BOOK REVIEW

bookart Night Comes to the Cretaceous
Dinosaur Extinction and the Transformation of Modern Geology
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by James Lawrence Powell
W.H. Freeman & Co., 1998

Reviewed by Blake Edgar

(Posted February 19, 1999 · Issue 48)

Review

Few scientific debates have generated as much acrimony and deep-seated conviction among the protagonists, or garnered as much attention from the public, as the debate over what killed off most of the dinosaurs - plus half the known vertebrates, the ammonites, and plenty of plants and plankton - at the close of the Cretaceous era 65 million years ago. As mass extinctions go, this most recent one was the weakest of the five past such catastrophes documented in the fossil record; for instance, it definitely lacked the punch of the extinction at the end of the Permian, which eliminated some 95 percent of all life. Still, because of our awe for its most celebrated victims, the Cretaceous-Tertiary (or K-T) mass extinction holds a special fascination for us. It even made the cover of Time.

Dinosaurs large and small, slow and fast, had dominated the continents for upwards of 160 million years. They were widespread, abundant, diverse; what could have ended their reign so abruptly?

Dozens of potential causes, from the plausible to the preposterous, have been proposed. They boil down, essentially, to a couple of issues: was the dinosaurs' extinction gradual or sudden, and was the agent of death a process that was familiar or foreign to life on Earth?

A slew of recent books, several written or co-authored by principal figures in the debate, have tackled the question of dinosaur extinction and championed a favorite theory. The latest offering, Night Comes to the Cretaceous, is by an outside observer, James Lawrence Powell.

Powell is a geologist, and president and director of the Natural History Museum of Los Angeles County. Though until now Powell had been a bystander in the controversy, he steadfastly argues that the great mystery has been solved by the suggestion that the impact from a giant comet or asteroid snuffed out the dinosaurs. For Powell, the impact theory is more than just the resolution of a scientific quandary. He sees the process by which it was developed, debated, and (as far as he is concerned) demonstrated as this century's best example of how science works to establish truth. Powell is convinced the theory itself will force a radical shift in our understanding of Earth's history and evolution.

Since the nineteenth century, geologists had been guided by the principle of uniformitarianism, which holds that the present is the key to the past. Whatever processes shaped the planet and its denizens in the past should be operating and observable today. Uniformitarianism had pushed aside the preceding ideas of catastrophists, and Powell charges that this change sent geologists down the wrong philosophical path for nearly a century. One consequence was that startling new ideas, whether continental drift or the presence of impact craters on Earth's surface, were roundly ridiculed and ignored.

Then, in the June 6, 1980 issue of Science, came the upstart impact hypothesis that called into question this core geological concept. If an extraterrestrial impact could be a harbinger of mass extinction, then rare, alien events - and catastrophic ones to boot - played decisive roles in the past that could not be fully or easily appreciated in the present. "Impact theory," writes Powell, "asks geologists to look, not down at the familiar terra firma that has always drawn their gaze, but up at the sky--for them an unfamiliar and uncomfortable posture."

Discomfort, furthermore, came not just from the message, but from one of the messengers. The impact theory has been most closely associated with the father-son team of Luis and Walter Alvarez of the University of California, Berkeley. While Walter is a respected geologist, his more vocal and combative father was a Nobel laureate physicist who had built an already distinguished career by flaunting conventional wisdom and pondering problems in unrelated fields. "Outsiders are like bees carrying vital scientific pollen from one disciplinary flower to another," writes Powell.

The elder Alvarez, however, rarely disguised his disdain for the lack of rigor in historical sciences, and that stuck in geologists' craws. (Powell points out one scientist's opinion that Luis Alvarez set out to sabotage and curtail his research career for championing a rival explanation for dinosaur extinction.)

While Powell addresses some of the controversy that has attended the dinosaur death debate, his primary concern is to lay out the predictions made by what he calls "the Alvarez theory." He then shows how the accumulating evidence supports both the occurrence of an extraterrestrial impact 65 million years ago, and its devastating effects that resulted in the mass extinctions. Along the way, he aims to refute the main catastrophic competitor for the K-T extinction - widespread and long-term volcanic eruptions.

Like a prosecuting attorney, Powell presents his argument in a deliberate, step-by-step fashion, often repeating key points. Although this approach doesn't help the flow or flair of his writing, the book will surely interest anyone seeking details of the impact theory, or the context of how it fits into our broader understanding of mass extinction.

Does Powell do a convincing job of assuring readers that a comet or asteroid did in the dinosaurs? Not entirely. He persuasively shows how numerous lines of evidence confirm the existence of a K-T impactor. First, there's the conspicuous clay layer enriched in iridium, an element rare in the crust but abundant in meteorites, at several sites around the globe. As scientists examined the K-T boundary layers, other clues emerged: shocked quartz grains (a feature known from other impact craters but not from volcanic rocks), shattered zircon crystals, and other intriguing rock types.

Most impressively, by the early 1990s pro-impactors could even point to the smoking crater, a circular structure called Chicxulub, Mayan for "the red devil," submerged off the coast of the Yucatan Peninsula. Between 170 and 300 kilometers in diameter, the Chicxulub basin fits with the ten-kilometer estimated size for the Alvarezes' lethal impactor, and it dates precisely to the end of the Cretaceous. (Ironically, Chicxulub had been identified by geologists for its possible connection to the Cretaceous extinction within months of the impact theory being proposed, but a decade passed before anyone else tracked clues back to this crater.)

No doubt the K-T impact constituted a cosmic catastrophe. At ten kilometers in diameter, the comet or asteroid would have packed the explosive power of 100 million megatons of TNT, unleashing a hell on Earth of infernos and dust clouds. But proving the impact occurred doesn't demonstrate that it killed the dinosaurs. Tellingly, Powell devotes at least twice as much space to bolstering the first half of his argument than he does to establishing the link from impact to extinction.

By far the best fossil evidence for the K-T extinction comes from the Hell Creek Formation of Montana, a limited area to be sure, but one that reveals how selective this extinction was. Why would a global catastrophe spare most fish, amphibians, mammals, and reptiles, while sacrificing just sharks, lizards, marsupials, and dinosaurs? No extinction theory appears to provide an answer to this anomaly.

Perhaps more than one cause ultimately brought about the dinosaurs' demise, and pro-volcanists may still have their day. The Deccan Traps, a vast field of basalt in India, probably peaked in eruption at the end of the Cretaceous, and whereas only one mass extinction can confidently be said to coincide with an extraterrestrial impact, a second, earlier extinction event has now been closely linked in time with major episodes of volcanism.

Indeed, the Permian-Triassic extinction 250 million years ago was the most destructive in Earth's history. In 1995, a team led by Paul Renne of the Berkeley Geochronology Center established that volcanic rocks from this extinction boundary in China and the Siberian Traps - the world's most extensive lava flows - date to exactly the same age. They propose volcanism and acid rain as the Permian extinction mechanism.

Powell raises a question about the precision of this dating (though he doesn't hesitate to accept the date of 64.98 million years for Chicxulub obtained by the same lab), but the coincidence is hard to ignore. Even Walter Alvarez, in his recent book, T. Rex and the Crater of Doom, hints that, based on the new Permian data, impacts and volcanoes may combine in still unknown ways to produce mass extinctions. Despite Powell's assertion in his preface, there's still a little mystery left to unravel.

Blake Edgar is the coauthor with Donald Johanson of From Lucy to Language. He is an associate editor of California Wild, the magazine of the California Academy of Sciences.

Excerpt

From that primordial chaos arose a comet or an asteroid that through the subsequent eons was intermittently pounded by impact and continually nudged by gravity. Had one collision been just a bit more or a bit less energetic, had gravity tugged a little more here or a little less there, the impactor would have had a different size and a different orbit. The dinosaur killer would have struck at some other time in the earth's history, or missed our planet entirely, and dinosaurs would not have become extinct when they did.

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