third...
. An airburst would have been similar to but many orders of magnitude larger than the
Tunguska eventThe Tunguska Event, or Tunguska explosion, was a powerful explosion that occurred near the Podkamennaya Tunguska River in what is now Krasnoyarsk Krai of Russia, at around 7:14 a.m...
of 1908. Animal and human life not directly killed by the blast or the resulting coast to coast
wildfireA wildfire is any uncontrolled fire that occurs in the countryside or a wilderness area. Reflecting the type of vegetation or fuel, other names such as brush fire, bushfire, forest fire, grass fire, hill fire, peat fire, vegetation fire, and wildland fire may be used to describe the same phenomenon...
s would have starved on the burned surface of the continent.
The scenario is the product of a team of geologists at American universities, among them James Kennett of the University of California Santa Barbara, Richard Firestone of
Lawrence Berkeley National LaboratoryThe Ernest Orlando Lawrence Berkeley National Laboratory , is a U.S. Department of Energy national laboratory conducting unclassified scientific research. It is located on the grounds of the University of California, Berkeley, in the Berkeley Hills above the central campus. It is managed and...
, as well as archaeologists Douglas Kennett and Jon Erlandson of the
University of OregonThe University of Oregon is a public, coeducational research university in Eugene, Oregon, United States. The second oldest public university in the state, UO was founded in 1876, and graduated its first class two years later. The University of Oregon is one of 60 members of the Association of...
.
Evidence
The evidence claimed for an impact event includes a charred carbon-rich layer of soil that has been found at some 50 Clovis-age sites across the continent. The layer contains unusual materials (nanodiamonds, metallic microspherules, carbon spherules, magnetic spherules,
iridiumIridium is the chemical element with atomic number 77, and is represented by the symbol Ir. A very hard, brittle, silvery-white transition metal of the platinum family, iridium is the second densest element and is the most corrosion-resistant metal, even at temperatures as high as 2000 °C...
, charcoal, soot, and
fullereneA fullerene is any molecule composed entirely of carbon, in the form of a hollow sphere, ellipsoid, or tube. Spherical fullerenes are also called buckyballs, and cylindrical ones are called carbon nanotubes or buckytubes...
s enriched in
helium-3Helium-3 is a light, non-radioactive isotope of helium with two protons and one neutron. It is rare on Earth, and is sought for use in nuclear fusion research...
) interpreted as evidence of an impact event, at the very bottom of the "black mat" of organic material that marks the beginning of the Younger Dryas.
Results
It is conjectured that this impact event brought about the extinction of many North American large mammals. These animals included
camelCamels are even-toed ungulates within the genus Camelus. The dromedary or Arabian camel has a single hump, and the Bactrian camel has two humps. They are native to the dry desert areas of western Asia, and central and east Asia, respectively...
s,
mammothA mammoth is any species of the extinct genus Mammuthus. These proboscideans are members of Elephantidae, the family of elephants and mammoths, and close relatives of modern elephants. They were often equipped with long curved tusks and, in northern species, a covering of long hair. They lived from...
s, the giant short-faced bear and numerous other species. The markers for the impact event also appear at the end of the
Clovis cultureThe Clovis culture is a prehistoric Paleoindian culture that first appears in the archaeological record of North America around 11,500 rcbp radiocarbon years ago, at the end of the last glacial period, characterized by a particular tool kit adapted to the hunting of large mammals...
.
History of the hypothesis
The British science journal
NatureNature is a prominent British scientific journal, first published on 4 November 1869. Most scientific journals are now highly specialized, and Nature is among the few journals that still publish original research articles across a wide range of scientific...
addressed the theory in a news story on 17 May 2007. On 24 May 2007, a session at the spring 2007 joint assembly of the
American Geophysical UnionThe American Geophysical Union is a nonprofit organization of geophysicists, consisting of over 50,000 members from over 135 countries. AGU's activities are focused on the organization and dissemination of scientific information in the interdisciplinary and international field of geophysics...
in Acapulco, Mexico was held to discuss this hypothesis and reveal the evidence. On 27 September 2007, a paper presenting the findings of the Acapulco group was pre-published online at the
Proceedings of the National Academy of SciencesThe Proceedings of the National Academy of Sciences of the United States of America, usually referred to as PNAS, is the official journal of the United States National Academy of Sciences...
website. According to the study, the impact event may have led to an immediate decline in human populations in North America at that time.
Less than a year later substantial support for the synchronous nature of the black mat was provided by leading Clovis archaeologist, C. Vance Haynes, also in the PNAS. Says Haynes:
Further analysis is in progress and other Clovis sites need independent study and verification of this evidence. Until then I remain skeptical of the ET impact hypothesis as the cause of the YD onset and the megafaunal extinction. However, I reiterate, something major happened at 10,900 B.P. that we have yet to understand.
The theory drew new scrutiny in March 2008 at the annual meeting of the
Society for American ArchaeologyThe Society for American Archaeology is the largest organization of professional archaeologists of the Americas in the world. The Society was founded in 1934 and today has over 7000 members. The Society holds an annual conference and publishes the flagship journal of American archaeology,...
in Vancouver, Canada. In August 2008, at the annual Pecos Archaeological Conference Allen West, a lead proponent of the Clovis Comet theory, and Ted Bunch, a co-author of the original PNAS paper and former
NASAThe National Aeronautics and Space Administration is an agency of the United States government, responsible for the nation's public space program. NASA was established by the National Aeronautics and Space Act on July 29, 1958, replacing its predecessor, the National Advisory Committee for...
chief of exobiology, presented new evidence, and participated in a panel discussion of the findings with Sandia Labs asteroid impact modeler
Mark BosloughMark Boslough is a physicist most famous for his April Fools' Day joke involving Pi. To poke fun at New Mexico's legislature for attempting to require schools to teach creationism, he wrote an article claiming that Alabama state legislature voted to change the value of the mathematical constant pi...
, and comet hunter Carolyn Shoemaker.
Independent verification of Firestone and West's identification of ET material in Clovis stratigraphy was presented by Mustafa Fayek and Sharon Hull of the
University of ManitobaThe University of Manitoba, in Winnipeg, Canada, is the largest university in the province of Manitoba. It is also Manitoba's most comprehensive and only research-intensive post-secondary educational institution.. It was founded in 1877, making it Western Canada’s first university. It placed in...
.
Most recently, In January 2009, transmission electron microscopy evidence showing nanodiamonds from the geologic moment of the event was published in the journal
Science and reviewed in the International Herald Tribune. Also, in the same issue, D.J. Kennett reported that:
These diamonds provide strong evidence for Earth's collision with a rare swarm of carbonaceous chondriteChondrites are stony meteorites that have not been modified due to melting or differentiation of the parent body. They formed when various types of dust and small grains that were present in the early solar system accreted to form primitive asteroids...
s or cometA comet is a Small Solar System Body that has coma and is bigger than a meteoroid. When close enough to the Sun, a comet exhibits a visible coma , and sometimes a tail, both because of the effects of solar radiation upon the comet's nucleus...
s at the onset of the Younger Dryas cool interval, producing multiple airbursts and possible surface impacts, with severe repercussions for plants, animals, and humans in North America.
Criticisms of the hypothesis
A study of Paleoindian demography published in August 2008 (almost a year after the first publication in PNAS) states "The results of the analyses were not consistent with the predictions of extraterrestrial impact hypothesis. No evidence of a population decline among the Paleoindians at 12,900 ± 100 calBP was found. Thus, minimally, the study suggests the extraterrestrial impact hypothesis should be amended."
Since the effects of the putative impact on Earth's biota would have been brief, all extinctions caused by the impact should have occurred simultaneously. However, there is evidence that the
megafaunal extinctionsThe Quaternary epoch saw the extinctions of numerous predominantly larger species, many of which occurred during the transition to the Holocene epoch in what is termed the Holocene extinction. Among the main causes hypothesized by paleontologists are the spread of disease, natural climate change,...
that occurred across northern Eurasia, North America and South America at the end of the
PleistoceneThe Pleistocene is the epoch from 2.588 million to 12 000 years BP covering the world's recent period of repeated glaciations. The name pleistocene is derived from the Greek and ....
were not synchronous. The extinctions in South America appear to have occurred at least 400 years after the extinctions in North America. The extinction of
woolly mammothThe woolly mammoth , also called the tundra mammoth, is an extinct species of mammoth. This animal is known from bones and frozen carcasses from northern North America and northern Eurasia with the best preserved carcasses in Siberia.This mammoth species was first recorded in deposits of the...
s in
SiberiaSiberia , is the vast region constituting almost all of Northern Asia and for the most part currently serving as the massive central and eastern portion of the Russian Federation, having served in the same capacity previously for the USSR from its beginning, and the Russian Empire beginning in the...
also appears to have occurred later than in North America. A greater disparity in extinction timings is apparent in island megafaunal extinctions that lagged nearby continental extinctions by thousands of years; examples include the survival of woolly mammoths on
Wrangel IslandWrangel Island is an island in the Arctic Ocean, between the Chukchi Sea and East Siberian Sea. Wrangel Island lies astride the 180° meridian. The International Date Line is displaced eastwards at this latitude to avoid the island as well as the Chukchi Peninsula on the Russian mainland...
until 3700 BP, and the survival of
ground slothsThe ground sloths of the genus Megalocnus were among the largest of the Caribbean ground sloths, with individuals estimated to have weighed up to 90 kilos when alive. Two species are known, M. rodens of Cuba, and M. zile of Hispaniola. Subfossils of M...
in the
AntillesThe Antilles refers to the islands forming the greater part of the West Indies in the Caribbean Sea...
until 4700 cal BP.
Some extant megafaunal species such as
bisonThe Wood Bison or Wood Buffalo is a distinct northern subspecies of the North American Bison. Its original range included much of the boreal forest regions of Alaska, Yukon, western Northwest Territories, northeastern British Columbia, northern Alberta, and northwestern Saskatchewan...
and
grizzliesThe brown bear is a large bear distributed across much of northern Eurasia and North America. It weighs 100 to 680 kilograms and its largest subspecies, the Kodiak Bear, rivals the polar bear as the largest member of the bear family, and as the largest land based predator.While the brown...
seem to have been little affected by the extinction event, while the environmental devastation caused by a bolide impact would not be expected to discriminate.
External reading
External links