Issue 7, April 2002
Catastrophic Events in the History of Life: Toward a New Understanding of Mass Extinctions in the Fossil Record - Part II
David B. Weinreb
Molecular Biophysics and Biochemistry & Geology and Geophysics, Yale University
In Part I of
this series, we traced the historical development of a theory stating
that the Cretaceous-Tertiary mass extinction resulted from the collision
of a meteorite near the Yucatán Peninsula in Mexico. The idea of
a K-T impact, first put forth by Luis and Walter Alvarez more than
two decades ago, came under heavy criticism from paleontologists
who had always thought the creatures who perished at the end of
the Mesozoic were wiped out slowly over millions of years, not in
a instant's time by a fiery meteorite.
By the late 1980s and early 1990s, the evidence supporting the
Impact Theory was almost overwhelming. Many geologists and paleontologists
began to wonder, "If it could happen once, could it happen
Here, in the second part of our series, we explore some very
recent research that suggests a meteorite may have been the culprit
not only in the K-T mass extinction, but also in some of the most
devastating biological crises in the history of the earth.
The Death Star Hypothesis
How often do mass extinctions occur? While the Phanerozoic - an
interval that lasted from 253 to 11 million years ago - was punctuated
by five major extinction events, there have also been numerous minor
events. The boundaries between geological periods (for instance,
the transition between the Jurassic and Cretaceous) are synchronized
with episodes of high extinction rates.
In 1982, David Raup and John Sepkoski, both of the University of
Chicago, examined marine invertebrate biodiversity in the Phanerozoic.
They divided this 242-million-year sequence into 39 equal-length
intervals, and defined an "extinction event" as an interval
where at least 2% of all known marine invertebrate families became
extinct. Raup and Sepkoski observed that such extinction events
occur about every 26 million years. Raup's initial reaction to the
data was, "That can't be right." Surprised by the
outcome of their analysis, the two researchers reasoned that there
must be some explanation for this pronounced periodicity:
A first question is whether we are seeing the effects of a purely
biological phenomenon or whether periodic extinction results from
recurrent events or cycles in the physical environment. If the forcing
agent is in the physical environment, does this reflect an earthbound
process or something in space? One possibility is the passage of
our solar system through the spiral arms of the Milky Way Galaxy,
which has been estimated to occur on the order of 108
years. Shoemaker has argued that passage through the galactic arms
should increase the comet flux and this could, following the Alvarez
hypothesis, provide an explanation for the biological extinctions.
Raup and Sepkoski had boldly suggested the apparent periodicity in
the marine extinction record was the result of cyclical astronomical
events that vastly increased the probability of the earth being pelted
In 1983, Marc Davis and Richard Muller, of the University of California
at Berkeley, proposed that the Sun has a yet-undetected companion
star with an eccentric orbit. The "unseen companion" is
generally about 2 light years away from the Sun, according to Davis.
As the companion star passes through the Oort cloud of comets surrounding
the Sun, it launches many of these comets in the general direction
of the earth. Davis includes a few reassuring words in his 1984 Nature
paper, noting that such a companion star, if it does indeed exist,
will not launch another comet toward Earth for at least another 15
million years. Davis confesses that:
The major difficulty with our model is the apparent absence of an
obvious companion to the Sun, and the existence of such an object
is its most important prediction. We take this prediction seriously
largely because of our inability to find any simpler explanation for
the periodicity consistent with known facts.
there is indeed a simpler explanation: The periodicity observed in
the marine fossil records is a statistical artifact. That is, extinctions
don't really occur every 26 millions; it just appeared that way because
of the way Raup and Sepkoski analyzed the data.
Silencing the Deathstar Hypothesis
Antoni Hoffman of
Columbia University dealt a tragic blow to the Raup and Sepkoski
hypothesis in 1985. Hoffman asked his colleagues to consider a sequence
of two consecutive geological stages. There are four changes in
species diversity that could occur in these stages: 1) There is
an increase in species diversity in the first stage, followed by
another increase in the second stage; 2) There is a decrease in
species diversity in the first stage and a decrease in the next
stage; 3) There is a decrease in the first stage and an increase
in the following stage; and 4) There is a increase in the first
stage and a decrease in the second stage. Raup and Seposki defined
an extinction event by the occurrence of an increase in the first
stage and a decrease in the second stage. Statistically, there is
a one in four probability of getting such an extinction peak between
any two stages. Moreover, the average length of each stage defined
in the Raup and Sepkoski study is 6.4 million years. For every 6.4-million-year
stage there is a 25% chance that the stage will end with extinction.
Therefore, extinctions will occur, on average, every 25.6 million
years. It is then merely a statistical coincidence that Raup and
Sepkoski find extinction events spaced 26 million years apart.
The work of Raup and Sepkoski was an attempt to find an extraterrestrial
cause for every extinction event in the marine fossil record.
Their claim was that, whenever many species die-out suddenly, the
culprit is a shower of meteorites that invading the earth. Although
their findings were widely challenged, many researchers have remained
optimistic that meteorite impacts may still hold the answer to not
only the K-T extinction, but also the Triassic-Jurassic event and
the most horrific episode of all, the Permo-Triassic mass extinction.
212 million years ago, a gigantic meteorite may have struck in Quebec,
Canada, creating the Manicouagan Crater. Unlike the Chixculub structure,
Manicouagan is mostly exposed at the earth's surface. The structure
is about 75 kilometers in diameter. The collision required to create
such a crater must have released approximately 1022 Joules
of energy - 100 times greater than the energy released if the world's
entire nuclear arsenal were to be detonated simultaneously.
The Manicouagan impact was so forceful that it ejected material out
of the atmosphere and sent it on a ballistic trajectory around the
earth. Like the Chicxulub impact, the Manicouagan impact left behind
a global geochemical signature in the rock record.
A little more than 200 million years ago, the earth's biota suffered
a fairly significant mass extinction. Although more than 75% of species
passed into extinction at the Triassic-Jurassic boundary, the event
is not considered to be as devastating as the Cretaceous-Tertiary
or the Permo-Triassic extinctions.
Did the Manicouagan impact result in the Triassic-Jurassic extinction?
Many paleontologists have argued that the two events - the impact
and the extinction - were unrelated. The impact event occurred more
than 10 million years before the end of the Triassic. According to
the current view, an impact occurred, but had no pronounceable effects
on the biosphere. Then, 10 million years later, the earth experienced
a mass extinction, comparable in magnitude to the K-T event that would
come 140 million years later. The situation here is basically the
reverse of the K-T debate: early skeptics of the Alvarez theory cited
the absence of a 65-million year old crater to claim the extinction
was not brought on by an extraterrestrial cause. At Manicouagan, there
is a crater, but no simultaneous mass extinction that it could have
Andrew Winslow, of the State University at Stony Brook in New York,
claims the best explanation may simply be that the age assigned to
the crater is incorrect, perhaps by as much as 10 million years. If
the impact at Manicouagan occurred 200 million years ago, then there
would be little debate as to whether it was responsible for (or at
least one of the major causes of) the Triassic-Jurassic extinction.
Since the precise date of the extinction event can only really be
estimated to an accuracy of 5 million years or so, Winslow can make
a good case for an extraterrestrial cause of the extinction by dating
the crater to anywhere between about 198 and 204 million years ago.
Why are craters so difficult to date? The answer has to do with how
they are formed. When the meteorite responsible for the Manicouagan
crater struck Quebec nearly a quarter of a billion years ago, the
force of the impact exhumed rocks that resided as deep as 9 kilometers
below the surface. The surface of the newborn crater consisted of
material brought to the impact site by the meteorite, rocks at the
surface at the time of impact, and minerals exhumed from far below
the earth's surface. At Manicouagan, some of the exhumed rocks may
have been 800 million years old.
The heat generated by the impact liquefied the rock and, in effect,
the crater became a melting pot for relatively young rocks at the
surface and much older minerals originally buried kilometers below
the site of impact. The heat released by the impact was so intense
that it took between 1,600 and 5,000 years before the melted rocks
Geochemists generally determine the age of a rock by measuring the
ratio of radioactive parent isotopes to radiogenic daughter
isotopes. Lead is produced by three pathways: the decay of 238Ur
(a process that occurs with a half-life of 4.5 billion years) to generate
206Pb; the decay of 235Ur to 207Pb
(half-life of 704 million years); and the decay of 232Th
to produce 208Pb (half-life of 14 billion years).
Isotope petrologists often rely on a mineral known as zircon to date
rocks. Although zircons occur in very low abundances in most rocks,
they are extremely resistant to weathering. When a zircon cools and
crystallizes, it will not incorporate any lead into its crystal structure.
Therefore, when geochemists shatter a zircon crystal, they know any
lead found trapped within the crystal lattice has been produced by
radioactive decay of uranium and thorium atoms previously trapped
within the crystal. Theoretically, knowing the ratios of the various
isotopes of uranium, thorium, and lead trapped within a zircon allows
geochemists to calculate its age.
However, the techniques for dating zircons are not this straightforward.
A zircon may be subjected to intense heat sometime after its initial
formation, causing it to re-crystallize and exclude lead from its
crystal structure, even lead atoms that may have been present in the
structure prior to melting.
This is a big part of the problem at Manicouagan. Very old zircons
(nearly a billion years old) buried deep within the crust were exhumed
to the surface, where they melted together with much younger surface
rocks. The intense heat from the impact caused the zircons to melt
and "donate" their radiogenic lead isotopes to other minerals.
These other minerals therefore have more radiogenic lead than would
be expected for minerals of their age. For this reasons, the dates
assigned to Manicouagan minerals are much too old - perhaps by 10
The same type of problem occurs in the Potassium-Argon dating system.
Potassium-40 decays to the noble gas, 40Ar, with a half-life
of 1.25 billion years. The argon remains trapped in the rock's crystal
structure until some disruption - violent shaking as in the case of
an earthquake, or intense heating - releases it. This process is known
as degassing, and it can make rocks seem younger than they really
are. For instance, a 70 million-year-old rock that has experienced
degassing twice - for instance, at 45 and 25 million years ago - will
be dated at only 25 million years old. This is simply because we are
only measuring the argon formed since the time of the most recent
The Manicouagan impact certainly would have triggered degassing of
the radiogenic argon found within the very old rocks exhumed by the
impact. The radiogenic argon released from these rocks may have been
trapped within the structures of other minerals during the rapid cooling
process that ensued. Normally, geochemists use the accumulation of
radiogenic argon as a chronometer for how much time has elapsed since
a rock cooled. However, under the extraordinary conditions of the
Manicouagan impact, many rocks may have formed with radiogenic argon
already trapped within their crystals. Winslow comments, "If
this impact actually occurred 210-212 million years ago, and it didn't
cause the Triassic-Jurassic extinction, then that would be incredible.
How could something this big not cause a mass extinction?"
If we hope to attribute extinction events to meteorite or comet impacts,
we need to learn to reliably date craters. The dating of such complex
geological structures, which form under such extraordinary conditions,
is far from simple.
The Permian-Triassic Extinction
If life on Earth has
ever come close to being snuffed out entirely, it happened 245 million
years ago. The Permian extinction was the most devastating crisis
in the history of life. Although many explanations have been put forward
to explain the rapid die-off of nearly 96% of all species, the fundamental
underlying cause of the Permian-Triassic event is still largely debated.
In 1994, Douglas Erwin, of the Smithsonian Institution's National
Museum of Natural History, suggested this mass extinction involved
"a tangled web rather than a single mechanism." Global sea
levels decreased at the end of the Permian, reducing the available
habitats for many marine organisms. Erwin addresses the possibility
that the eruption of the Siberian traps may have contributed to the
end-of-Permian extinction. However, most of the basalts from the Siberian
eruptions formed after the boundary (thus making their environmental
effects an unlikely suspect in the search for a mechanism of extinction).
Erwin warns fellow earth scientists:
Few complex events stem from a single cause; more common
is a complex web of causality, a web that can be difficult to untangle.
My own view is that the cause of the end-Permian extinction lies
in such a tangled web. The most plausible explanation would appear
to be a three-phase model combining elements of several mechanisms
described previously. The extinction began with the loss of habitat
area as the regression dried out many marine basins. The increased
exposure of Pangea as the regression progressed exacerbated climatic
instability. This instability, coupled with the effects of continuing
volcanic eruptions and an increase in atmospheric carbon dioxide
(with some degree of global warming), led to increasing environmental
degradation and ecological collapse …The final phase of the extinction
occurred in the earliest Triassic. The rapid transgression destroyed
near-shore terrestrial habitats.
Certainly, it seems possible to explain the Permo-Triassic event without
invoking an extraterrestrial cause as proposed for the K-T and T-J
boundaries. However in February 2001, Luann Becker of the University
of Washington published compelling evidence that an impact had indeed
occurred in the final hours of the Permian.
Becker extracted a type of organic molecule known as fullerenes (C60
to C200) from clay sediments from Permo-Triassic sites
in China, Hungray, and Japan. Fullerenes are molecules composed of
60 or more carbon atoms arranged to form a spherical, hollow, cage-like
structure. These fullerenes, nicknamed "buckyballs" for
their round shape, are constructed from 60 carbon atoms and are common
in clays from the Permo-Triassic sites as well as Cretaceous-Tertiary
sites; they are also found in meteorites. Extraordinarily large fullerenes,
constructed of as many as 400 carbon atoms, were found in the 4.6-billion-year-old
Allende meteorite that crashed in Mexico in the 1970s.
Figure 4. Incorporation
of noble gases into fullerene molecules.
will need RealPlayer
to view this
video demonstration of figure 4.
Buckyballs which have formed in outer space may be
transported to Earth by meteorites. The isotopic composition
of noble gases sequestered in buckyballs in sediments
at the P-T boundary provide evidence for a bolide
impact at the end of the Permian.
Inert gases, including helium and argon, are often trapped inside
the cage of the buckyballs. Becker has hypothesized about the origin
of the buckyballs themselves by measuring the ratio of helium isotopes
trapped inside these gigantic carbon molecules. The helium isotopic
composition of the fullerenes from the Permo-Triassic localities is
similar to the signature in carbonaceous chrondrites, suggesting that
the buckyballs were delivered to Earth in a meteorite or comet. The
ratio of helium isotopes is dramatically different from that found
in the earth's atmosphere, for instance. Becker and her colleagues
propose that the buckyballs found at the Permo-Triassic sites were
transported to Earth by the meteorite that triggered the most catastrophic
extinction of all time.
While many questions
remain unanswered, we now have come to recognize that the history
of life has been profoundly affected by meteorite impacts. The 1980
Alvarez theory, that a meteorite impact brought about the demise
of the dinosaurs, stunned most paleontologists and ultimately compelled
them to re-examine the nature of the driving forces of evolution.
Today, researchers have announced discoveries strongly suggesting
that the mass extinctions at the Permo-Triassic and Triassic-Jurassic
boundaries, among others, may also have been caused by meteorite
impacts. The riddle of evolution demands that we search for answers
perhaps beyond the realm of our imaginations. Life as it exists
in our modern world is the product of 600 million years of metazoan
evolution - and the earth's biota will continue to evolve, following
a course yet to be chartered, for eons to come. As Stephen Jay Gould
states in his book Wonderful Life, "The pageant of evolution
is a staggeringly improbable series of events, sensible enough in
retrospect and subject to rigorous explanation, but utterly unpredictable
and quite unrepeatable."
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Archibald, J. D. Dinosaur Extinction and the End of an Era: What
the Fossils Say. New York: Columbia University Press, 1996.
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boundary: Evidence from extraterrestrial noble gases in fullerenes".
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of Young Investigators. 2002. Volume Five.
Copyright © 2002 by David Weinreb and JYI. All rights reserved.