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Impact from the Deep C REDI T 64 SCIENTIFIC AMERICAN OCTOBER 2006 COPYRIGHT 2006 SCIENTIFIC AMERICAN, INC. GREEN AND PURPLE sulfur bacteria colonizing a hot spring thrive in water depleted of oxygen but rich in hydrogen sulfi de. Widespread ocean blooms of these organisms during ancient periods of mass extinction suggest similar conditions prevailed at those times. IMPACTFROMFROM THEDEEP Strangling heat and gases emanating from the earth and sea, not asteroids, most likely caused several ancient mass extinctions. Could the same killer-greenhouse conditions build once again? By Peter D. Ward hilosopher and historian Thomas S. Kuhn has suggest- extinctions underwent a Kuhnian revolution when a team at ed that scientifi c disciplines act a lot like living organ- the University of California, Berkeley, led by geologist Walter isms: instead of evolving slowly but continuously, they Alvarez proposed that the famous dinosaur-killing extinction enjoy long stretches of stability punctuated by infre- 65 million years ago occurred swiftly, in the ecosystem catas- P quent revolutions with the appearance of a new species—or in trophe that followed an asteroid collision. Over the ensuing the case of science, a new theory. This description is particu- two decades, the idea that a bolide from space could smite larly apt for my own area of study, the causes and consequenc- a signifi cant segment of life on the earth was widely em- es of mass extinctions—those periodic biological upheavals braced—and many researchers eventually came to believe that when a large proportion of the planet’s living creatures died cosmic detritus probably caused at least three more of the fi ve off and afterward nothing was ever the same again. largest mass extinctions. Public acceptance of the notion crys- Since fi rst recognizing these historical mass extinctions tallized with Hollywood blockbusters such as Deep Impact more than two centuries ago, paleontologists believed them and Armageddon. to have been gradual events, caused by some combination of Now still another transformation in our thinking about climate change and biological forces such as predation, com- life’s punctuated past is brewing. New geochemical evidence petition and disease. But in 1980 the understanding of mass is coming from the bands of stratifi ed rock that delineate mass www.sciam.com SCIENTIFIC AMERICAN 65 COPYRIGHT 2006 SCIENTIFIC AMERICAN, INC. extinction events in the geologic record, including the exciting Mass Extinctions,” by Douglas H. Erwin; Scientific Amer- discovery of chemical residues, called organic biomarkers, ican, July 1996]. Worldwide death happened again 201 mil- produced by tiny life-forms that typically do not leave fossils. lion years ago, ending the Triassic period, and the last major Together these data make it clear that cataclysmic impact as extinction, 65 million years ago, concluded the Cretaceous a cause of mass extinction was the exception, not the rule. In with the aforementioned big bang. most cases, the earth itself appears to have become life’s worst In the early 1990s paleontologist David Raup’s book Ex- enemy in a previously unimagined way. And current human tinctions: Bad Genes or Bad Luck? predicted that impacts activities may be putting the biosphere at risk once again. ultimately would be found to be the blame for all these major mass extinctions and other, less severe events as well. Evi- After Alvarez dence for impact from the geologic boundary between the to understand the general enthusiasm for the impact Cretaceous and Tertiary (K/T) periods certainly was and re- paradigm, it helps to review the evidence that fueled it. The mains convincing: in addition to the Chicxulub crater and the scenario advanced by Alvarez, along with clear iridium layer, impact debris, includ- his father, physicist Luis W. Alvarez, and ing pressure-shocked stone scattered across nuclear chemists Helen V. Michel and Frank The earth can, the globe, attests to the blow. Further Asaro, contained two separate hypotheses: chemical clues in ancient sediments docu- first, that a fairly large asteroid—estimated and probably ment rapid changes in the world’s atmo- to have been 10 kilometers in diameter— spheric composition and climate that soon struck the earth 65 million years ago; sec- did, exterminate followed. ond, that the environmental consequences its own. For several other extinction periods, the of the impact snuffed out more than half signs also seemed to point “up.” Geologists of all species. They had found traces left had already associated a thin iridium layer by the blow in a thick layer of iridium—rare on the earth with the end Devonian extinctions in the early 1970s. And by but common in extraterrestrial materials—that had dusted 2002 separate discoveries suggested impacts at the end Trias- the globe. sic and end Permian boundaries. Faint traces of iridium reg- Within a decade of this prodigious announcement the istered in the Triassic layer. And for the Permian, distinctive killer’s thumbprint turned up, in the form of the Chicxulub carbon “buckyball” molecules believed to contain trapped crater hiding in plain sight on the Yucatán Peninsula of Mex- extraterrestrial gases added another intriguing clue [see “Re- ico. Its discovery swept aside most lingering doubts about peated Blows,” by Luann Becker; Scientific American, whether the reign of the dinosaurs had ended with a bang. At March 2002]. Thus, many scientists came to suspect that as- the same time, it raised new questions about other mass ex- teroids or comets were the source of four of the “big five” tinction events: If one was caused by impact, what about the mass extinctions; the exception, the end Ordovician event, rest? Five times in the past 500 million years most of the was judged the result of radiation from a star exploding in our world’s life-forms have simply ceased to exist. The first such cosmic neighborhood. event happened at the end of the Ordovician period, some 443 As researchers continued to probe the data in recent years, million years ago. The second, 374 million years ago, was however, they found that some things did not add up. New near the close of the Devonian. The biggest of them all, the fossil analyses indicated that the Permian and Triassic extinc- Great Dying, at the end of the Permian 251 million years ago, tions were drawn-out processes spanning hundreds of thou- wiped out 90 percent of ocean dwellers and 70 percent of sands of years. And newly obtained evidence of the rise and plants, animals, even insects, on land [see “The Mother of fall of atmospheric carbon, known as carbon cycling, also seemed to suggest that the biosphere suffered a long-running Overview/Mass Extinctions series of environmental insults rather than a single, cata- strophic strike. ■ More than half of all life on the earth has been wiped out, repeatedly, in mass extinctions over the past Not So Sudden Impact ) 500 million years. t he l esson of the K/T event was that a large-body impact ■ One such disaster, which included the dinosaurs’ is like a major earthquake leveling a city: the disaster is sud- disappearance, is widely attributed to an asteroid den, devastating, but short-lived—and after it is over, the city impact, but others remain inadequately explained. quickly begins rebuilding. This tempo of destruction and sub- preceding pages ■ New fossil and geochemical evidence points to a shocking sequent recovery is reflected in carbon-isotope data for the environmental mechanism for the largest of the ancient K/T extinctions as well as in the fossil record, although verify- mass extinctions and possibly several more: an oxygen- ing the latter took the scientific community some time. The depleted ocean spewing poisonous gas as a result of expected sudden die-off at the K/T boundary itself was in- global warming. deed visible among the smallest and most numerous fossils, those of the calcareous and siliceous plankton, and in the NATIONAL PARK SERVICE ( 66 SCIENTIFIC AMERICAN OCTOBER 2006 COPYRIGHT 2006 SCIENTIFIC AMERICAN, INC. spores of plants. But the larger the fossils in a group, the more gradual their extinction looked. Patterns of Destruction Slowly, paleontologists came to understand that this ap- Carbon 13 (13C) isotopes found in geologic strata suggest parent pattern was influenced by the sparsity of large-fossil longer-acting mechanisms behind two of three ancient samples for most of the soil and rock strata being studied. To extinction events. 13C is more abundant in the atmosphere address this sampling problem and gain a clearer picture of the when land and sea plants are thriving. When plant life dies on pace of extinction, Harvard University paleontologist Charles a massive scale, 13C drops as a proportion of atmospheric Marshall developed a new statistical protocol for analyzing carbon. Comparing ancient samples with a common carbon ranges of fossils. By determining the probability that a par- standard reveals multiple large drops in 13C leading up to the ticular species has gone extinct within a given time period, this end Permian (top) and end Triassic (middle) boundaries. The analytical method teases out the maximum amount of infor- dips imply multiple extinction crises occurring over hundreds mation yielded by even rare fossils. of thousands of years. In contrast, a 13C plunge for the period In 1996 Marshall and I joined forces to test his system on around the Cretaceous-Tertiary boundary (bottom) depicts K/T stratigraphic sections and ultimately showed that what one abrupt ecological cataclysm. had appeared to be a gradual extinction of the most abundant End Permian extinction of the larger marine animals, the ammonites (molluscan fossils related to the chambered nautilus) in Europe, was instead con- 0 Permian Triassic sistent with their sudden disappearance at the K/T boundary itself. But when several researchers, including myself, applied –5 the new methodology to earlier extinctions, the results dif- fered from the K/T sections.
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