Johns Hopkins Magazine - November 1994 Issue

Science & Technology

Double pulse in the Paleozoic, etc.

The case of the hidden extinction

Just before the start of the Cenozoic era, 65 million years ago, a meteorite crashed into Earth. The event was so dire that it created instant winter, a worldwide storm of dust and debris that shut out the sun for months--and, most paleontologists now agree--killed nearly half of existing species, including all the big dinosaurs.

Now, that's dramatic. You wouldn't think the fossil record could hold another extinction that big, unrecognized. It does, though, according to Steven Stanley, professor of earth and planetary sciences at Hopkins. Stanley has discovered a major extinction that happened some 250 million years ago, near the end of the Paleozoic. The event was hidden like Poe's purloined letter--in plain sight.

In a paper recently accepted by Science magazine, Stanley and former student Xiangning Yang (PhD '86), now a professor at Nanjing University, argue that the Paleozoic era ended with two worldwide die-offs. "It was a double event," says Stanley: two abrupt pulses of major extinction, separated by five million years.

Until now, scholars had thought there was only one closing event, a massive extinction at the end of the Tatarian interval (the last of the Paleozoic), which was said to have wiped out as much as 96 percent of species.

Stanley agrees that final event occurred. "The terminal event remains the biggest extinction of all time," he says. "But according to our calculations, only about 80 percent of species died out at that time." Stanley and Yang conclude that species had already been seriously impoverished by an earlier pulse, at the end of what is called the Guadalupean interval.

The fossil record looks that way, on the face of it. "We find a lot of species in the Guadalupean that are not found later, in the Tatarian," says Stanley. But scholars had explained this away on grounds that the fossil record was poor. They argued that most of the species that appeared to die out in the Guada-lupean had indeed been present in the Tatarian; they just hadn't been found because there were too few fossils.

Stanley wondered about that. What if the fossil record were basically correct, as it stood? He thought of several ways to test the idea, including study of fusulinacea, the best-preserved group of the whole late Paleozoic.

Fusulinacea were small, amoeba-like creatures that lived on the sea floor, secreted skeletons, and throve so mightily that their skeletons dominate many limestones of the period. Being useful to date rocks, they are widely studied; the data are excellent, worldwide. In short, if the known fossil record were accurate at all, it would be accurate for fusulinacea. And something terrible had happened to the fusulinacea, quite suddenly, at the end of the Guadalupean. Of 59 known genera (a genus is a group of related species), only 14 survived into the Tatarian. All fusulinacea genera larger than 6 mm. died out, as did all the ones with complex, internal wall structures. Then, at the very end of the Tatarian, all the remaining fusulinacea died out in another big event.

That pattern could not possibly be chance, argues Stanley. "This could never be an artifact of an imperfect fossil record. If you missed some parts of the [Tatarian] fossil record, you'd have a mixed picture--some big, some small." Stanley and Yang's other intricate, statistical arguments point to the same conclusion: two events--both sudden, both big extinctions. -- EH

Screening for "green chemicals"

Some 1,000 new chemicals come into production each year, most of them organic, and for the most part, these chemicals do the useful things they were designed to do. But then their remnants and by-products get out into the environment, and 20 years from now, they do--what?

That is what A. Lynn Roberts, assistant professor of geography and environmental engineering, would like to know, preferably without the uncontrolled experiment of just using the chemicals without discrimination. Long-term fallout from some of the 70,000 synthetic chemicals now in use includes more than a thousand Superfund sites; some organic chemicals seem to play a role in rising cancer rates and the depleted ozone layer.

Luckily, says Roberts, organic chemicals are "fairly well behaved." She thinks a compound's fate in the environment can be predicted, eventually, simply through knowing the compound's structure. "The hope is that when people synthesize a new chemical, they will know in advance how it's going to behave," Roberts says. Will its breakdown products be relatively benign, or not? How soon will they appear? What is their likely half-life?

Already, based solely on structure, the environmental chemist has developed a model that predicts how fast halogenated alkanes, such as DDT or chlorinated solvents, will be transformed into something else once they reach an aquatic environment. These compounds all have a hydrogen and a halogen on adjacent carbon atoms; exposed to H2O, they undergo dehydro-halogenation. Roberts found that electron-withdrawing substituents facilitate the reaction. The more of these substituents there are, and the closer they are to a hydrogen, the faster the compound is transformed into something else.

"The something else may be more toxic than the parent compound," notes Roberts. "There is some evidence that this is generally so." In addition, the second-generation chemicals tend to resist degradation. They persist. Some persist in fatty tissue, for instance, moving up the food chain from plankton to fish to birds and people.

Many problem compounds descend from chlorine, currently used worldwide in dozens of ways, including pesticides, pharmaceuticals, refrigerants, and disinfectants. Forty of the top 50 pharmaceutical drugs in the United States contain chlorine. As a water treatment alone, chlorine saves millions of lives a year.

At the same time, some chlorinated compounds are suspected carcinogens. Others, says Roberts, "if we include chlorofluorocarbons, are actually the bulk of the problem" in destroying stratospheric ozone. And some can mimic estrogen in animal bodies, which contributes to developmental sexual deformation. For instance, a spill polluted Florida's Lake Apopka with DDE, the dehydrohalogenation breakdown product of the now-banned pesticide DDT. Since that time, fertility and infant survival of the lake's alligators are very low; of the surviving young, the males have feminized genitalia, while the females have deformed eggs. Effects of similar compounds on human fertility are "debatable," says Roberts, but possible. Worldwide, sperm counts dropped in this century.

"Should chlorinated compounds be banned?" asks Roberts, rhetorically. "I don't think I can answer that yet. Because if we ban chlorine, we'll want to replace it with something that confers the same good features. But if a particular chemical structure behaves a lot like chlorine as a pharmaceutical or a disinfectant or a pesticide, isn't it likely to behave a lot like chlorine when it enters the environment?"

Very subtle structural changes can make all the difference, and that's where Roberts's new predictive tool enters the picture, she hopes. That test, and others like it, may eventually be used as a rapid way to screen chemicals, picking out those most friendly to the environment. --EH

The stories that shards tell

Blythe McCarthy caresses a shard of ancient pottery that has the fuzzy turquoise cast of a corroded penny. Another shard, dull brown and coated by a misty film, rests on the desk in front of her. "I really enjoy looking at these things," she says, holding up the bluish piece. "I find them so beautiful."

Small mountains of the corroding fragments, each shard sealed in its own plastic pouch, teeter on McCarthy's desk at the Smithsonian Institution's Conservation Analytical Laboratory. These broken bits of figurines, oil lamps, bricks, burial jars, and coffins were manufactured in southern Mesopotamia between 100 B.C. and 1400 A.D. They were excavated from Nippur, an archaeological site 100 miles south of Baghdad.

McCarthy, who is just finishing her PhD through Hopkins's Conservation Science Program, spent the past four years examining the shards using high-tech instruments. "If you can document how they've degraded you can make statements about how they used to be," she explains. "Many glazed ceramics you see in a museum are quite different in appearance from what they originally were."

Glazes degrade while under the earth, more after they are dug up. When glazeware is buried beneath the earth, it is in a relatively moist environment, she explains. "When you bring it into the air, you desiccate it. Cracks form." The process is mercilessly quick.

McCarthy steps into a windowless room, darkens the lights, and positions a thin section of a shard under the scanning electron microscope. A magnified black-and-white image appears on a screen above the microscope. It looks as though an angry abstract artist had flung blobs of black paint and odd-shaped splats of white paint on canvas. McCarthy, however, has defined and quantified the chaos.

The blobs and splats are heterogen-eities, she says. Spherical black spots are gas-filled pores. They occur where the raw materials of the ceramic, such as plant ashes, had released carbon dioxide and other gases. The bubbles then got trapped in the viscous glaze. "Pores, bubbles, and pits will act as initiators where corrosion will start," she explains.

Bright triangular blobs, she continues, are particles of iron chromate, a black pigment. Jagged-edged white blobs are crystals of calcium-magnesium silicate, which precipitated out of the glaze as it cooled. McCarthy counted the number and type of hetero- geneities in scores of samples, then made replicas of the samples and real-time observations of how they corrode.

The heterogeneities have also taught McCarthy something about how ancient cultures produced the pottery. For example, in shards dating to pre-Islamic times, the glazes vary little. In early Islamic times, however, from about 700 A.D. to 900 A.D., there is tremendous variation from one piece to another. That finding indicates that pottery was manufactured at several different sites throughout southern Mesopotamia, she concludes. Each site may have had its own particular glaze recipe, firing time, and firing temperature. It also suggests that artisans were experimenting with the technology.

McCarthy has since moved on to investigate artwork from a more contemporary era--she's testing new pro- tective coatings for Limoges enamels, at the Fraunhofer Institute for Silicate Science in Wurzburg, Germany. --MH

Written by Elise Hancock and Melissa Hendricks.

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