Disappearance of dinosaurs offers environmental lessons
ONE thing should be stated clearly at the outset. No one knows for sure what did in the dinosaurs. But accumulating evidence strongly suggests a global catastrophe was involved. United States Geological Survey (USGS) scientists have laid out the latest indication. They find that shocked quartz - quartz grains subjected to extremely high pressures - are widely associated with the 65 million-year-old geological stratum separating the Cretaceous and Tertiary periods. This so-called K-T boundary marks the end of the dinosaur age.
The USGS work strongly supports the hypothesis that a massive asteroid hit Earth at that time. Such an impact would have released energy on a scale to match or exceed the simultaneous explosion of the all the world's nuclear weapons at a single spot. The environmental disruption - including a sun-dimming dust pall - could have given the coup de gr^ace to the most massive extinction of species in the fossil record. That extinction could well reflect the greatest ecological catastrophe our planet has known until today.
This last caveat is important because many ecologists suspect that man-made environmental disruption has begun a mass extinction of living species on a similar scale. Indeed, one reason to try to understand what happened then is to get a better perspective on what's happening today.
Ever since Walter and Luis Alvarez of the University of California at Berkeley and a group of colleagues first suggested it eight years ago, linking the Cretaceous extinctions to an asteroid impact has stirred wide scientific interest and controversy. The Alvarez group and other investigators have shown that the K-T boundary stratum is rich in iridium. This element is rare on Earth's surface. Geochemists think our planet's iridium supply was carried deep into the interior early in Earth's evolution. But the element should be common in asteroids and comets. Hence, the iridium in the K-T boundary can be taken as an extraterrestrial signature.
More recently, shocked quartz turned up at a K-T site in Montana, suggesting a second possible extraterrestrial signature. Bruce F. Bohor, Peter J. Modreski, and Eugene E. Foord of the USGS followed up that clue. They isolated more shocked quartz from five K-T sites in Europe, from a site in New Zealand, and from a sediment core in the North-Central Pacific Ocean. Detailed study of these quartz grains shows that they have been subjected to instantaneous pressures some 90,000 times sea level air pressure or higher. That strongly suggests the impact of a massive asteroid hitting Earth at a speed of 33,000 miles an hour or faster.
While this helps the asteroid case, it doesn't settle the question. Some geophysicists have insisted right along that you don't need to look to space to explain what happened 65 million years ago. They cite evidence for massive volcanism. They argue that this, too, could have caused global ecological disruption, including a sky-obscuring dust veil. They say it also could have brought iridium from deep within the planet to enrich the K-T layer. As for shocked quartz, they insist that this could be associated with the volcanism, although the mechanism for producing it is unclear.
In any event, no single catastrophe, however massive, can fully account for the Cretaceous extinctions. As paleobiologist Steven M. Stanley of Johns Hopkins University explains in the May issue of the Bulletin of the American Academy of Arts and Sciences, Earth's global ecosystem had been deteriorating for several million years before the asteroid (or volcanic outbreak) would have arrived. Such a final catastrophe would only have finished off a decline that, Dr. Stanley says, very likely was due to global climatic cooling.
This emerging picture of the global ecosystem, already under stress, being pushed to catastrophe by a cataclysmic event provides an instructive allegory for our own time. As Stanley notes, the natural factors that brought mass extinctions in the past now are matched by the destructive acts of humans. Removal of forests, spread of deserts, and general loss of habitat devastate many species. Wide use of pesticides and other polluting chemicals take their toll.
Reviewing the impact that volcanoes could have had at the end of the Cretaceous, Charles B. Officer of Dartmouth College and colleagues explained in Nature: ``The volatile emissions from this volcanism would lead to acid rain, ... global atmospheric temperature changes, and ozone layer depletion.'' That sounds uncomfortably similar to what many geochemists suspect humanity is doing to the global environment today.
A Tuesday column. Robert C. Cowen is the Monitor's natural science editor.