Princeton University 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 courtesy of Gerta Keller)
by Morgan Kelly
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 University 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) in October. 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 said, 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," said 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 in 2010. 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 said. "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 University Diderot, said that the Princeton papers are based on a closer examination of Deccan volcanism and its aftermath than has been conducted previously. As such, he said, 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 said.
"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 said. "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 said, 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 said. "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 said.
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