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Home > News > Killer Asteroids May Not Have Been the Cause of Mass Extinctions After All
Geologists had previously considered Chiliques, a simple 5,778-meter (18,957-foot) stratovolcano with a 500-meter (1,640-foot) diameter circular crater in northern Chile, to be dormant. However, images from NASA's TERRA Earth Observing System (EOS) are leading scientists to believe that previously dormant volcanoes such as this one, in widely separated areas of the Pacific "Ring of Fire," are starting to show signs of life. (Image
Credit: NASA/TERRA/ASTER)
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Scientists are coming to the conclusion that extraterrestrial-based theories of great periods of mass extinction on Earth are flawed and that more down to earth factors could have accounted for these devastating events.
Earth history has been punctuated by several mass extinctions rapidly wiping out nearly all life forms on our planet. What causes these catastrophic events? Are they really due to asteroid impacts as is widely believed today? Current research suggests that the cause may come from within our own planet – through the eruption of vast amounts of lava and deadly gases from deep inside the Earth that vent into the atmosphere, knocking the Earth’s delicate equilibrium into a tailspin.
University of Leicester geologists, Professor Andy Saunders and Dr. Marc Reichow, are taking a fresh look at what may actually have wiped out the dinosaurs 65 million years ago and caused other similarly cataclysmic events. They are keenly aware that they may end up exploding a few popular myths along the way.
The idea that a massive asteroid impact caused a mass extinction has been in vogue over the last 25 years, since Louis Alverez’s research team at the University of California in Berkeley 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 -- that is a large asteroid hitting the Earth and ultimately wiping out the dinosaurs – and many other organisms - off the Earth’s surface.
According to Professor Saunders, “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 e.g., at the end of the Permian, 250 million years ago, and at the end of the Triassic, 200 million years ago. The evidence that has been found does not seem large enough to have triggered an extinction at these times.”
Flood basalt eruptions are – he says - an alternative planet killing mechanism that should be studied more closely. These do correspond with all main mass extinctions. Furthermore, they may have released enough greenhouse gases (SO2 and CO2) to dramatically change the climate. The largest flood basalts on Earth (known as the Siberian Traps and the Deccan Traps) coincide with the largest extinctions (at the end of the Permian and the end of the Cretaceous periods). “Pure coincidence?” asks Saunders.
While this is unlikely to be pure chance, the Leicester researchers are interested in precisely what the kill mechanism may have been. One possibility is that the gases released by volcanic activity lead to a prolonged volcanic winter induced by sulphur-rich aerosols, followed by a period of CO2-induced warming.
The Siberian Traps are the largest known continental flood basalts. Eruptions about 250 million years ago at high latitude in the northern hemisphere caused these continental flood basalts - vast outpourings of lava that covered large areas of the Earth's surface. A major debate is now underway concerning their origins and their environmental impact.
Using radiometric dating techniques, the scientists 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. 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.
From these data, Professor Saunders and his team hope to link the volcanism to the extinction event. According to Professor Saunders, “If we can show, for example, that the full extent of the Siberian Traps was erupted at the same time, we can be confident that their environmental effects were powerful. Understanding the actual kill mechanism is the next stage.”
In related, but independent research, Dr. Mark Sephton from Imperial College in London said: "The cause of the end Permian extinction has been highly controversial. We show that the terrestrial ecosystem was the first to suffer. The continent-wide nature of the event implies that it was caused by something in the atmosphere. The unique chemical data indicates that something fast and catastrophic happened on land."
However, analysis of a unique set of molecules found in rocks taken from the Dolomites in Italy has enabled scientists to build up a picture of what actually happened. The molecules are the remains of polysaccharides, large sugar-based structures common in plants and soil, and they tell the story of the extinction.
The molecules date from the same time as the major volcanic eruption that caused the great outpouring of basalt lava over vast swathes of land in present day Siberia. The researchers believe that the volcanic gases from the eruption, which would have depleted Earth's protective ozone layer and acidified the land and sea, killed rooted vegetation. This meant that soil was no longer retained and it washed into the surrounding oceans.
The chemistry of the rocks reveals that although the sugar molecules were found in marine sediments, they were derived from land, supporting the theory that massive soil erosion caused them to end up in the sea.
Soil materials in the oceans would have blocked out light and soaked up oxygen. Analysis of rock chemistry suggests that after the soil crisis on land, the marine ecosystem succumbed to the stresses of environmental change and oceanic life faltered, completing a global catastrophe.
Professor Henk Visscher of Utrecht University, also part of the research team, commented: "Similar to the 'Dead Zone' nowadays spreading in the Gulf of Mexico, the soil crisis could have caused a worldwide expanse of uninhabitable low-oxygen conditions in shallow marine waters. So what began on land ended in the sea. It seems there was no place to hide at this time of great dying."
For More Information:
http://www2.le.ac.uk/ebulletin/news/press-releases/2000-2009/2006/03/nparticle-cjk-wh
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http://www.imperial.ac.uk/P7101.htm
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