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The findings may provide an opportunity for scientists to study the cause and effect of an event that wiped out animal life similar to species on Earth today.
"Unlike what impacts did to dinosaurs and other prehistoric creatures, this was not an event that led to global extinctions," said principal investigator Peter Schultz, professor of geological sciences at Brown University and an impact specialist. "We've found something linked to much more recent land history. The advantage to studying something this young is that you can really examine the forensics.
"This is a threshold event. It may have been small enough to cause regional damage and extinctions and may have triggered a climate change. El Niño or a volcanic eruption produces small tweaks to the climate compared to what one of these impacts can do." The cyclical cooling of the Earth's temperatures that began soon after the impact 3.3 million years ago continues today, he said.
The study is published in this week's Science magazine. Its co-authors are Argentinean scientists Marcelo Zarate and Cecilia Camilion; Willis Hames, an Auburn University geologist; and John King, a researcher in the Graduate School of Oceanography at the University of Rhode Island. The team studied an 18-mile-long narrow layer of greenish glass and red brick-like materials found in the high ocean cliffs of southeastern Argentina. Called escoria, the glass had puzzled scientists since it was first described in 1865. [Editors: A color image of the escoria is available at the News Bureau's web site.]
The glass and surrounding red-baked powder bear the signatures of a powerful ancient blast archived in the thick Argentine dust, say the researchers. They describe a half-dozen physical signs, from the twisted and folded shapes of the glass to its isolation from other potential sources such as volcanoes. Chemical analysis of the glass produces all the right impact signatures: unusually high levels of magnesium oxide and calcium oxide, significant amounts of iridium and chromium, and only the tiniest traces of water.
The study shows the glass occurs just below a layer of dusty deposits containing fossil evidence of a 3-million-year-old disappearance of 36 local types of animals. Extinct species include large armadillo-like creatures, ground sloths, hoofed groups of related mammals and a flightless carnivorous bird. Other fauna later appeared in their place.
By using a laser fusion technique to measure heavy to light argon atoms in the glass, and by comparing the magnetic readings of the glass layer to published records of magnetic-field changes over the eons, the researchers date the glass as 3.3 million years old, just prior to the extinctions.
Using research by other scientists that compared heavy to light oxygen isotopes in sediment cores from the nearby ocean floor, Schultz and colleagues offer evidence of a sudden drop in both atmospheric and water temperatures almost 3.3 million years ago. The finding indicates that a climate change occurred shortly after the glass appeared and just prior to the animal life turnover.
"This research is analogous to comparing several time clocks," said Schultz. "We compared a clock in the glass to a clock in the soil to a clock in the deep-sea cores. This told us the conditions at the time. We were surprised to find that the appearance of the glasses and the turnover of the fauna coincided with a temperature drop."
The research began as a simple project to determine the origin and age of the escorias. However, the work identified a series of coincidences that strongly suggest a major, ecosystem-altering event took place relatively recently, geologically speaking, he said.