[meteorite-list] A Volcanic Dinosaur Debate

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Mon May 9 16:00:46 2005
Message-ID: <200505091928.j49JSRX17432_at_zagami.jpl.nasa.gov>

http://www.astrobio.net/news/modules.php?op=modload&name=News&file=article&sid=1552

A Volcanic Dinosaur Debate
Astrobiology Magazine

Summary (May 09, 2005): Scientists can recite a long list of the devastating
environmental consequences of a large meteorite impact, but they cannot
prove these effects have led to the simultaneous loss of life around the
globe. Answering the question of how and why such a large variety of
species died out at the same time is one of the greatest mysteries in
paleontology.


A Volcanic Dinosaur Debate
by Leslie Mullen

Outgassing lava flows: did they cause heat-induced reproductive failure
in cold-blooded dinosaurs?

At least 50 percent of the world's species, including the dinosaurs,
perished 65 million years ago. A large meteorite struck Mexico's Yucatan
Peninsula around the same time, and most scientists blame this impact
for the mass extinction.

Yet there is nothing that directly links meteorite impacts with the
extinction of entire species. Scientists can recite a long list of the
devastating environmental consequences of a large meteorite impact, but
they cannot prove these effects have led to the simultaneous loss of
life around the globe. Answering the question of how and why such a
large variety of species died out at the same time is one of the
greatest mysteries in paleontology.

While the exact reason for the Cretaceous-Tertiary (K-T) mass extinction
is still under debate, other past extinctions have been clearly linked
with climate change. As species become increasingly specialized to their
environment, a substantial or sudden change will tend to threaten their
survival. The Earth has gone through many cycles of extreme warming and
cooling in its history, with an associated rise and fall of species.

Could such climate change have played a role in the K-T extinction? One
proponent of this theory is Dewey McLean, a geologist at Virginia
Polytechnic University who first published the idea in the 1970s. McLean
thinks the Chicxulub impact in Mexico just added more stress to an
environment that was already upset by the release of copious amounts of
volcanic gases.

His culprit for the outgassing is the Deccan Traps, an ancient lava flow
in west central India. This flood basalt volcanism, says McLean, upset
the Earth's carbon cycle and led to long-term global warming. McLean
suspects the dinosaurs gradually became extinct through heat-induced
reproductive failure. He says that the higher temperatures, along with
pH changes in ocean water, led to the extinctions seen in marine life at
the time.

But Simon Kelley, a geologist at Open University in England, disagrees
that the volcanic gas from the Deccan Traps could have caused such
warming. He says the traps could have released, at most, only 2 percent
of the carbon dioxide (CO2) already in the atmosphere -- not enough to
trigger global warming. In addition, he notes that volcanoes release
sulfur dioxide (SO2), which causes cooling rather than heating.

"The most recent example of massive volcanism, the Laki eruption of
1783-4 in Iceland, caused cooling in Europe and the northern USA, not
heating," says Kelley. "The bitter winter in Paris was documented by
Benjamin Franklin, envoy from the newly formed United States of America.
Although SO2 is washed out rapidly, the signal of volcanism should be a
combination of the both cooling and heating."

Kevin Pope, a geologist with Geo Eco Arc Research, says there is no
evidence for global warming following the K-T extinction. "In fact, the
best records show an abrupt cooling in the earliest Tertiary," says Pope.

Although McLean says that oxygen isotopes in ancient rocks indicate the
Earth endured long-term global warming from the Cretaceous through the
Tertiary eras, he admits the climate signal is mixed overall, with some
rocks indicating cooling instead of warming. "We have much work to do in
straightening out the K-T climate record," McLean says.

Dinosaurs first appeared and flourished during the Mesozoic, a generally
warm era that lasted from 248 to 65 million years ago. Geologists split
that large chunk of time into the Triassic, Jurassic and Cretaceous
periods.

In the Triassic, all the continents were joined together in one huge
landmass called Pangea. Such large swaths of land tend to influence
their own climate, resulting in very dry conditions, with greater
seasonal fluctuations than in coastal areas. Deserts spread across the
continent during the Triassic, but there were still oases of tropical
and temperate forests.

In the Jurassic, the great continent Pangea broke in half, creating
Laurasia in the north and Gondwanaland in the south. Rainfall increased,
ocean levels rose, and lush rain forests began to displace the deserts.
These tropical forests eventually blanketed much of two continents.

By the end of the Cretaceous, the two continents had separated into even
smaller landmasses that were well on their way toward their present
continental shapes. The late Cretaceous experienced extreme climate
fluctuations, where temperatures would drop and then rebound. This
stressed the environment and likely resulted in the extinction of many
species.

Not only did the breakup of large continents into smaller chunks of land
alter the global climate, but all that tectonic movement also must have
affected the ocean cycles that help regulate climate. The El Ni?o and La
Ni?a ocean cycles of our own time are testament to how strong this
marine influence can be.

After the extinction of the dinosaurs and 50 percent of the world's
species, the early Tertiary (or Paleocene) era begins. Temperatures
continued to fluctuate during this time period, although they were
generally cooler than the end of the Mesozoic.

This cooling may have been due in part to the gasses and debris that was
thrown into the atmosphere by the Chicxulub meteorite impact. But the
continental shuffling in the late Mesozoic also suggests a great deal of
volcanic activity must have been occurring at the time, throwing out
gases that could have changed the balance of atmospheric gases.

The greatest accumulation of lava on the Earth's surface at the time was
in the Deccan Traps of India. The Deccan lava first appeared millions of
years before the K-T extinctions. Sankar Chatterjee, a paleontologist at
Texas Tech University in Lubbock, Texas, says the fossil evidence shows
that dinosaurs lived quite happily right near these lava flows.

"There are layers of sediment, lava, sediment, lava, and so on,
indicating the lava stopped and then started again over a long period of
time," says Chatterjee. "We find dinosaur eggs and bones throughout
these layers, right up to the K-T layer. So they lived around the Deccan
Traps while this lava was erupting."

But then, right about 65 million years ago, the intermittent trickle of
lava became a vast flood. Geologists estimate that 90 percent of the
lava in the Traps was released at that time.

The K-T extinction is not the first mass extinction event to coincide
with a large outpouring of lava. An extensive lava flow in Siberia
occurred about 250 million years ago, around the same time as the
Permian-Triassic (P-Tr) extinction event, the largest extinction of life
in Earth's history.

The P-Tr extinction is often referred to as the "Great Dying," because
90 percent of marine and 70 percent of land species perished.

In 2004, a group of scientists announced that the Bedout crater, buried
off the northwestern coast of Australia, is about 250 million years old,
and therefore may coincide with the P-Tr extinction. The scientists say
the Bedout crater was created by a meteorite similar to the one that
made the Chicxulub crater in Mexico's Yucatan Peninsula.

If Bedout does prove to be an impact crater, it would point to another
instance where both a giant meteorite impact and massive flood basalt
volcanism occurred around the same time as a mass extinction.

Yet the evidence indicating Bedout is an impact crater is not as firmly
established as the impact evidence for Chicxulub, and some scientists
say there is little proof Bedout is anything but a volcanic structure.

Peter Ward, a geologist with the University of Washington in Seattle,
doesn't think the P-Tr extinction was caused by impact. He recently
published a paper in the
journal Science that blames global warming on the Permian extinction.
Just as McLean thinks the Deccan Traps led to the K-T extinctions, Ward
thinks the Siberian lava flows could have eventually led to the P-Tr
mass extinction.

Ward says that the extinction rate in the P-Tr was much more gradual
than the K-T, occurring almost imperceptibly over millions of years. Yet
McLean believes the K-T extinction rate also was slow and incremental.

"I use the term K-T transition' because the biological turnover actually
began during the Late Cretaceous, and extended into the Early Tertiary,"
says McLean. "As indicated by the geobiological record, there was no
global catastrophic extinction of most of Earth's life at the K-T
boundary 65 million years ago."

Most paleontologists disagree, however. The consensus is that the K-T
extinctions took place over a relatively brief point in time.

"The best paleontological data bases, like marine foraminifera
and dinoflagellates and terrestrial pollen, all point to an abrupt
catastrophic event at the K-T boundary," says Pope.

These microscopic fossils are preferable to dinosaur bones when it comes
to determining the time scale of the K-T extinction, says Pope, because
dinosaur fossils are so rare that they can not reliably indicate whether
an extinction was sudden or gradual. (A "short" period or "sudden" event
in the fossil record can describe something that occurred over hundreds
or thousands of years, due to the margin of error in the dating methods.)

Looking at the bone record we do have, it's clear that many dinosaur
species died out as a part of the natural extinction cycle long before
the Chicxulub meteorite hit the Earth, and other species were in
decline. But species that seemed to have robust populations before the
impact suddenly disappear as the fossil record enters the Tertiary.

In fact, the fossil record indicates that dinosaurs achieved their
greatest species variety only a few million years before they became
extinct. This suggests that something dramatic must have occurred to
cause such a definitive end to the reign of the dinosaurs.

Most scientists who study the K-T believe the Chicxulub impact alone
caused the extinction, because the preponderance of evidence suggests
the two events are closely linked in time. But, says Kelley, "we still
have a lot to learn." He notes that, historically, "there is a better
correlation between volcanism and mass extinctions than impact and mass
extinction."

Even if the K-T was triggered by atmospheric changes due to massive
volcanic outgassing, that does not answer many other questions about the
extinction event. For instance, while the dinosaurs and many other
species perished 65 million years ago, a variety of other animals
survived, including the rodent-like mammals that eventually became human
beings. If the environment became so hostile that half of all life on
Earth died, then how did animals like birds, frogs, crocodiles, and
mammals live on?
Received on Mon 09 May 2005 03:28:26 PM PDT