The Extinction Debate
by Evan Hadingham
From its first public announcement in May 2007, the hypothesis
that a comet devastated North America 12,900 years ago has
aroused intense skepticism and debate among scientists. The
debate revolves around several key questions:
-
Is there solid, unambiguous evidence for an
extraterrestrial impact or airburst at this time?
-
Did the impact cause the dramatic extinction of some 35
types of large animals, or megafauna, in North America,
also dated roughly to this period?
-
Or should we blame these extinctions on the Clovis people,
the first well-documented prehistoric hunters in North
America, who had entered the continent only two or three
centuries before?
-
If the Clovis hunters were not to blame, could a sudden,
sharp "deep freeze"—an abrupt reversal of the
climate back to Ice Age conditions—have been the
culprit?
-
Finally, could this sudden chill somehow be connected to
the claimed impact?
To dig deeper into the clashing arguments, follow the links
highlighted below, all of which connect to sources freely
available on the Web and do not require special academic
access. We also provide
full references
to each journal article or book.
The Cosmic-Impact Hypothesis
The impact hypothesis was first publicly presented at the
Spring 2007 joint assembly of the American Geophysical Union
in Acapulco, Mexico, by its leading proponents: James Kennett
of the University of California, Santa Barbara, Richard
Firestone of Lawrence Berkeley National Laboratory, geological
consultant Allen West, and others:
New insights into extraterrestrial impacts, Younger Dryas
cooling, mass extinction, and the Clovis people I
Video of the press conference related to this session, which
was shot in Acapulco on May 23, 2007, can be viewed on
YouTube:
Younger Dryas (YD) impact AGU press conference
Rex Dalton, a correspondent for the journal Nature,
summarized the immediate response to the AGU session:
Blast in the past?
[PDF]
A more critical report appeared shortly afterwards in the
journal Science:
Mammoth-killer impact gets mixed reception from Earth
scientists
[PDF]
The impact team's main arguments, based on nearly a dozen
different types of evidence at 26 sites from the U.S. West
Coast to Belgium, were first published in October 2007 in the
Proceedings of the National Academy of Sciences (PNAS):
Evidence for an extraterrestrial impact 12,900 years ago
that contributed to the megafaunal extinctions and the
Younger Dryas cooling
A strongly critical report appeared in March 2008 in
Science, which cited inconsistencies in many of the
team's claimed impact markers, including a failure to
replicate finds of elevated iridium, and the argument that the
alleged "E.T." materials could have been deposited gradually
by meteoritic dust "fallout" from the upper atmosphere rather
than by a sudden impact:
Experts find no evidence for a mammoth-killer impact
[PDF]
Later in 2008, an interesting exchange of views appeared in
the online journal GSA Today, beginning with a highly
critical commentary by Nicholas Pinter and Scott Ishman:
Impacts, mega-tsunami, and other extraordinary claims
In response to Pinter and Ishman, the impact hypothesis team
posted two comments:
Comment by Firestone and West
Comment by Bunch, Kennett, and Kennett
Since these initial criticisms, the impact hypothesis team has
focused mainly on a single type of evidence, microscopic
nanodiamonds, which it claims to be a clear signature of a
catastrophic event in the atmosphere. The nanodiamond
arguments were reported in Science:
Nanodiamonds in the Younger Dryas boundary sediment
layer
This Science article was accompanied by another
commentary by Richard Kerr:
Did the mammoth slayer leave a diamond calling card?
A press release summarized this Science paper:
Six North American sites hold 12,900-year-old
nanodiamond-rich soil
NOVA's program "Megabeasts' Sudden Death" reports on the
team's latest round of research on the Greenland ice sheet,
carried out mainly by Paul Mayewski of the University of
Maine. As shown on the program, the initial results of this
fieldwork indicate strikingly high levels of impact-shocked
diamonds in the relevant layer of the Greenland ice sheet.
This latest work is currently awaiting publication as follows:
Nanodiamonds discovered in the Greenland ice sheet within
the Younger Dryas boundary layer
The Megafauna Extinction Debate
For more than four decades, scientists have debated why so
many types of large animals, or megafauna, were driven into
extinction at the end of the last ice age. For an
authoritative and up-to-date introduction to the arguments,
see the following essay by archeologist Gary Haynes, who
favors the "overkill" theory that the impact of prehistoric
human hunting was the crucial factor:
Introduction to the volume
In
American Megafaunal Extinctions at the End of the
Pleistocene
Anthropologist Donald Grayson argues a contrasting
viewpoint—that climate change was decisive. His survey
compares the evidence from North America with the very
different patterns of extinction in Europe and Asia:
Deciphering North American Pleistocene extinctions
[PDF]
A spirited debate between Haynes and Grayson and their
colleagues can be followed in these three papers:
A requiem for North American overkill
[PDF]
A premature burial: comments on Grayson and Meltzer's
"Requiem for overkill"
[PDF]
North American overkill continued?
[PDF]
Countering both the climate and human "overkill" theories,
Ross MacPhee, a curator at the American Museum of Natural
History, argues that the megafauna could have been wiped out
by an infectious "hyperdisease," perhaps spread by contact
with colonizing human populations:
What killed the mammoths and other behemoths that once
roamed the Americas? This mammalogist thinks it may have
been hyperlethal disease
Origins of hyperdisease hypothesis
Taking a broader perspective on the controversy, biologist
Anthony Barnosky relates the megafauna extinction debate to
the present-day context of global warming and worldwide
extinctions:
Megafauna biomass tradeoff as a driver of Quaternary and
future extinctions
Among many books covering the megafauna debate, see the
following:
Heatstroke: Nature in an Age of Global Warming
by Anthony D. Barnosky
(Chapter 5 specifically)
Twilight of the Mammoths: Ice Age Extinctions and the
Rewilding of America
by Paul S. Martin
American Megafaunal Extinctions at the End of the
Pleistocene
by Gary Haynes, editor
The Early Settlement of North America: The Clovis Era
by Gary Haynes
The Younger Dryas Mystery: What Caused the Climate to Flip?
From around 16,000 years ago, the world began to emerge from
the deep freeze of the last ice age. As the global climate
warmed up, it suddenly flipped back to glacial conditions for
a thousand years or so before the present age of relatively
warm conditions finally took hold. The cause of that
mysterious flip has been the focus of intense debate ever
since ice-core research in Greenland revealed that the cold
interval (named the Younger Dryas after an alpine wildflower,
Dryas octopetala) both started and ended in a decade or
less.
More recent work indicates that the time frame may have been
even more abrupt, flipping in and out of the deep freeze in a
mere one to three years:
Did you say fast?
[PDF]
What could cause such a sudden flip? Columbia University Earth
scientist Wally Broecker pioneered the leading contender among
Younger Dryas theories during the 1980s. His theory involves a
disruption or shutdown of the great Atlantic Ocean conveyor,
the northeast-flowing current that brings warm water from the
tropics up to the northern latitudes. Broecker proposes that
at the end of the Ice Age, Lake Agassiz, a lake over 700 miles
wide fed by the melting glaciers and covering much of
present-day North Dakota, Minnesota, Manitoba, Ontario, and
Saskatchewan, suddenly drained into the St. Lawrence valley in
what is now eastern Canada. A huge pulse of freshwater would
have interfered with the Atlantic circulation, plunging the
Northern Hemisphere into icy conditions. Broecker summarizes
the theory in the following paper:
What if the conveyor were to shut down? Reflections on a
possible outcome of the great global experiment
A popular account of the theory and its implications appeared
in The Atlantic Monthly:
The great climate flip-flop
Although a shutdown of the Atlantic conveyor remains a widely
accepted explanation, hard evidence for a flood from Lake
Agassiz remains elusive, as Broecker has recently
acknowledged:
Was the Younger Dryas triggered by a flood?
[PDF]
In North America, the onset of the Younger Dryas is marked by
a "black mat" layer at more than 50 prehistoric sites. The
black mat is thought to consist mostly of dried algae forming
in stagnant pools as rainfall and the water table rose with
the onset of the Younger Dryas. Recently, archeologist C.
Vance Haynes, Jr. summarized his many years of investigating
these black mat sites:
Younger Dryas "black mats" and the Rancholabrean
termination in North America
[PDF]
According to the scientists who are proposing the comet-impact
hypothesis, a thin layer at the base of the black mat also
contains charcoal, nanodiamonds, and other materials claimed
to be the signature of a cosmic explosion. The scientists have
suggested that the comet or its fragments may have hit the
great ice sheet covering Canada and destabilized it, or
perhaps launched the Lake Agassiz flood, either of which could
have led to the shutdown of the Atlantic conveyor and plunged
the climate into the deep freeze of the Younger Dryas.
However, until an impact crater or other evidence is found,
this aspect of the impact hypothesis remains only a
speculation. The idea is reviewed, along with more general
background about the Younger Dryas, in the blog at
Realclimate.org:
Younger Dry-as dust?
The Younger Dryas comet-impact hypothesis: gem of an idea
or fool's gold?
Finally, today's concerns about global warming are fueling an
intense scientific quest to understand the causes of the
Younger Dryas and many other similar abrupt climate shifts in
earlier periods. The evidence of these ancient climate shifts
poses a disturbing question: Will today's steady build-up of
carbon dioxide emissions lead to a predictable, equally steady
gradual rise in global temperatures? Or could the climate
system suddenly "flip" to a disastrously warm state?
Among many popular books examining this question and reporting
on the evidence for rapid climate change, the following are
particularly useful:
The Ice Chronicles: The Quest to Understand Global Climate
Change
by Paul A. Mayewski and Frank White
The Two-Mile Time Machine: Ice Cores, Abrupt Climate
Change, and Our Future
by Richard B. Alley
The Long Thaw: How Humans Are Changing the Next 100,000
Years of Earth's Climate
by David Archer
Thin Ice: Unlocking the Secrets of Climate in the World's
Highest Mountains
by Mark Bowen
See a
list of references
for all journal articles and books mentioned in this article.
To join the debate yourself, see our
discussion board.
