Enjoy
a good mystery story?
The geologic
record provides five of the best.
Mass extinction
events brought much of the world's existing life to an untimely
end in the late Ordovician, Devonian, Permian, Triassic and Cretaceous
periods.
Now science
sleuths are taking a new look at global extinctions — with findings
that could change the way we see life on Earth today.
Kenneth
G. MacLeod is an assistant professor of geology at the University
of Missouri-Columbia, and co-editor of the Geological Society of
America's special paper volume, Catastrophic Events and Mass Extinctions:
Impacts and Beyond.
He understands
the allure of extinction events for paleontology puzzle-solvers.
"The geologic
column is based on these things," MacLeod noted. "The nice thing
about mass extinctions is that they're a huge signal. They really
stand out."
There's
no mystery why geologists appreciate all this detective work.
Research
related to mass extinctions pays a huge bonus in advancing stratigraphy
and improving knowledge of geochronology.
On Second
Thought …
Investigations
into the sudden, widespread disappearance of species took a new
turn in the early 1990s.
Based on
the theories of Walter and Luis Alvarez, and the fieldwork and research
of Alan Hildebrand, the Chicxulub impact crater in Mexico's Yucatan
Peninsula became linked to the mass extinction at the Cretaceous-Tertiary
(K-T) boundary.
As evidence
for this link accumulated through the decade, more and more scientists
accepted that a major impact by an asteroid or comet had produced
a global K-T catastrophe.
The impact
theory caught the public imagination, since it implied a fiery and
dramatic end for the last of the world's dinosaurs.
For scientists,
the Chicxulub strike offered a novel but viable explanation for
sudden extinction.
"Catastrophic
explanations aren't a last resort now. They are a legitimate alternative
hypothesis," MacLeod said.
Researchers
began searching in earnest for impacts coeval with other extinctions,
although they could still dip into a grab-bag of potential culprits.
Those include:
Global
cooling, global warming, carbon dioxide, sulfuric acid, atmospheric
changes, eutrophication, euxinic conditions, anoxia, eustacy, flood
basalt, volcanoes, earthquakes, orogenies and other major tectonic
shifts.
To name
a few.
As work
progressed, evidence seemed to point away from impact-driven catastrophic
events outside the K-T extinctions.
True believers
insisted that the tell-tale craters hadn't been discovered yet.
Other researchers
looked to new theories.
And with
so much talk going on about global warming, some began to focus
on ancient atmospheric and climatic changes.
Read, "greenhouse
effect."
Today,
that approach "has replaced asteroid impacts as the sexiest explanation
invoked" for mass extinctions, MacLeod said.
Hot Stuff
Peter Ward
spent years in South Africa looking into possible extinction causes,
his latest work involving the massive die-out at the end of the
Permian.
The Permian
catastrophe brought the Paleozoic era to a screeching halt and may
have wiped out more than 90 percent of then-existing life, the largest
extinction known.
"I went
there fully expecting to find evidence of an impact," he said. "But
it didn't look that way."
Ward, professor
of biology and earth and space science at the University of Washington,
has written extensively about mass extinctions.
His latest
book, published in January, is Gorgon: Paleontology, Obsession and
the Greatest Catastrophe in Earth's History.
It combines
a memoir of Ward's experiences with an account of his research into
the Permian extinction.
That event's
victims included the Gorgon, a fearsome, mammal-like, lizard predator.
Ward said
growing evidence for the K-T impact intrigued him in the 1990s.
Like many other researchers, though, he began to doubt that the
impact explanation could be applied universally.
"We've
had a paradigm change. About 2000, things sort of turned around
again," he recalled.
For other
extinctions, theories began to shift from instantaneous catastrophe
to sudden but slower change, from meteor strikes to climate spikes.
"K-T may
be unique," he said. "As we get better precision going back in time,
we realize there can be really big changes in temperature."
In particular,
Ward found evidence of barren, desert-like conditions, with braided
streams in a desolate landscape.
"Once you
strip away the plants, you get enormous changes in sedimentation,"
he said. "If you removed the plants now, you'd have a major mass
extinction in the oceans."
The eruptions
of the Siberian Traps flood basalt occurred at this same time, and
could have played a major role, Ward noted.
He also
thinks atmospheric change came from more than a release of carbon
dioxide.
"The one
word that's been used more and more during the past five years is
'methane,'" he said.
"It looks
like sudden global warming where, when the planet got hot, it got
a lot hotter," Ward explained.
"It's not
sudden a-day-to-a-month, and it's not over millions of years. It's
a 10,000-year type of thing," he added.
If ocean
levels fell and hydrocarbon gas from clathrates began to enter the
atmosphere, the Permian would have ended in "a positive feedback
loop causing ever-increasing temperatures," according to Ward.
"The Permian
was a runaway greenhouse effect," he said. "We didn't become Venus,
but we got real hot."
War of the
Words
As researchers
moved beyond meteor strikes in explaining other extinctions, a new
debate evolved over the Chicxulub impact.
Gerta Keller,
a professor of geosciences at Princeton University, led a group
of researchers who came to challenge the accepted theory.
Those researchers
included Wolfgang Stinnesbeck and Doris Stuben of Karlsruhe University,
Germany, and Thierry Adatte of the University of Neuchatel, Switzerland.
"Let's
start with where we agree," Keller said. "We agree there was a major
impact and it coincides with major extinctions at the K-T boundary.
Where we don't agree is that this major impact was Chicxulub."
In fact,
Keller places the Chicxulub impact about 300,000 years before the
final K-T extinction.
She sees
evidence for several large impacts during the end of the Cretaceous,
building her extinction case on multiple meteor strikes, Deccan
volcanism and a K-T greenhouse effect.
"The greenhouse
warming that occurred between about 200,000-400,000 years before
the K-T boundary had a profound effect on marine plankton and other
life forms, and resulted in severe biotic stress that caused species
dwarfing, reduced populations and reduced species diversity," she
said.
Keller
served as lead author for the paper "Multiple Impacts Across the
Cretaceous-Tertiary Boundary," published last September in Earth-Science
Reviews.
The authors
followed with a paper on K-T spherule deposits, published in the
Journal of the Geological Society, and an article, "The Non-Smoking
Gun," in Geoscientist magazine.
This led
to an online debate of comments, queries and ripostes on the Geological
Society's Web site, a Chicxulub discussion that extended from November
2003 through January 2004.
Keller's
case rests largely on examination of deposits between the Chicxulub
ejecta layer and the identified K-T boundary layer.
She and
her associates claim to find planktic foraminifera in these lithological
units indicative of the last 300,000 years of the Maastrichtian,
as well as fossil burrows.
They also
claim to identify multiple glass spherule layers, as well as zeolite-enriched
zones associated with smectite.
Special
attention was given to greenish layers that might have been glass
altered to clay, according to Keller.
But an
independent expert "found that, lo and behold, it was not (altered)
glass. It was glauconite — glauconitic clay with burrows in it,"
she said.
"We found
more evidence of bioturbation, which indicates there was repeated
colonization of the ocean floor by invertebrates in the interval
after the impact ejecta was deposited and prior to the K-T boundary.
"Whether
in northeastern Mexico or the Chicxulub crater, the bioturbated
and glauconitic sediments therefore indicate the impact occurred
well before the K-T boundary and was followed by a long period of
normal marine sedimentation," she added.
Keller's
revisionist bait drew few nibbles in the online discussion.
The primary
counter-argument came from Dutch paleontologist Jan Smit of Free
University in Amsterdam, who cited tsunami-wave deposits or tsunami-triggered
deposition, altered by slumping.
"A lot
of people are surprised that we have not heard from the very large
support community that Chicxulub has. I expected to be inundated
with attacks," Keller said.
Researchers
now are studying core samples obtained at the crater site in early
2002, providing an opportunity for new interpretations of the event.
The final
word on Chicxulub remains to be written.
A study
recently published in Geology found little charcoal in K-T layers,
casting doubt on a global firestorm theory.
A Fine
Line
New expertise
is pushing stratigraphic resolution to finer levels than anyone
imagined, according to MacLeod.
"It has
opened the door to time scales that were not considered before,"
he said. "People are looking at the boundaries with a new sense
of detail."
In the
future, extinction research may benefit from techniques that provide
even better resolution.
MacLeod
talked about analyzing stratigraphy on less than a 1,000-year timescale.
"Is the
stratigraphic record well enough preserved so you can pull that
out? It's really pretty amazing that we all think we can," he commented.
Future
research also will draw on better integration among fields, with
"a whole range of experts brought to bear," MacLeod predicted.
And he
foresees a focus on boundaries that have gotten less attention,
like the Ordovician and Frasnian-Fammenian extinction events.
Greater
and lesser extinctions pepper geologic history. Researchers tend
to concentrate on one or two of them, leaving plenty of room for
other investigators to join the game.
"There
aren't many people who work up and down the column on mass extinctions,"
MacLeod noted.
Given the
possibilities, the potential and the publicity, research into extinction
events — with attending controversies — seems certain to continue
undiminished.
"We are
always fascinated by death," Ward said. "Look at how interested
we are in serial killers. Death writ large is even more intriguing."
Natural
cataclysms have threatened the existence of life before, and "it's
our own, deep-seated nightmare that we'll do the same thing," he
observed.
For Keller,
extinction research opens a window into the world's past.
"I'm interested
in seeing what happened in the history of the Earth," she said.
"I'm basically a historian."
Then there's
the drive to sort out an ancient puzzle, to play detective on a
grand scale, to read the final chapter in one of Earth's great murder
mysteries.
"Probably,"
MacLeod said, "most people are driven by curiosity more than anything
else."