Volcanic Gases, Not Meteors, May Have Caused Mass Extinctions
LEICESTER, UK, March 13, 2006 (ENS) - Earth's history has been punctuated by mass extinctions that have rapidly wiped out nearly all life forms on the planet. To determine what caused these events, British geologists are challenging the currently held theory that meteorite impacts are to blame for wiping out the dinosaurs 65 million years ago and other mass extinctions.
Professor Andy Saunders and Dr. Marc Reichow are testing the theory that gases released by volcanic activity led to a prolonged volcanic winter brought on by sulphur-rich aerosols, followed by a period of warming induced by carbon dioxide in the atmosphere.
Flood basalt eruptions correspond with all main mass extinctions, says Saunders, within error of the techniques used to determine the age of the volcanism. Flood basalt eruptions are vast outpourings of lava that covered large areas of the Earth's surface, creating what geologists call flood basalt provinces.
Saunder says, these flood basalt events may have released enough greenhouse gases - sulfur dioxide (SO2) and carbon dioxide (CO2) - to "dramatically change the climate."
The largest flood basalt provinces on Earth, known as traps, coincide with the largest extinctions. The Siberian Traps correspond with the end of the Permian era some 251 million years ago when around 95 percent of all living species died out. The Deccan Traps in India correspond with the end of the Cretaceous era, some 65 million years ago when the dinosaurs disappeared, leaving only fossils.
The Siberian Traps are the largest known continental flood basalt province. Erupted about 250 million years ago at high latitude in the northern hemisphere. A scientific debate is underway concerning the origin of these provinces and their environmental impact that this research team hopes to clarify.
Using radiometric dating techniques, they hope to constrain the age and, combined with geochemical analysis, the extent, of the Siberian Traps.
Measuring how much gas was released during these eruptions 250 million years ago is "a considerable challenge," Saunders says.
The researchers will study microscopic inclusions trapped in minerals of the Siberian Traps rocks to estimate the original gas contents. Using these data they hope to be able to assess the amount of SO2 and CO2 released into the atmosphere 250 million years ago, and whether or not this caused climatic havoc, wiping out nearly all life on Earth.
By studying the composition of sedimentary rocks laid down at the time of the mass extinction, they also hope to detect changes to seawater chemistry that resulted from major changes in climate.
The idea that meteorite impacts caused mass extinctions has been in fashion over the last 25 years, since Louis Alverez’s research team in Berkeley, California published their work about an extraterrestrial iridium anomaly found in 65 million year old layers at the Cretaceous-Tertiary boundary.
This anomaly only could be explained by an extraterrestrial source, a large meteorite, hitting the Earth and ultimately wiping out the dinosaurs and many other species, according to Alverez.
Professor Saunders observed, “Impacts are suitably apocalyptic. They are the stuff of Hollywood. It seems that every kid’s dinosaur book ends with a bang. But are they the real killers and are they solely responsible for every mass extinction on Earth?"
There is scant evidence of impacts at the time of other major extinctions such as at the end of the Permian, 251 million years ago, and at the end of the Triassic, 200 million years ago. Saunders says the evidence that has been found does not seem large enough to have triggered an extinction at these times.
Dr. Michael Benton, a professor of paleontology at Bristol University, is also studying the extinctions of 251 million years ago. “Over the past decade or so, new discoveries in the field and important progress in dating techniques have given us a more precise picture of this period,” he wrote in April 2005.
The suddenness and the scale of the event first suggested a collision with an asteroid or other meteor. Recent paleontological data suggest a different cause, Benton wrote in "RTD Info," a magazine on European research.
Volcanic eruptions over 600, 000 years shook the only continent existing 251 million years ago, Pangaea. Huge eruptions occurred in what is now Siberia, spreading basaltic lava onto the Earth’s surface across an area equivalent to the size of Europe to a depth of between 400 and 3,000 meters - the Siberian Traps. Recent dating techniques place these eruptions at the end of the Permian era, Benton and Saunders' team both say.
“Volcanism, and the colossal CO2 emissions that accompany it, are today seen by many scientists as the element that triggered the array of factors at the origin of this brutal crisis,” Benton wrote.
The work by Paul Renne of the Institute of Human Origins in Berkeley, California, and Asish Basu of the University of Rochester indicates that the volcanic activity occurred very rapidly.
"This eruption occurred over an extremely short period of time, geologically speaking," said Renne in 1991. "We believe we've nailed the date down fairly precisely. It's even conceivable that it occurred over an interval as short as 200,000 years."
There are other suggestions that atmospheric hydrogen sulfide and carbon dioxide led to the mass extinctions.
In February 2005, Pennsylvania State geoscientist Dr. Lee Kump, studying bacteria in deep sea sediments, said the volcanic eruptions in Siberia 250 million years ago in the end-Permian period, may have started a cascade of events leading to high hydrogen sulfide levels in the oceans and atmosphere and precipitating the largest mass extinction in Earth's history.
"The recent dating of the Siberian trap volcanoes to be contemporaneous with the end-Permian extinction suggests that they were the trigger for the environmental events that caused the extinctions," wrote Kump in 2005. "But the warming caused by these volcanoes through carbon dioxide emissions would not be large enough to cause mass extinctions by itself."
As the levels of atmospheric oxygen fell and the levels of hydrogen sulfide and carbon dioxide rose, the upper levels of the oceans could have become rich in hydrogen sulfide. Kump says this would kill most the oceanic plants and animals and the hydrogen sulfide dispersing in the atmosphere would kill most terrestrial life.
"A hydrogen sulfide atmosphere fits the extinction better than one enriched in carbon dioxide," says Kump. "Carbon dioxide would have a profound effect on marine life, but terrestrial plants thrive on carbon dioxide, yet they are included in the extinction."
Another piece in the puzzle surrounding the end-Permian extinction is that hydrogen sulfide gas destroys the ozone layer. Recently, Dr. Henk Visscher of Utrecht University and his colleagues suggested that there are fossil spores from the end-Permian that show deformities that researchers suspect were caused by ultraviolet light.
"These deformities fit the idea that the ozone layer was damaged, letting in more ultraviolet," says Kump.
Once this process is underway, methane produced in the ample swamps of this time period has little in the atmosphere to destroy it. The atmosphere becomes one of hydrogen sulfide, methane and ultraviolet radiation.
More about the research on Siberian Traps is online at: http://www.le.ac.uk/gl/ads/SiberianTraps/Index.asp