Title: Earth's near-death experience. Subject(s): EXTINCTION (Biology) Source: Earth, Jan94, Vol. 3 Issue 1, p42, 10p, 3 diagrams, 3 graphs, 10c Author(s): Alper, Joseph Abstract: Speculates on the possible causes of the mother of all mass extinctions which took place on Earth 250 million years ago. Almost all species of life simply vanished. Life on Earth at the time of this catastrophe; Permian crisis struck down an estimated 96 percent of all marine species; Possible shower of meteor impacts or impact of one giant meteor; Theories concerning violent volcanic eruptions; Possibility life was already declining; General scenario for mass extinctions. AN: 9310277508 ISSN: 1056-148X Note: This title is not owned by Tucson-Pima Public Library. Database: MasterFILE Elite EARTH'S NEAR-DEATH EXPERIENCE The mother of all mass extinctions occurred 250 million years ago. Was it caused by a huge meteor impact? A giant volcanic eruption? Or was it just plain dumb luck? Something nearly sterilized our planet 250 million years ago. In the worst series of mass extinctions in Earth's history, almost all species of life simply vanished. By the time of this mysterious catastrophe at the end of the Permian Period, life on land had become richly diverse. Enormous tree ferns and club mosses rising a hundred feet or more were common, as were many primitive conifers. Myriad species of insects and animals populated the land. Giant dragonflies buzzed through the brush. Squat, human-sized amphibians stalked prey along the edges of swamps. And horse-sized reptiles grazed placidly on lush vegetation. Then disaster struck. According to one recent estimate, nearly 80 percent of all four-legged land animals disappeared at the end of the Permian. Large or small, widespread or regional in distribution, it mattered not - species died in unprecedented numbers. Typical was the fate of the amphibians. These animals had dragged themselves out of the oceans 370 million years ago to become the first vertebrates to colonize land. Facing little competition at first, they thrived and diversified. By the middle of the Permian, about 270 million years ago, their many species had spread throughout the world. But other animal groups were evolving to take advantage of terrestrial niches too. Toward the end of the period, amphibians faced intense competition from a group of evolutionary newcomers, the mammal-like reptiles. Fading slowly, the amphibians were then almost knocked out of the evolutionary race:Only one out of the four existing orders of these animals survived through the end of the Permian to see the dawn of the next geologic period, the Triassic. But their ecological adversaries, the mammal-like reptiles, fared no better. Of the 50 genera of these creatures that lived during the Permianperiod, only one, the genus Dicynodon, made it into the Triassic. Were it not for these lone survivors, our own evolutionary line would have been cut off at its roots: It was the mammal-like reptiles that later gave rise to our own mammalian ancestors. As disastrous as the great Permian crisis was for creatures on land, it was even worse for life in the sea. A quarter of a billion years ago, the oceans seethed with life, ranging from trilobites to magnificent coral reefs. But life in the oceans came crashing down at the end of the Permian: An estimated 96 percent of all marine species disappeared forever. The Permian crisis was so overwhelming it struck down entire groups of sea creatures. For example, all of the many species of tabulate and rugose corals went extinct. Also gone for good were the three existing orders of crinoids, or sea lilies - flowerlike invertebrate animals that attached themselves to the seafloor with slender stalks and gathered food with outstretched tentacles. The ammonoids, elegant spiral-shelled creatures whose bodies resembled those of modern-day squid, nearly disappeared forever. And the brachiopods, a phylum of marine invertebrates comprising numerous species of creatures with clamlike shells, similarly came within a hair's breadth of oblivion. Over the years, the Permian crisis has attracted far less attention than the mystery of the less-severe extinctions that did in the dinosaurs 65 million years ago. But now that scientists have found what they believe to be the smoking gun of the dinosaur extinctions - a massive asteroid impact - some researchers have reopened the case of the cataclysmic Permian die-off. They certainly suffer no lack of suspects. The view held by the overwhelming majority of researchers is that the Permian extinctions were not so much murder as death by natural causes. These scientists believe the die-off occured gradually as environmental conditions changed. But in recent scientific meetings and papers, a few researchers have been promoting catastrophic causes for the most catastrophic extinctions ever. If one of these theories is correct, our understanding of the events that shaped the Earth's geologic and biologic past will be profoundly altered. The most radical theories - dismissed by most researchers - were proposed by Verne Oberbeck and his colleagues at the NASA Ames Research Center and Michael Rampino, of New York University and the NASA Goddard Institute for Space Studies. In Oberbeck's view, a shower of meteor impacts may have rained down on Earth over a period of time, weakening the crust enough to allow huge amounts of lava to flood the land. By altering the climate, these eruptions may have led to the mass extinctions. In Rampino's view, one giant meteor - an asteroid or comet - crashed into southern Gondwanaland (the southern portion of the supercontinent Pangea). This object, at least 10 miles in diameter, scooped out a crater some 125 to 200 miles wide and 25 miles deep. The impact released enough heat to trigger a worldwide forest fire and vaporize a significant amount of water from the ocean. Shock waves reverberating through the Earth touched off intense volcanism. Airborne soot and dirt from the impact and volcanism blanketed the Earth and stopped photosynthesis, triggering a mass extinction of species. If all that weren't enough, Rampino believes the impact was so severe it triggered the breakup of Gondwanaland, setting free the landmasses that would much later become South America, Africa, Antarctica and Australia. "I don't believe it," says Gerald Czamanske, of the U.S. Geological Survey in Menlo Park, Calif. His studies have led him to propose that the trigger for the Permian extinctions was a violent series of volcanic eruptions in Siberia totally unrelated to meteor impacts. In this scenario, enormous volumes of basaltic lava rose from deep in the Earth and passed through layer upon layer of coal and other sulfur-rich sedimentary rocks three miles thick. As the lava erupted explosively at the surface, it ejected volcanic debris and sulfur dioxide, as well as coal and sulfate-rich sedimentary materials, into the stratosphere. This worldwide cloud of dust and sulfur dioxide not only cooled the Earth rapidly, triggering a brief period of glaciation and a rapid fall in sea level, but also produced highly acidic rain. The rapid climate change, lowering of sea level and poisonous acid rain spelled doom for most living creatures. "I think all these ideas are rubbish," says Grant Young, a glaciologist at the University of Western Ontario who advances a more traditional view. This school of thought holds that the breakup of the Pangean supercontinent triggered widespread climate change and glaciation, which caused the mass extinction to occur over millions of years. "We know glaciation tied to the breakup of continents occurs throughout the Earth's history, and there's good evidence that it occurred at the end of the Permian. These guys are throwing a spanner in the works with ideas and no sound data. There's absolutely no evidence to support that either volcanism or a meteor impact triggered a mass extinction then." But both Oberbeck and Rampino say the same thing about glaciation. That is, they both question the whole idea that periodic glaciation is the norm on Earth. They believe, instead, that periodic meteor bombardment is the norm. "The geological evidence used to support glaciation is purely circumstantial," Oberbeck says. He has published scientific papers showing that meteor impacts can produce the same sediment and rock patterns that geologists have long believed are produced solely by glaciers as they pick up and deposit debris. "There isn't any conclusive evidence for ancient glaciation," Rampino says, "but we're supposed to believe that glaciation is a normal event? On the other hand, we know that large objects have been striking the Earth throughout its history, but we're supposed to ignore that evidence? In addition, we know that the major forces acting on every other planet are large body impacts and basaltic volcanism. But we're supposed to believe that on Earth the major forces are glaciation and movement of tectonic plates? Why should the Earth be special?" Rampino asks. "In fact," he says, "I think there's a case to be made that large impacts may be the triggers for large-scale tectonic movements and massive volcanic eruptions. If this is true, then to a large degree, huge impacts control the geology and biology of Earth." In Permian times, the Pangean supercontinent covered about 30 percent of the globe, with its northern part, Laurasia, and southern region, Gondwanaland, separated by the triangular Tethys Sea. Sediments that accumulated on the shores of the Tethys, along with the fossils they preserved, form the most complete record discovered so far of the late Permian and early Triassic periods. These deposits are visible today as stratified limestone in the Italian and Austrian Alps, Greece, the Tibetan Himalaya, southeast China, and elsewhere. Though the exact location of the boundary between the Permian and Triassic periods in the layered sedimentary rocks is still uncertain, scientists generally agree that it lies near a layer where fossils of a spiral-shaped mollusk called Otoceras suddenly disappear. By dating zircons isolated from this layer, a group of Australian and Chinese scientists in 1991 determined that the sediments at or close to this suspected Permian-Triassic boundary are 251.2 million years old, give or take 3.4 million years. The prevailing belief among paleontologists is that life was declining slowly during the last stages of the Permian. These scientists believe that some gradual process - global cooling, global warming, dropping sea levels, a change in ocean chemistry, or possibly some combination of factors like these - was making life on Earth increasingly difficult. Then the declining Permian life-forms reached some kind of fatal threshold, and they fell over the edge into mass extinction. This notion comes from the observation that many Permian species disappear from the fossil record well before the Permian-Triassic boundary. Certainly, such a pattern in the record could have resulted from a slowly rising extinction rate. But some paleontologists say that just because a species disappears from the fossil record doesn't mean it actually went extinct at that point. Paul Wignall of England's University of Leeds and Erik Flfgel of Erlangen University in Germany point out that Permian fossils are, on the whole, relatively rare. So it's possible that creatures presumed to have gone extinct prior to the end of the Permian actually lived on to the very end. We don't find their fossils in late-Permian deposits simply because very few of their remains were preserved, Flfgel and Wignall say. In some areas of the world, however, Permian fossils are found in abundance in deposits right at the Permian-Triassic boundary. Flfgel, working with unusually rich deposits in Greece and China, and Wignall, studying similar sediments in northern Italy, have found that reef communities had indeed reached a peak of diversity at the very end of the Permian Period. Of course, it's entirely possible that these regions may have somehow served as temporary, protected refuges where many Permian species made a last, desperate stand before fading out. But Wignall believes otherwise. He says the abundance and diversity of the late-Permian reefs suggests that marine life was thriving until it was decimated suddenly by some catastrophe. A 10-mile-diameter meteor striking the Earth would certainly have been a catastrophe, and not just for life at the impact site. According to Rampino, the blast from such a collision would have been in the range of 100 million megatons of TNT, sufficient to produce an earthquake of magnitude greater than 12. Even 600 miles from the impact site, the Earth's surface would have moved up and down 300 feet or so, Rampino says, enough to rupture the crust. The explosion would have launched 10 million billion tons of excavated rock and vaporized asteroid more than 60 miles into the air. The Earth would have become a very dark place for several months, enough time without sunlight to destroy the food chain and trigger widespread death. Rampino was not the first to theorize that the Permian extinctions were caused by such an impact. Scientists have actually been debating the theory, off and on, for more than a decade. In the early 1980s, David Raup and Jack Sepkoski, paleobiologists at the University of Chicago, found evidence in the fossil record that mass extinctions over the past 250 million years occurred about every 26 million to 32 million years. (Rampino and Richard Stothers of NASA's Goddard Space Flight Center later confirmed this observation.) The finding suggested that some cyclical phenomenon, like periodic meteor bombardment, was responisble for the extinctions. Raup then found circumstantial evidence to support the meteor hypothesis. He programmed a computer with the known rate of meteor impacts during Earth's 4.5-billion-year history. Then he asked the computer to determine the frequency of mass extinctions that might be caused by that rate of bombardment. The conclusion?"We were able to generate an extinction pattern remarkably similar to the one we found in the fossil record," Raup says. Based on this work he proposed that a cycle of large-body impacts could be culprits in the known episodes of mass death. His theory is plausible: Rampino and Stothers noted in a paper published in the journal Nature that the solar system passes through the debris-strewn galactic plane every 30 million to 33 million years. Computer models are not rocks and fossils, however, and that's where Oberbeck and Rampino come in. Verne Oberbeck is quiet, meticulous and cautious, a geologist who has made a career of studying the effects of large-body impacts on planets other than the Earth. Michael Rampino is outspoken and daring, unafraid to rock the boat - a geologist who uses his thorough familiarity with the scientific literature to propose theories that are perhaps one step ahead of the available data. At a geoscience meeting late in 1992, Oberbeck and Rampino made an unlikely pair as each in his own way asked geologists to thoroughly reexamine much of what has been held true about the past 600 million years of Earth's history. Both scientists, working independently, are questioning the origin of rock deposits known as tillites, which geologists have long considered to be signposts of glaciation. Tillites are sedimentary rocks made up of hardened debris of all sizes, from fine mud to large boulders. They resemble the jumbled and laminated boulder clays called tills that are produced by modern glaciers. When trying to determine whether particular sedimentary layers represent tillite deposits, geologists look for stones with faceted shapes and scratched surfaces, scratched and sculpted bedrock, fine sediments surrounding large solitary stones (inferred to have been dropped by melting glaciers), and several other features that presumably could have resulted only from glacial action. There is a major problem, however, in ascribing all tillites to glaciation. These deposits are widespread in Precambrian sediments, leading geologists to conclude that this early period in Earth's history saw widespread glaciation. Yet temperatures during the Precambrian are thought to have been warmer than those in today's non-glacial world. Glaciers in a warm world? "This makes no sense," Rampino says. He believes instead that the tillites were produced by meteor impacts, not glaciers. The first solid piece of evidence that meteor impacts might have produced tillites came from moon rocks brought back by Apollo program astronauts. These rocks show the same features as Earthly tillites, and since no one believes the moon ever had glaciers, that leaves but one possible cause: impacts. This finding prompted Oberbeck and colleagues John Marshall and Hans Aggarwal to look at deposits from known impact sites on Earth, such as the Ries crater in Germany, the Azuara crater in Spain and the Brazos River in Texas. Again, these deposits showed the same features ascribed to tillites. "The textures of impact crater deposits are the same as deposits produced by modern glaciers, which to me says we need to rethink the origin of tillites," Oberbeck says. "We're not saying that all tillites came from impacts and that none came from glaciers but that we need to reexamine these deposits because there's a chance that we've come to the wrong conclusion." In 1992, Oberbeck and Aggarwal calculated the distribution and thickness of tillite deposits that should have been formed over the past two billion years, given the known rates of meteor impacts. "These calculations matched the observed thickness distribution of tillites now attributed to glaciation," Oberbeck says. In other words, the thickness and pattern of tillite deposits seen in the world today matches very well the thickness and pattern you would expect to get from meteor impacts. "What we need to do now," Oberbeck says, "is look through tillites for grains of shocked quartz. These grains are only formed by impacts. " Discovery of shocked quartz would be convincing evidence that many "glacial" tillites actually came from impacts. And if that's the case, then scientists would be forced to conclude that impacts have played a much more significant role in shaping the history of life on Earth than they have been willing to admit. At just about the time Oberbeck was making his calculations concerning the distribution of tillites, Rampino was in South Africa visiting the Cape Fold Belt, a range of mountains made of highly deformed rock. Rampino believes the fold belt may actually be a remnant of a giant impact. (Unknown to him at the time, Oberbeck was thinking the same.) According to Rampino, a technique called argon-argon dating has shown that rocks from the Cape Fold Belt were intensely deformed about 250 million years ago - the time of the Permian extinctions. In addition, a geologic diagram by a South African colleague illustrating how the mountains probably looked in cross section just after they were deformed looks remarkably like a mammoth wave of debris - just the kind of wave you'd expect to be generated by a meteor impact. The "wave" curls over toward the north, suggesting that it was pushed up from the south. At the bottom of the Atlantic Ocean, between the former constituents of Gondwanaland that today are called South America and South Africa, is a craterlike feature 200 miles in diameter. During the Permian, prior to the breakup of Gondwanaland, this site would have been on land, just south of today's Cape Fold Belt. According to Rampino, anomalous gravity readings clearly define the outer edge of the craterlike feature. And granite recovered from its bottom appears to have been deformed 250 million years ago, the time of the great die-off. To the west of the crater, just off the east coast of South America, Rampino says he discerns a second, smaller crater defined by another anomalous gravity reading. Rocks from a region in South America that would have been adjacent to this crater in the late Permian were also deformed about 250 million years ago. Rampino's conclusion: A comet or asteroid 6 to 12 miles in diameter slammed into the Earth at an angle in southern Gondwanaland, forming the larger crater about 250 million years ago. The top of the object sheared off on impact, flew west and crashed to Earth near present-day South America, forming the smaller crater. The impacts triggered the breakup of Gondwanaland and led to the Permian mass extinctions. Almost at the same time Rampino was crawling around the Cape Fold Belt, Gerry Czamanske was on the other side of the Earth in Siberia. He was studying an unusually rich deposit of platinum metals associated with the Siberian Traps, a thick series of basaltic lava flows that cover much of the region north of Lake Baikal. The traps are of interest not only because of their relationship to the ore deposit, but also because their volume of 500,000 to 750,000 cubic miles makes them the biggest outpouring of lava in the continental geologic record. They reach thicknesses of 12,000 feet and were erupted over an area of 750,000 to 1.1 million square miles. Czamanske, a jovial man, was thinking more about the origin of platinum deposits than the causes of mass extinctions when he arranged to have colleagues in Menlo Park and Australia determine the age of rocks from the Siberian Traps. But when the date proved to be that seemingly magical 250 million years, he immediately came to the conclusion that the violent series of eruptions that produced the traps could have been the coup de grace for Permian life forms. "If you're going to paint a scenario for basalt volcanism's playing a major role in mass extinction, this would be the candidate," Czamanske says. Based on studies of the magnetic properties of rocks from the traps, Czamanske and his colleagues calculated that the eruptions lasted only 600,000 years. Thus, a remarkably destructive period of volcanism was concentrated in a very short time on the geologic timescale - a finding consistent with a sudden demise of species. "We're looking at an explosive source of sulfur dioxide and soot that could have easily shut down photosynthesis and killed off plant life with toxic acid rains," he says. The sulfur dioxide and soot would have also obscured the sun, reducing temperatures. "We suggest that following the eruptions in Siberia a brief ice age caused sea level to fall dramatically as the polar ice caps expanded. When the volcanic cloud dissipated, acid rain fell on the exposed continental shelves. All this decimated marine life." Rampino and Oberbeck actually disagree with Czamanske on only one major point: Whereas Czamanske is convinced that the volcanism was a result of Earth's internal processes, they believe in an extraplanetary trigger. Oberbeck thinks that the object that crashed into southern Gondwanaland was only one of several extraplanetary missiles that struck Earth over some limited period of time. He says that one such object, striking in Siberia, could have scooped out enough crust to allow trapped magma to finally erupt. "Magma would be solid about 100 kilometers below the surface simply because of the pressure caused by the weight of the crust," Oberbeck explains. "But an impact scoops much of this out, releasing the overburden pressure and allowing the magma to melt and erupt." Traces of the crater would vanish under later lava flows. Rampino believes that the Gondwanaland impact itself may have triggered the Siberian eruption. In the Permian, the Siberian Traps were almost at the antipode - the point on precisely the opposite side of the globe - of the impact site, Rampino says. It is his contention, supported by some theoretical modeling, that the interior of Earth transmits shock waves from an impact site to its antipode. If a plume of magma from the mantle happened to be poking up at the underside of the crust at this antipodal point in Siberia, Rampino says the shock from the meteor impact could have been enough to breach the crust here. This would have allowed the magma to rush up through the sedimentary rocks and cause lava to erupt onto the surface. He also contends that the asteroid that did in the dinosaurs 65 million years ago probably sent shock waves racing to the antipode of the impact site near Chixchulub in the Yucatan. That area was the Deccan Traps, a thick layer of basalt that covers much of west-central India. The age of the Deccan Traps is in accordance with this idea: 65 million years. "In fact," Rampino says, "we have shown that the dates of other major flood basalts and mass extinctions correlate exactly." Piecing these findings together, Rampino, who admits that this is a stretch, proposes a general scenario for mass extinctions: A large meteor strikes the Earth and sends shock waves to the antipodal point, triggering flood volcanism there; the debris from the impact crater, combined with the dust and other material ejected by the volcanic eruption, blocks enough sunlight to shut down photosynthesis and cool the globe. "There are too many coincidences for there not to be a connection between impacts, basalt volcanism and mass extinction," Rampino asserts. But could coincidence actually be the culprit in the Permian extinctions? Climatologist Thomas Crowley of the Applied Research Corporation in College Station, Texas, thinks so. His idea, based on principles of chaos theory, is that gradual climate change unrelated to meteors or volcanoes can build up to a dangerous threshold. At this point it only takes a very small random event to make the climate go haywire. "We have found that an event 400 times smaller than the eruption of Mt. Pinatubo can trigger dramatic climate change that could set off a mass extinction," Crowley says. In short, bad luck, on a truly cosmic scale, could have caused the worst mass extinction ever. GRAPH: The mass extinctions at the end of the Permian 250 million years ago were far more severe than any others, extinguishing at least 60 percent of all genera. By contrast, only 40 percent of all genera were wiped out during the extinctions that killed off the dinosaurs at the end of Cretaceous, 65 million years ago. (EARTH: Steven G. Davis. Source: Michael Rampino) GRAPHS (2): Evidence of the devastation at the end of the Permian is seen in the graph at right, which tracks carbon-13 in marine carbonate rocks deposited during the Permian and Triassic Periods. Low carbon-13 means low biological activity in the oceans. Carbon-13 plummeted at the end of the Permian, suggesting a mass die-off. NYU's Michael Rampino says the graph at left suggests that the extinctions occurred when a large extraplanetary object crashed to Earth. It depicts the concentration of iridium in rocks recovered from the Alps. The iridium spikes at exactly the same time that marine life was devastated. Iridium, a substance rare on Earth but common in extraplanetary objects, may be deposited globally after a large impact. Rampino says the concentration of iridium shown here is consistent with an impact by a comet. (EARTH: Steven G. Davis. Source: Michael Rampino) DIAGRAM: Anomalous gravity readings define the edges of a 200-mile-wide craterlike feature on the seafloor off the eastern coast of South America - the geologic fingerprint, NYU's Michael Rampino says, of a giant meteor impact. (Courtesy Michael Rampino) DIAGRAM: This map shows the positions of South America, Africa and Antarctica at the end of the Permian Period, when they were united in the southern part of Gondwanaland. (The blue areas between the continents were then high and dry.) The center of the right set of rings marks Rampino's proposed impact site. The center of the left set of rings marks the spot where he thinks a fragment of the meteor may have impacted. The numbers around the rings indicate when, in millions of years before the present, rocks recovered from those sites were subjected to intense deformation (as determined by argon-argon dating). All the rocks appear to have been deformed around 250 million years ago - the time of the extinctions. (According to Rampino, differences in the dates are within the margin of error of the dating technique.) (Courtesy Michael Rampino) DIAGRAM: A reconstructed cross section of the Cape Fold Belt (diagram, upper left) shows how it existed just after it was uplifted 250 million years ago. South is to the left, toward the proposed impact site. The wavelike form of the overturned rock layers could have been caused by the shock from a mammoth impact, Rampino says. (Courtesy Michael Rampino) MAP: Erupting lava created the steplike volcanic rock formations called the siberian Traps, which cover a substantial portion of Russia. (EARTH: Steven G. Davis) PHOTO: From the muddy banks of a stream during the Permian Period, a six-foot-long amphibian called Eryops stalks its prey: Diplocaulus, a three-foot insect eater. Grazing on plants in the background are eight-foot reptiles called Scutosaurs. It was during the later stages of the Permian that four-footed plant eaters like Scutosaurus found a place in the terrestrial ecosystem. The structure of that ecosystem survives to this day, with plants at the bottom of the food pyramid, plant-eating insects and animals in the middle and carnivores at the top. But the animals in this painting did not survive. They were among the many species that perished in the worst mass extinction in Earth's history. (Matt Groshek) PHOTO: This diorama of a Permian reef at Chicago's Field Museum of Natural History depicts sponges, corals, spiral-shelled ammonoids and clamlike brachiopods. All suffered severely during the great Permian crisis. (Neg. #GEO 80873.1c, Field Museum of Natural History, Chicago) PHOTOS (2): Geologists gaze south toward the mountains of South Africa's Cape Fold Belt. (Courtesy Michael Rampino) PHOTO: According to Gerald Czamanske of the U.S. Geological Survey, a plume of magma rising from the mantle beneath Siberia passed through three miles of coal and other sulfur-rich sediments, causing lava to erupt explosively onto the surface 250 million years ago. Czamanske proposes that the eruption ejected vast quantities of sulfur dioxide and soot into the stratosphere. This cooled the Earth, triggering a brief glaciation, and produced a highly acidic rain. The environmental catastrophe spelled doom for most living creatures. (Matthew Groshek) PHOTO: This Cambrian Period trilobite fossil, found in Newfoundland, is 15 inches long. Trilobites, which first appeared in the oceans more than 550 million years ago, met their end at the close of the Permian Period. (Riccardo Levi-Setti, Trilobites, Second Edition, The University of chicago Press) ILLUSTRATIONS (5) ~~~~~~~~ By Joseph Alper Joe Alper, a contributing correspondent for the journal Science, has written frequently for Earth. _________________ Copyright of Earth is the property of Kalmbach Publishing Co. and its content may not be copied or emailed to multiple sites or posted to a listserv without the copyright holder's express written permission. However, users may print, download, or email articles for individual use. Source: Earth, Jan94, Vol. 3 Issue 1, p42, 10p, 3 diagrams, 3 graphs, 10c. Item Number: 9310277508