[meteorite-list] Latest from Gerta Keller - Chixilub didn't really do it...

From: David R. Vann <drvann_at_meteoritecentral.com>
Date: Fri, 18 Nov 2011 11:37:36 -0500
Message-ID: <C430E565E30B45AEB57885BB86B3B703_at_sas.upenn.edu>

Not sure how much I agree with all this, but it sures seems the end Cretaceous
would have been a bad time to be on planet Earth.

One-Two Punch Caused Mass Extinction
November 18, 2011

Princeton Univ. researchers found that massive, prolonged eruptions of the
Deccan Traps in India gradually eliminated species and resulted in the
Cretaceous-Tertiary mass extinction that killed the dinosaurs 65 million years
ago. Marine sediment trapped between Deccan lava flows revealed that a species
known as planktonic foraminifera-widely used to gauge the severity of
prehistoric disasters-succumbed to lava mega-flows and volcano-induced
environmental stress such as acid rain and drastic climate changes. As
conditions on Earth worsened, large, variedspecies (left) were eliminated. The
no more than seven or eight smaller species (right) that remained dwarfed
further. Image: Gerta Keller
A cosmic one-two punch of colossal volcanic eruptions and meteorite strikes
likely caused the mass-extinction event at the end of the Cretaceous period that
is famous for killing the dinosaurs 65 million years ago, according to two
Princeton Univ. reports that reject the prevailing theory that the extinction
was caused by a single large meteorite.

Princeton-led researchers found that a trail of dead plankton spanning half a
million years provides a timeline that links the mass extinction to large-scale
eruptions of the Deccan Traps, a primeval volcanic range in western India that
was once three-times larger than France. A second Princeton-based group
uncovered traces of a meteorite close to the Deccan Traps that may have been one
of a series to strike the Earth around the time of the mass extinction, possibly
wiping out the few species that remained after thousands of years of volcanic
activity.

Researchers led by Princeton professor of Geosciences Gerta Keller report this
month in the Journal of the Geological Society of India that marine sediments
from Deccan lava flows show that the population of a plankton species widely
used to gauge the fallout of prehistoric catastrophes plummeted nearly 100
percent in the thousands of years leading up to the mass extinction. This
eradication occurred in sync with the largest eruption phase of the Deccan
Traps-the second of three-when the volcanoes pumped the atmosphere full of
climate-altering carbon dioxide and sulfur dioxide, the researchers report. The
less severe third phase of Deccan activity kept the Earth nearly uninhabitable
for the next 500,000 years, the researchers report. A substantially weaker first
phase occurred roughly 2.5 million years before the second-phase eruptions.

Another group based in Keller's lab found evidence in Indian sediment of a
meteorite strike from the time of the mass extinction that would have been
sufficient to finish off the few but weakened species that survived the Deccan
eruptions, according to a report in the journal Earth and Planetary Science
Letters (EPSL). This same sediment-located in Meghalaya, India, more than 600
miles east of the Deccan Traps-portrayed the Earth during this period as a harsh
environment of acid rain and erratic global temperatures.

Taken together, Keller says, the Princeton findings could finally put to rest
the theory that the mass-extinction event-known as the Cretaceous-Tertiary, or
KT, for the periods it straddles-was triggered solely by a large meteorite
impact near Chicxulub in present-day Mexico. That impact -which occurred around
the time of the second-phase Deccan eruptions-is thought to have been 2 million
times more powerful than a hydrogen bomb and generated an enormous dust cloud
and gases that radically altered the climate. Keller has long held that the
Chicxulub impact was not catastrophic enough to cause the KT mass extinction-the
newest work from her lab, however, shows that the largest Deccan eruptions were.


"Our work in Meghalaya and the Deccan Traps provides the first one-to-one
correlation between the mass extinction and Deccan volcanism," says Keller, who
is lead author of the Geological Society paper and second author of the EPSL
paper after lead author Brian Gertsch, who earned his Ph.D. from Princeton.
Gertsch is now a postdoctoral researcher at the Massachusetts Institute of
Technology.

"We demonstrate a clear cause-and-effect relationship that these massive
volcanic eruptions were far more destructive than previously thought and could
have caused the KT mass extinction even without the addition of large meteorite
impacts," Keller says. "But given the environmental instability caused by the
massive Deccan eruptions, an impact could easily have killed off the few
survivor species at the end of the Cretaceous. It would have been a double
whammy."

Vincent Courtillot, a geophysicist and professor at Paris Univ. Diderot, says
that the Princeton papers are based on a closer examination of Deccan volcanism
and its aftermath than has been conducted previously. As such, he says, the
researchers' "impressive analysis" confirms the timing of the Deccan eruptions
and environmental fallout reported in recent years by various research teams,
including his own.

Courtillot, who is familiar with the Princeton work but had no role in it, led
the team that reported in the Journal of Geophysical Research in 2009 that
Deccan volcanism occurred in three phases, the second and largest of which
coincides with the Cretaceous-Tertiary mass extinction; the Keller-led study
published in the Journal of the Geological Society of India confirms the second
and third phases, he says.
 
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"The significance of this recent work is that the analysis was conducted in
important sections near the volcanic action, and not thousands of kilometers
away as had been the case previously," Courtillot says. "They provide support
for the idea that carbon and sulfur dioxide emissions were the principal agents
of environmental change and stress, and conclude that the characteristics of the
second-phase eruptions were such that it could alone have caused the mass
extinction."

In addition, Courtillot says, the approach the teams used could prove valuable
to understanding the part volcanoes played in other extinction events in Earth's
history. "Exceptional, massive volcanism, I am now quite sure, is the general
cause of mass extinctions," he says. "But in order to be considered as proven
and quantitatively explained, we need the kind of extensive, detailed work
described by these teams to be conducted for all other extinctions."

The case for Deccan over the Chicxulub impact as the cause of the KT extinction

Keller is prominent among scientists who reject the Chicxulub impact's role in
the end-Cretaceous mass extinction. She is well known for leading a team of
researchers who announced in 2003 that a sediment core from the Chicxulub crater
revealed that the impact predated the mass-extinction event by about 300,000
years.

Keller and her co-authors published their findings in the journal Proceedings of
the National Academy of Sciences in 2004 and suggested that the Chicxulub
meteorite was instead one of several meteorite strikes that occurred in the
several hundred thousand years leading up to the mass-extinction event. They
concluded that while destructive, the Chicxulub impact was not powerful enough
to have caused widespread annihilation. Keller and her collaborators have since
supported these findings with additional evidence from Texas and northeastern
Mexico published in EPSL in 2007 and the Journal of the Geological Society of
London in 2009, respectively.

Keller has joined other scientists in focusing her research on the 30-year-old
idea first championed by Virginia Tech geologist Dewey McLean that Deccan
volcanism was the root of the Cretaceous mass extinction. Until recently, the
theory was in question because the eruptions were thought to have been stretched
out over a period of more than 1 million years, leaving plenty of time for the
Earth to recover between eruptions, Keller says.

Improved dating technology, however, allowed scientists-particularly the team
led by Courtillot-to narrow the time of the largest eruptions to a few hundred
thousand years at the end of the Cretaceous. Known as Deccan phase-2, this
period accounted for 80 percent of the total volcanism. The first and weakest
phase of activity occurred about 67.5 million years ago; the third and final
eruption phase began about 300,000 years after the KT mass extinction.

In 2008, Keller and her team reported in EPSL the first direct link that the KT
extinction coincided with the end of the second phase of Deccan eruptions. She
explained that marine sediments preserved between lava flows from the second-
and third-phase eruptions contained evidence of the KT boundary, a thin,
worldwide geological layer that marks the mass-extinction event.

Deccan volcanism behind the mass extinction, so say the plankton

The work published Nov. 1 by the Geological Society of India builds on Keller's
2008 paper in EPSL. She and her co-authors examined cores from Deccan lava flows
near Rajahmundry in the Krishna-Godavari Basin, the remnant of an ancient sea on
the Bay of Bengal coast, and found that lava flows from the second and third
Deccan phases are separated by marine sediments.

Keller worked with scientists with India's government-operated Oil and Natural
Gas Corporation, which owns the sediment cores. Also included is Thierry Adatte,
a geologist with the Univ. of Lausanne in Switzerland, who is Keller's long-time
collaborator and a co-author on the papers challenging the time of the Chicxulub
impact, as well as previous papers on Deccan volcanism.

The team examined the basin's sediment layers to determine the size and number
of a species known as planktonic foraminifera that remained following each
eruption phase. These plankton are single-celled micro-organisms ranging in size
from the point of a needle to a pinhead that are highly sensitive to changes in
oxygen, salinity, temperature and nutrients, Keller says. Their sensitivity to
environmental changes and their near extinction at the end of the Cretaceous
makes the species key to determining the timespan, pace and severity of the mass
extinction.

After studying microplankton remains in sediment from below, between and above
the second-phase lava flows, the researchers observed that the number of living
species dropped 50 percent at the onset of eruptions. The species count plunged
by another 50 percent after the first of what would be four lava mega-flows. No
more than seven to eight of the species that were most tolerant to environmental
changes survived after the first mega-flow, and no recovery occurred between
subsequent mega-flows. By the end of the fourth mega-flow the mass extinction
was complete, the researchers wrote.

The vast amounts of carbon dioxide and sulfur dioxide poured into the atmosphere
by the end of the second volcanic phase-estimated to be 30-times more than the
levels produced by the Chicxulub impact-resulted in, among other crises, heavy
acid rain, acidic oceans and global temperatures that swung between scorching
and frigid, the researchers report. The third eruption phase prolonged these
conditions.

Thus, the number of species evolving remained low, and existing species dwarfed
during the 500,000-year period after the mass extinction, although no
significant extinctions occurred again, Keller and her co-authors found. New,
larger marine species did not appear until after the third phase when Deccan
eruptions went dormant, suggesting that life began to recover as the atmosphere
became less poisonous.

"In my work, I had always observed evidence of marked changes in species
abundance with gradually higher levels of stress and extinction during the last
several hundred thousand years, rather than one single instantaneous
annihilation," Keller says. "For lack of better evidence, scientists had
interpreted this steady decline as the result of climate and sea-level changes."


Evidence that a large meteorite helped finish the job

For the paper published in EPSL, Keller and her co-authors provide a supporting
and more nuanced depiction of conditions during the Deccan period. They examined
sediments from an ancient shallow sea in Meghalaya where rock layers are known
to contain among the clearest fossil records of the Cretaceous-Tertiary mass
extinction, Keller says.

Analysis of the Meghalaya sediment revealed an inhospitable planet rife with
high humidity, severe storms and massive blooms of the plankton species
Guembelitria cretacea, a disaster opportunist that flourished in devastated
environments when few other species survived.

At the same time, the team detected large amounts of iridium, an element
typically associated with meteorite impacts, Keller says. Iridium is rare on
Earth yet is found in high concentrations in the KT boundary, a phenomenon known
as the iridium anomaly. Remnants of iridium at the KT boundary in Meghalaya
coincide with the global KT boundary iridium anomaly, she says.

The new evidence of a meteorite strike at Meghalaya that coincides with the KT
mass extinction supports the theory Keller proffered in 2003 that multiple
meteorites struck the Earth around the time of the Deccan eruptions, adding to
the volcano-fueled misery of the mass-extinction era.

"Our data suggest that the mass extinction of the dinosaurs and other species
was caused by the harsh conditions resulting from massive Deccan eruptions and
the coincidence of multiple meteorites," Keller says. "In light of this new
evidence, the single-impact story seems more like an article of faith at this
point."

The study published in the Journal of the Geological Society of India about the
Deccan eruption and the meteorite research published in EPSL were both supported
by grants from the National Science Foundation.

Source: Princeton Univ.


David R. Vann, Ph.D.
Department of Earth and Environmental Science
THE UNIVERSITY of PENNSYLVANIA
240 S. 33rd St.
Philadelphia, PA 19104-6316
drvann at sas.upenn.edu
office: 215-898-4906
FAX: 215-898-0964
Received on Fri 18 Nov 2011 11:37:36 AM PST