EMBARGOED for 2 p They call them the
Big Five -- a handful of unfathomable mass extinctions over the past 500 million years, each estimated to have obliterated somewhere between 50 and 96 percent of all species on the planet.That much we know, because Earth recorded the mass deaths in layers of ancient soil, where crowds of miniscule corpses and other evidence show wholesale destruction of the smallest critters, on which larger animals depend.
What we don't know, except in one case, is what caused these five mass
extinctions. Nor is there solid evidence showing how rapidly the catastrophes occurred. Such knowledge would be a window not only to the past, but to the future: How likely is it that future Earth dwellers will meet with an inescapable catastrophic fate, much like the dinosaurs did? And how much time will there be to adjust or perish?
While there are no firm answers, a three-page study in the May 11 issue of the journal Science adds modestly to a mounting stack of reports suggesting that
asteroids and comets are the leading cause of terrestrial death, delivering immensely fatal blows every 100 million years or so that wipe the slate of life frighteningly close to clean in remarkably rapid fashion.Death came quick
The new study involved the fifth largest known mass extinction, in which roughly half of all species were wiped out. It occurred about 200 million years ago at the boundary of the Triassic and Jurassic Periods in geologic history. This T-J boundary, as it is called, marks the dawn of the dinosaurs.
In exposed soil layers on islands off the coast of British Columbia, researchers found that droves of marine plankton kicked their watery buckets at that time. Simultaneously, plants were disappearing rapidly, as seen in a quick drop in the rate at which organic carbon was created through processes such as photosynthesis.
The extinction occurred in 50,000 years or less, the study's authors write, possibly within as few as 10,000 years -- the blink of an eye in geological terms. And far faster than previous estimates, which ranged up to 10 million years.
The paper did not speculate about a cause. But Peter D. Ward, a University of Washington Earth and space sciences professor, and lead author of the study, told SPACE.com that the evidence makes it look very much like the later Cretaceous-Tertiary extinction, or K-T event, which wiped out the dinosaurs. The K-T event is the lone mass extinction for which researchers have a definitive smoking gun: A
crater in the Yucatan Peninsula excavated by an asteroid."The very rapidity of the [T-J] event and the geochemical and paleontological similarity to K-T boundary sections makes it look like an impact," Ward said. "My gut feeling is that it was impact."
But he quickly added that there are other possible causes, such as rapid climate change due to heavy volcanic activity.
The world's number-one killer?
Noted University of Chicago paleontologist David Raup told SPACE.com that several studies have shown "reasonably good evidence" linking other mass extinctions to impacts, but these reports are "almost always ignored."
Raup produced a study in 1992 suggesting that roughly 60 percent of all species extinction may have been caused by impacts.
"I strongly suspect that in a few years this will be the conventional wisdom, but it is strangely slow in coming," Raup said. "Ward's paper and a couple of others recently on the Permian extinction may get people to rush to the other side of the boat."
Other scientists agreed there remain other possible causes for Earth's greatest mass extinctions. But the case for cosmic impacts is growing.
"It seems that several lines of evidence based on new data and careful statistical analysis are now showing that at least some of the great mass extinctions were geologically instantaneous, leading us to look for catastrophic causes such as asteroid impacts," said David Morrison, an asteroid researcher at NASA's Ames Research Center in California. "We know that impacts have occurred throughout Earth's history, so the connection seems plausible."
Morrison cautioned that there is "much research yet to be done" before the connection can be made clear.
Next Page: Clues in craters
advertisement
