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Impact from the Deep
Strangling heat and gases emanating from the earth and sea, not asteroids, most likely caused several ancient mass
extinctions. Could the same killer-greenhouse conditions build once again?
By Peter D. Ward | September 18, 2006 |
Philosopher and historian Thomas S. Kuhn has suggested that scientific disciplines act a lot like
living organisms: instead of evolving slowly but continuously, they enjoy long stretches of stability
punctuated by infrequent revolutions with the appearance of a new species--or in the case of science,
a new theory. This description is particularly apt for my own area of study, the causes and
consequences of mass extinctions--those periodic biological upheavals when a large proportion of
the planet's living creatures died off and afterward nothing was ever the same again.
Since first recognizing these historical mass extinctions more than two centuries ago, paleontologists
believed them to have been gradual events, caused by some combination of climate change and
biological forces such as predation, competition and disease. But in 1980 the understanding of mass
extinctions underwent a Kuhnian revolution when a team at the University of California, Berkeley,
led by geologist Walter Alvarez proposed that the famous dinosaur-killing extinction 65 million
years ago occurred swiftly, in the ecosystem catastrophe that followed an asteroid collision. Over the
ensuing two decades, the idea that a bolide from space could smite a significant segment of life on
the earth was widely embraced--and many researchers eventually came to believe that cosmic
detritus probably caused at least three more of the five largest mass extinctions. Public acceptance of
the notion crystallized with Hollywood blockbusters such as Deep Impact and Armageddon.
Now still another transformation in our thinking about life's punctuated past is brewing. New
geochemical evidence is coming from the bands of stratified rock that delineate mass extinction
events in the geologic record, including the exciting discovery of chemical residues, called organic
biomarkers, produced by tiny life-forms that typically do not leave fossils. Together these data make
it clear that cataclysmic impact as a cause of mass extinction was the exception, not the rule. In most
cases, the earth itself appears to have become life's worst enemy in a previously unimagined way.
And current human activities may be putting the biosphere at risk once again.
After Alvarez
To understand the general enthusiasm for the impact paradigm, it helps to review the evidence that
fueled it. The scenario advanced by Alvarez, along with his father, physicist Luis W. Alvarez, and
nuclear chemists Helen V. Michel and Frank Asaro, contained two separate hypotheses: first, that a
fairly large asteroid--estimated to have been 10 kilometers in diameter--struck the earth 65 million
years ago; second, that the environmental consequences of the impact snuffed out more than half of
all species. They had found traces left by the blow in a thick layer of iridium--rare on the earth but
common in extraterrestrial materials--that had dusted the globe.
Within a decade of this prodigious announcement the killer's thumbprint turned up, in the form of the
Chicxulub crater hiding in plain sight on the Yucatan Peninsula of Mexico. Its discovery swept aside
most lingering doubts about whether the reign of the dinosaurs had ended with a bang. At the same
time, it raised new questions about other mass extinction events: If one was caused by impact, what
about the rest? Five times in the past 500 million years most of the world's life-forms have simply
ceased to exist. The first such event happened at the end of the Ordovician period, some 443 million
years ago. The second, 374 million years ago, was near the close of the Devonian. The biggest of
them all, the Great Dying, at the end of the Permian 251 million years ago, wiped out 90 percent of
ocean dwellers and 70 percent of plants, animals, even insects, on land [see "The Mother of Mass
Extinctions," by Douglas H. Erwin; Scientific American, July 1996]. Worldwide death happened
again 201 million years ago, ending the Triassic period, and the last major extinction, 65 million
years ago, concluded the Cretaceous with the aforementioned big bang.
The earth can, and probably did, exterminate its own.
In the early 1990s paleontologist David Raup's book Extinctions: Bad Genes or Bad Luck? predicted
that impacts ultimately would be found to be the blame for all these major mass extinctions and
other, less severe events as well. Evidence for impact from the geologic boundary between the
Cretaceous and Tertiary (-K/T) periods certainly was and remains convincing: in addition to the
Chicxulub crater and the clear iridium layer, impact debris, including pressure-shocked stone
scattered across the globe, attests to the blow. Further chemical clues in ancient sediments document
rapid changes in the world's atmospheric composition and climate that soon followed.
For several other extinction periods, the signs also seemed to point "up." Geologists had already
associated a thin iridium layer with the end Devonian extinctions in the early 1970s. And by 2002
separate discoveries suggested impacts at the end Triassic and end Permian boundaries. Faint traces
of iridium registered in the Triassic layer. And for the Permian, distinctive carbon "buckyball"
molecules believed to contain trapped extraterrestrial gases added another intriguing clue [see
"Repeated Blows," by Luann Becker; Scientific American, March 2002]. Thus, many scientists came
to suspect that asteroids or comets were the source of four of the "big five" mass extinctions; the
exception, the end Ordovician event, was judged the result of radiation from a star exploding in our
cosmic neighborhood.
As researchers continued to probe the data in recent years, however, they found that some things did
not add up. New fossil analyses indicated that the Permian and Triassic extinctions were drawn-out
processes spanning hundreds of thousands of years. And newly obtained evidence of the rise and fall
of atmospheric carbon, known as carbon cycling, also seemed to suggest that the biosphere suffered
a long-running series of environmental insults rather than a single, catastrophic strike.
Not So Sudden Impact
The lesson of the K/T event was that a large-body impact is like a major earthquake leveling a city:
the disaster is sudden, devastating, but short-lived--and after it is over, the city quickly begins
rebuilding. This tempo of destruction and subsequent recovery is reflected in carbon-isotope data for
the K/T extinctions as well as in the fossil record, although verifying the latter took the scientific
community some time. The expected sudden die-off at the K/T boundary itself was indeed visible
among the smallest and most numerous fossils, those of the calcareous and siliceous plankton, and in
the spores of plants. But the larger the fossils in a group, the more gradual their extinction looked.
Slowly, paleontologists came to understand that this apparent pattern was influenced by the sparsity
of large-fossil samples for most of the soil and rock strata being studied. To address this sampling
problem and gain a clearer picture of the pace of extinction, Harvard University paleontologist
Charles Marshall developed a new statistical protocol for analyzing ranges of fossils. By determining
the probability that a particular species has gone extinct within a given time period, this analytical
method teases out the maximum amount of information yielded by even rare fossils.
In 1996 Marshall and I joined forces to test his system on K/T stratigraphic sections and ultimately
showed that what had appeared to be a gradual extinction of the most abundant of the larger marine
animals, the ammonites (molluscan fossils related to the chambered nautilus) in Europe, was instead
consistent with their sudden disappearance at the K/T boundary itself. But when several researchers,
including myself, applied the new methodology to earlier extinctions, the results differed from the
K/T sections. Studies by my group of strata representing both marine and nonmarine environments
during the latest parts of the Permian and Triassic periods showed a more gradual succession of
extinctions clustered around the boundaries.
That pattern was also mirrored in the carbon-isotope record, which is another powerful tool for
understanding rates of extinction. Carbon atoms come in three sizes, or isotopes, with slightly
varying numbers of neutrally charged particles in the nucleus. Many people are familiar with one of
these isotopes, carbon 14 (
14
C), because its decay is often used to date specific fossil skeletons or
samples of ancient sediments. But for -interpreting mass extinctions, a more useful type of
information to extract from the geologic record is the ratio of
12
C to
13
C isotopes, which provides a
broader snapshot of the vitality of plant life at the time.
That is because photosynthesis largely drives changes in the
12
C-
13
C ratio. Plants use energy from the
sun to split carbon dioxide (CO
2
) into organic carbon, which they exploit to build cells and provide
energy; happily for us animals, free oxygen is their waste product. But plants are finicky, and they
preferentially choose CO
2
containing
12
C. Thus, when plant life--whether in the form of
photosynthesizing microbes, floating algae or tall trees--is abundant, a higher proportion of CO
2
remaining in the atmosphere contains
13
C, and atmospheric
12
C is measurably lower.
By examining the isotope ratios in samples from before, during and after a mass extinction,
investigators can obtain a reliable indicator of the amount of plant life both on land and in the sea.
When researchers plot such measurements for the K/T event on a graph, a simple pattern emerges.
Virtually simultaneously with the emplacement of the so-called impact layer containing
mineralogical evidence of debris, the carbon isotopes shift--
13
C drops dramatically--for a short time,
indicating a sudden die-off of plant life and a quick recovery. This finding is entirely consistent with
the fossil record of both larger land plants and the sea's microscopic plankton, which underwent
staggering losses in the K/T event but bounced back rapidly.
In contrast, the carbon records revealed by my group in early 2005 for the Permian, and more
recently for the Triassic, document a very different fate for plants and plankton during those two
mass extinctions. In both cases, multiple isotope shifts over intervals exceeding 50,000 to 100,000
years indicate that plant communities were struck down, then re-formed, only to be perturbed again
by a series of extinction events. To produce such a pattern would take a succession of asteroid
strikes, thousands of years apart. But no mineralogical evidence exists for a string of impacts during
either time span.
Indeed, further investigation of the evidence has called into question the likelihood of any impacts at
those two times. No other research groups have replicated the original finding of buckyballs
containing extraterrestrial gas at the end Permian boundary. A discovery of shocked quartz from that
period has also been recanted, and geologists cannot agree whether purported impact craters from the
event in the deep ocean near Australia and under ice in Antarctica are actually craters or just natural
rock formations. For the end Triassic, the iridium found is in such low concentrations that it might
reflect a small asteroid impact, but nothing of the planet-killing scale seen at the K/T boundary. If
impacts are not supported as the cause of these mass extinctions, however, then what did trigger the
great die-offs? A new type of evidence reveals that the earth itself can, and probably did, exterminate
its own inhabitants.
Ghastly Greenhouse
About half a decade ago small groups of geologists began to team up with organic chemists to study
environmental conditions at critical times in the earth's history. Their work involved extracting
organic residues from ancient strata in search of chemical "fossils" known as biomarkers. Some
organisms leave behind tough organic molecules that survive the decay of their bodies and become
entombed in sedimentary rocks. These biomarkers can serve as evidence of long-dead life-forms that
usually do not leave any skeletal fossils. Various kinds of microbes, for example, leave behind traces
of the distinctive lipids present in their cell membranes--traces that show up in new forms of mass
spectrometry, a technique that sorts molecules by mass.
This biomarker research was first conducted on rocks predating the history of animals and plants, in
part to determine when and under what conditions life first emerged on the earth. But within the past
few years scientists began sampling the mass extinction boundaries. And to the great surprise of
those doing this work, data from the periods of mass extinction, other than the K/T event, suggested
that the world's oceans have more than once reverted to the extremely low oxygen conditions, known
as anoxia, that were common before plants and animals became abundant.
Among the biomarkers uncovered were the remains of large numbers of tiny photosynthetic green
sulfur bacteria. Today these microbes are found, along with their cousins, photosynthetic purple
sulfur bacteria, living in anoxic marine environments such as the depths of stagnant lakes and the
Black Sea, and they are pretty noxious characters. For energy, they oxidize hydrogen sulfide (H
2
S)
gas, a poison to most other forms of life, and convert it into sulfur. Thus, their abundance at the
extinction boundaries opened the way for a new interpretation of the cause of mass extinctions.
Scientists have long known that oxygen levels were lower than today around periods of mass
extinction, although the reason was never adequately identified. Large-scale volcanic activity, also
associated with most of the mass extinctions, could have raised CO
2
levels in the atmosphere,
reducing oxygen and leading to intense global warming--long an alternative theory to the impacts;
however, the changes wrought by volcanism could not necessarily explain the massive marine
extinctions of the end Permian. Nor could volcanoes account for plant deaths on land, because
vegetation would thrive on increased CO
2
and could probably survive the warming.
But the biomarkers in the oceanic sediments from the latest part of the Permian, and from the latest
Triassic rocks as well, yielded chemical evidence of an ocean-wide bloom of the H
2
S-consuming
bacteria. Because these microbes can live only in an oxygen-free environment but need sunlight for
their photosynthesis, their presence in strata representing shallow marine settings is itself a marker
indicating that even the surface of the oceans at the end of the Permian was without oxygen but was
enriched in H
2
S.
In today's oceans, oxygen is present in essentially equal concentrations from top to bottom because it
dissolves from the atmosphere into the water and is carried downward by ocean circulation. Only
under unusual circumstances, such as those that exist in the Black Sea, do anoxic conditions below
the surface permit a wide variety of oxygen-hating organisms to thrive in the water column. Those
deep-dwelling anaerobic microbes churn out copious amounts of hydrogen sulfide, which also
dissolves into the seawater. As its concentration builds, the H
2
S diffuses upward, where it encounters
oxygen diffusing downward. So long as their balance remains undisturbed, the oxygenated and
hydrogen sulfide-saturated waters stay separated, and their interface, known as the chemocline, is
stable. Typically the green and purple sulfur bacteria live in that chemocline, enjoying the supply of
H
2
S from below and sunlight from above.
Yet calculations by geoscientists Lee R. Kump and Michael A. Arthur of Pennsylvania State
University have shown that if oxygen levels drop in the oceans, conditions begin to favor the deep-
sea anaerobic bacteria, which proliferate and produce greater amounts of hydrogen sulfide. In their
models, if the deepwater H
2
S concentrations were to increase beyond a critical threshold during such
an interval of oceanic anoxia, then the chemocline separating the H
2
S-rich deepwater from
oxygenated surface water could have floated up to the top abruptly. The horrific result would be
great bubbles of toxic H
2
S gas erupting into the atmosphere.
Their studies indicate that enough H
2
S was produced by such ocean upwellings at the end of the
Permian to cause extinctions both on land and in the sea. And this strangling gas would not have
been the only killer. Models by Alexander Pavlov of the University of Arizona show that the H
2
S
would also have attacked the planet's ozone shield, an atmospheric layer that protects life from the
sun's ultraviolet (UV) radiation. Evidence that such a disruption of the ozone layer did happen at the
end of the Permian exists in fossil spores from Greenland, which display deformities known to result
from extended exposure to high UV levels. Today we can also see that underneath "holes" in the
ozone shield, especially in the Antarctic, the biomass of phytoplankton rapidly decreases. And if the
base of the food chain is destroyed, it is not long until the organisms higher up are in desperate straits
as well.
Kump and Arthur estimate that the amount of H
2
S gas entering the late Permian atmosphere from the
oceans was more than 2,000 times the small amount given off by volcanoes today. Enough of the
toxic gas would have permeated the atmosphere to have killed both plants and animals--particu-larly
because the lethality of H
2
S increases with temperature. And several large and small mass
extinctions seem to have occurred during short intervals of global warming. That is where the ancient
volcanic activity may have come in.
Around the time of multiple mass extinctions, major volcanic events are known to have extruded
thousands of square kilometers of lava onto the land or the seafloor. A by-product of this tremendous
volcanic outpouring would have been enormous volumes of carbon dioxide and methane entering the
atmosphere, which would have caused rapid global warming. During the latest Permian and Triassic
as well as in the early Jurassic, middle Cretaceous and late Paleocene, among other periods, the
carbon-isotope record confirms that CO
2
concentrations skyrocketed immediately before the start of
the extinctions and then stayed high for hundreds of thousands to a few million years.
But the most critical factor seems to have been the oceans. Heating makes it harder for water to
absorb oxygen from the atmosphere; thus, if ancient volcanism raised CO
2
and lowered the amount
of oxygen in the atmosphere, and global warming made it more difficult for the remaining oxygen to
penetrate the oceans, conditions would have become amenable for the deep-sea anaerobic bacteria to
generate massive upwellings of H
2
S. Oxygen-breathing ocean life would have been hit first and
hardest, whereas the photosynthetic green and purple H
2
S-consuming bacteria would have been able
to thrive at the surface of the anoxic ocean. As the H
2
S gas choked creatures on land and eroded the
planet's protective shield, virtually no form of life on the earth was safe.
Kump's hypothesis of planetary killing provides a link between marine and terrestrial extinctions at
the end of the Permian and explains how volcanism and increased CO
2
could have triggered both. It
also resolves strange findings of sulfur at all end Permian sites. A poisoned ocean and atmosphere
would account for the very slow recovery of life after that mass extinction as well.
Finally, this proposed sequence of events pertains not only to the end of the Permian. A minor
extinction at the end of the Paleocene epoch 54 million years ago was already--presciently--
attributed to an interval of oceanic anoxia somehow triggered by short-term global warming.
Biomarkers and geologic evidence of anoxic oceans suggest that is also what may have occurred at
the end Triassic, middle Cretaceous and late Devonian, making such extreme greenhouse-effect
extinctions possibly a recurring phenomenon in the earth's history.
Most troubling, however, is the question of whether our species has anything to fear from this
mechanism in the future: If it happened before, could it happen again? Although estimates of the
rates at which carbon dioxide entered the atmosphere during each of the ancient extinctions are still
uncertain, the ultimate levels at which the mass deaths took place are known. The so-called thermal
extinction at the end of the Paleocene began when atmospheric CO
2
was just under 1,000 parts per
million (ppm). At the end of the Triassic, CO
2
was just above 1,000 ppm. Today with CO
2
around
385 ppm, it seems we are still safe. But with atmospheric carbon climbing at an annual rate of 2 ppm
and expected to accelerate to 3 ppm, levels could approach 900 ppm by the end of the next century,
and conditions that bring about the beginnings of ocean anoxia may be in place. How soon after that
could there be a new greenhouse extinction? That is something our society should never find out.
ABOUT THE AUTHOR(S)
PETER D. WARD is a professor in the University of Washington's biology department and its earth
and space sciences division, where he investigates both realms. His terrestrial research centers on
ancient mass extinction events as well as the evolution and ultimate extinction of the nautiluslike
marine animals known as ammonites, which he described in his first article for Scientific American
in October 1983. Ward also applies principles gleaned from studying the earth's earliest life-forms to
research for the NASA Astrobiology Institute into potential habitats for life elsewhere. He discussed
those environments in an October 2001 Scientific American article, "Refuges for Life in a Hostile
Universe," written with Guillermo Gonzalez and Donald Brownlee, as well as in a popular book co-
authored with Brownlee, Rare Earth: Why Complex Life Is So Uncommon in the Universe (Springer,
2000).
From Scientific AmericanMagazine
http://www.scientificamerican.com/article.cfm?id=impact-from-the-deep
