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By Peter Rekcjek
Dinosaurs, asteroids and death — mass extinctions
don’t get more exciting than the most recent one
of 65 million years ago. That’s when an asteroid
slammed into the Earth, wiping out non-avian dinosaurs,
but assuring them a starring role in a string of Hollywood
epics.
Known as the Alvarez hypothesis for the father-and-son
team who published the paper in the journal Science in
1980, the theory suggests that the impact sent up a cloud
of particles into the atmosphere, drastically cutting
down solar energy and hence photosynthesis. Plants and
marine algae withered and died, followed by the herbivores
that munched on them, and reverberating up the food chain
to the predators that depended on the plant-loving animals.
Makes a great movie, and case closed, right? Maybe not.
A science team led by paleontologist Peter Ward with
the University of Washington in Seattle doesn’t dispute
that an asteroid scored a lucky shot on this planet at
the end of the Cretaceous Period. The evidence is overwhelming,
with sedimentary rock layers at the 65-million-year mark
revealing a concentration of iridium many times greater
than normal — the only source possible being from
an outer space rock.
What Ward and his colleagues do suggest is that the planet
was already on a downward spiral, an overheated greenhouse
that was snuffing out life before the asteroid arrived.
In boxing parlance, the asteroid didn’t deliver
a first-round knockout but instead arrived at the match
in round 12, after the planet’s biological life had
taken a series of environmental blows. The extraterrestrial
rock merely offered the final knockout punch, the coup
de grace.
“The evidence for there being an impact near the
K-T boundary is extremely good,” conceded Joe Kirschvink,
a geobiologist at the California Institute of Technology
and principal investigator for an expedition with Ward
and Eric Steig of the University of Washington to the
Antarctic Peninsula this coming field season. “But
that’s not the whole story.”
A murder mystery
The scientists instead point to a major volcanic event
that occurred roughly around the same time as the so-called
Cretaceous-Tertiary extinction event, also known as the
K-T extinction boundary.
Biodiversity and extinction events over the last 500
million years.The Deccan Traps in India represent one
of the largest volcanic features on Earth. Formed between
60 and 68 millions years ago from a series of volcanic
eruptions, the Deccan Traps consist of multiple layers
of solidified flood basalt, a large area covered by basalt
lava that today is about 2,000 meters thick, encompassing
an area of 500,000 square kilometers. (The term traps
comes from the Swedish word for stairs, referring to the
step-like hills forming the landscape of the region.)
The massive release of volcanic gases, particularly carbon
dioxide, from the eruptions may have created a greenhouse
effect, heating up the planet and stressing the giant
dinosaurs and many other species to extinction.
“It may have been one of those weird instances where
the environment was deteriorating drastically because
of the flood basalts, and then this impact bopped it all
and did them in,” Kirschvink said. “In science,
you try to find underlying principles that can explain
many things. The impact hypothesis was once thought to
be a universal explanation for all of these mass extinctions.
“It’s one of these murder mysteries that scientists
love to get tangled up in,” he added.
Five major extinctions have occurred in the past 500
million years — the Ordovician, the Devonian, the
Permian, the Triassic and the Cretaceous — as well
as many minor ones. All of the big five except the K-T
extinction appear to be greenhouse extinctions, according
to Ward.
“How can we say all of these other mass extinctions
are associated with flood basalts — big flood basalts
— and then right across K-T we have one of the biggest
flood basalts in history of the planet, the Deccan Traps,
but this one we say had no effect,” he said.
“In other words, flood basalts cause extinctions
every time except during the K-T. Hmm. Something is not
right about this picture.”
Home Contact Station Times HomeScienceKT Extinction Study
in Antarctica (page 2)
Clues at high latitudes
Ward has devoted the last 15 years of his career to studying
greenhouse extinctions. He recently published a book,
“Under a Green Sky,” that details the competing
hypotheses for these massive die offs. The Alvarez hypothesis
held sway for many years, and Ward’s own work in
the lower latitudes studying fossilized ammonites, shelled
cephalopods that thrived prior to 65 million years ago,
backed up the impact theory.
To prove that K-T extinction was already under way before
the Alvarez asteroid hit, the scientists need evidence
that species diversity was already in decline. Such evidence
of extinctions leading up to the K-T boundary may exist
at the higher and colder latitudes, which are more sensitive
to climate change, according to the scientists.
Noted Tom Wagner, program manager of Earth Sciences at
the National Science Foundation’s Office of Polar
Programs , which is funding the study, “The Earth
sciences advance through discovery, and with so little
of it explored, Antarctica consistently offers new finds.
“But its polar position is important, too. In this
case, it will help characterize the extinction process,”
he added. “Did extinction spread gradually across
the globe? Were creatures in polar oceans affected differently
than in the tropics? Are the poles a refuge or the first
to go? Questions like this can only be answered by research
in Antarctica.”
“You really need to know what’s going on in
the Southern Ocean to really understand what’s happening
globally, to tie the rocks there with the rest of the
world,” Kirschvink explained.
And that’s the plan. The scientists will join colleagues
from Argentina at James Ross and Seymour islands to collect
samples for analysis in the lab. Kirschvink visited Seymour
last year on something of a reconnaissance mission at
the invitation of Eduardo Olivero, a paleontologist with
the Argentine Antarctic program .
“Seymour Island section [of exposed rock] has the
best time resolution I’ve ever seen for this K-T
extinction boundary,” Kirschvink said. One meter
represents about 4,000 years in geologic time.
“It gives us the most incredible ability to go in
and get high-resolution data with what was going on with
this extinction than anywhere else I’ve seen,”
he explained. “Not only that, the rocks are completely
unheated, almost unburied, almost no alteration. …
It’s an incredibly exciting sequence to work through.
And fossils? You just whack on a few of these things,
and boom, out come ammonites.”
The scientists, along with graduate students from both
countries, will spend about a month from mid-February
to March digging out those fossils for analysis in this
first field season. “We’re going to do some
of the most horribly boring work on the face of the continent,
any continent,” Ward said.
Using a diamond-tipped coring drill, driven by a modified
chainsaw, the scientists will extract small cores of rock,
about 10 centimeters deep and about three centimeters
in diameter, Ward explained. They will measure the spatial
orientation of the cores with sun and magnetic compasses,
and a bubble level, for the paleogmagnetic work. They
will also record the cores’ position in the rock
sequence relative to the mass extinction layer to understand
the stratigraphy.
Back at the lab, the scientists will measure the direction
of its ancient magnetism, which can tell them what position
the continent was in 65 million years ago. “More
importantly, the Earth’s magnetic field reverses
back and forth in a random pattern, and, as the history
of these ‘geomagnetic reversals’ is well known
during this time interval, the pattern of magnetic reversals
can provide accurate age constraints on the sediments,”
Kirschvink said.
It will take at least a year before there are any results,
Ward said. “The field work is the fun part and the
lab work is slow.”
Aside from Kirschvink’s trip to Seymour, both scientists
are new to the U.S. Antarctic Program . “One of the
USAP’s goals is broadening participation,” Wagner
said.
The project is not only about solving a murder mystery
for Ward. He sees disturbing parallels between the past
greenhouse extinctions and today’s climate change.
Previous studies have suggested as much as 25 percent
of the world’s species today may vanish before the
end of the century unless greenhouse gases are curtailed.
“I’m personally worried about sea level change,”
Ward said. “Even a one-meter rise is going to be
a disaster in the world. … It’s a good time
for me to go back to the first [mass extinction] I ever
studied after seeing what the greenhouse [extinctions]
can do.”
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