Peter Schultz's discovery began by chance during a visit to Argentina.
You never know what will turn up on a trip. Military buffs comb battlefields along their travel routes. Amateur archeologists poke through every abandoned home site they find. Peter Schultz digs impacts.
Whenever he travels, Schultz's eyes are trained to spot the tiniest clues that suggest an ancient collision between asteroid and Earth.
Schultz's latest, and possibly most important impact discovery, began by chance during a visit to Argentina in 1992. That's when he was shown small pieces of green glass by a young woman who had picked them up as a child years earlier. She could not remember their origin and Schultz was unfamiliar with the material. Yet he saw that the glass bore the geophysical signs of having formed via tremendous heating followed by sudden cooling. Back at Brown, Schultz had the pieces analyzed chemically and found all the right signatures of an Earth-object collision: unusually high levels of magnesium oxide and calcium oxide, significant amounts of iridium and chromium, and only the tiniest traces of water.
A year later, Schultz, a professor of geological sciences, was back in Argentina searching for the origin of the glass. There he met Marcelo Zarate, an Argentinean geologist. Zarate had spent years trying to explain an 18-mile-long narrow layer of glass and red brick-like materials found in the ocean cliffs of southeastern Argentina. Called escoria, the glass had puzzled scientists since first described in 1865. Schultz looked at a sample of the escoria and told Zarate that this was from a field of impact glass just like the green glass.
Schultz and Zarate visited the coast to examine the great stretches of escoria. Together, the researchers noted a half-dozen distinctive impact signatures, from the twisted and folded shapes to the isolation from other potential sources such as volcanoes. The findings strongly suggested that both the glasses and the surrounding red baked powder were the result of a powerful ancient blast archived in the thick Argentine dust.
Despite a lack of grant funding, Schultz and Zarate enlisted the help of three other scientists, who agreed to donate their time and resources to describe the impact materials and site. Because scientific evidence linked the disappearance of dinosaurs 65 million years ago to an asteroid-Earth impact, the researchers wanted to know precisely when the escoria layer was formed and if there were any measurable environmental effects associated with its appearance. In the process, the researchers unearthed some remarkable coincidences.
First, with the help of Argentinean mineralogist Cecilia Camilion, the researchers confirmed that the glass occurred just below a layer of dusty deposits containing fossil evidence of an unexplained 3-million-year-old disappearance of 36 local types of animals. Extinct species included large armadillo-like creatures, ground sloths, hoofed groups of related mammals and a flightless carnivorous bird. Other creatures eventually appeared in their place.
Then, geologist Willis Hames of Auburn University provided a date for the escoria's formation. A specialist in finding and attaching historical timings to rocks and soil strata, Hames used a laser fusion technique to measure heavy to light argon atoms in the glass, dating it at about 3.3 million years.
Hames' finding was validated by John King, a specialist in the use of magnetic measurements to date layers in which materials occur. The researcher in the Graduate School of Oceanography at the University of Rhode Island compared magnetic readings of the layers to published records of magnetic-field changes over the eons. King's result: the glass layer was 3.3 million years old.
The parallel between the appearance of the glass and the turnover of the region's animal life led the researchers to wonder how an impact may have affected the region's climate.
Using research by other scientists that compared the heavy to light oxygen isotopes in sediment cores from the nearby ocean floor, Schultz and colleagues found evidence of a sudden drop in both atmospheric and water temperatures almost 3.3 million years earlier. Something had changed the climate. Maybe it was the impact.
"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." The most surprising finding was that the appearance of the glasses and the turnover of animal life both coincided with a temperature drop, he said.
What began for Schultz as a personal quest to determine the origin and age of several marble-sized pieces of green glass became a series of coincidences that strongly suggest a major ecosystem-altering event took place relatively recently, geologically speaking.
These coincidences are described in a paper that will be published in Science magazine Dec. 11. The paper says that a previously unknown impact from an asteroid or comet coincided with the disappearance of 35 different types of ancient mammals and a flightless bird 3.3 million years ago. The impact may have directly caused the regional extinctions or triggered a climate change that led to the disappearance of the animals in what is now southeastern Argentina. Other animals eventually appeared in their place.
"Unlike what impacts did to dinosaurs and other prehistoric creatures, this was not an event that led to global extinctions," said Schultz. "But now 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.
"What makes this interesting is that it's a threshold event. It may have been small enough to cause regional damage and extinctions and may have triggered a climate change." In fact, the cyclical cooling of the Earth's temperatures that began soon after the impact continues today, he said.
The findings may provide an opportunity for scientists to study the cause and effect of an event that wiped out animals similar to species now on Earth, Schultz said. The research may also fuel the fires of people who ponder the emergence of our own species, because the impact and subsequent cooling both occurred at about the time homo sapiens appeared.
"Makes you wonder if the cold and related stress could have driven the evolutionary change from small apes to the emergence of human beings," he said.
Down the road, that may be one more amazing coincidence in a series that began with a handful of green glass.