The Theory

Theory Page

A Chronology of the Scientific Theories and Discoveries that Brought Us to Italy.

1980 Luis and Walter Alvarez, Frank Asaro, and Helen Michel at UC Berkeley advance the hypothesis that the mass extinction at the end of the Cretaceous era was caused by an asteroid or comet impact. Their chief evidence was the enhanced abundance of iridium in the clay layer that, in various parts of the world, marks the boundary between the last rocks of the Creatceous period, rich in dinosaur fossils, and the earliest Tertiary rocks, devoid of dinosaur fossils. The K/T boundary layer was formed 65 million years ago.
Samples from the K/T layer in Italy showed a 30x normal amount of iridium. They speculated that the iridium was of extraterrestial origin, probably the result of the impact of a large (6-14 km diameter) asteroid or comet, which would have injected 60 times its mass in dust into the atmosphere. The resulting darkness would surpress photosynthesis for many years, leading to the extinction of many life forms, including the dinosaurs. The authors suggested that there be a search for a large impact crater which could then be tied to the extinction. Published in Science magazine.
1981 Glen Penfield and Antonio Camargo, petroleum geophysicists, discover a large crater at Chicxulub ("the devil's tail, in the local Mayan dialect) (pronounced "shick-zoo-loob"), located in the northwestern Yucatan peninsula in Mexico, and suggest that it is the one left by the dinosaur-killing impact, but few heard of this discovery and it was forgotten for nearly a decade. Published in Sky & Telescope magazine, March 1982, p. 249.
1990 Jay Melosh of the University of Arizona calculated that the heating of the atmosphere that would have been due to the sudden rise and fall of the meterotic debris could have broiled above-ground animals, except those protected by dense rainstorms or snowstorms. Afterward, the dust deposited in the stratosphere would have blocked sunlight for months ... similar to a nuclear winter.
1990 Alan Hildebrand and William Boynton of the University of Arizona used seismic imaging to identify a 300 km (180 mi) wide undersea depression that may have been the impact site. Thick layers of mud and debris on Cuba and Haiti indicate that a comet struck somewhere in the Columbian Basin of the Carribbean Sea, between North and South America. Reported in Science.
1991 Two different groups tie the Chicxulub crater to the K/T layer. In May, Adriana Ocampo of NASA JPL and Kevin Pope of Geo Eco Arch Research correlated a LanSat image mosaic showing a ring of sinkholes around Chicxulub with magnetic and stratigraphical information from the Pemax (petroleum company) data. In September, Alan Hildebrand of the University of Arizona looked at the thickness of the iridium layer within the K/T clay and found that it increased in proximity to Chicxulub.
1994 Carl Swisher III of the Institute of Human Origins in Berkeley used the radiometric technique that relies on the decay of potassium-40 to argon-40 and found that rocks from inside the Chicxulub circle formed exactly 65 million years ago and are the same age as the impact debris found around the Caribbean/Gulf of Mexico region. Reported in Science in August.
1995 Adriana Ocampo and Kevin Pope lead an expedition of Planetary Society members to Belize, about 364 km (226 mi) from Chicxulub, and examine an outcrop of Chicxulub ejecta for 12 days. Walter Alvarez was part of the expedition. Analysis of the relative amounts of heavy and light isotopes of carbon and oxygen in the carbonates will help to determine if fractionalization due to vaporization occured.

A Summary of Impact Theory
An Abstract by Sandro Montanari
August 1996

INVESTIGATING A SERIAL KILLER:
GEOLOGIC TESTIMONY OF EXTRATERRESTIAL (E.T.) IMPACTS AND THEIR VICTIMS
Alessandro Montanari
Osservatorio Geologico di Coldigioco, 62020 Frontale, Italia,
and Ecole des Mines de Paris
When the discovery of an iridium anomaly in an inconspicuous clay layer marking the Cretaceous/Tertiary (K/T) boundary at Gubbio (Italy) was first published by the Berkeley Group (Alvarez et al., 1980), few imagined that it would have opened a new exciting field of scientific research and debate, leading to a significant discontinuity in the vision of how life may evolve on Earth. The 5 ppb Ir anomaly (a couple of orders of magnitude higher than background in deep water marine sediments) was considered as a strong hint that an extraterrestrial object collided against the Earth at the end of the Cretaceous Period, causing a global catastrophe and the extinction, on land and in the oceans, of numerous life forms, from unicellular marine plankton to the mighty dinosaurs. This chemical anomaly, subsequently found in numerous other K/T sites around the world exactly where the fossil record changes dramatically, permitted the Berkeley group to make a first, yet remarkably precise identikit of the suspect E.T. killer: a comet or asteroid with a chondritic composition and a diameter in the order 10 km which would have excavated a crater with a diameter in the order of 200 km releasing an explosion energy equivalent to 108 megatons of TNT.
This subtle geochemical anomaly was, at the time, the only hint supporting a cause and effect relationship between an E.T. event and the terminal Cretaceous mass extinction; it had to be backed by more tangible evidence that could be understood and verified by the Earth science community as a whole. It was so that an ever increasing, international community of Earth scientists started an incredible investigation campaign about the most disparate, interdisciplinary, and integrated aspects of the K/T boundary (i.e., stratigraphy, petrography, mineralogy, paleontology, geochemistry, geophysics, ...you name it), leading to an unprecedented precise and detailed definition of this boundary compared to any other boundary event recorded in the stratigraphic record. As early as 1981, Smit and Klever linked some microscopic mineral spherules contained in the K/T boundary clay in Spain to the distal fallout of an extraterrestrial impact cloud. These spherules, eventually found in every K/T clay fingerprinted by the Ir anomaly, were further studied and interpreted as quenched melt droplets produced in an impact cloud (Montanari et al., 1983), and then altered by diagenetic processes. Other discoveries followed at a fast pace supporting the hypothesis of the killer impact. Skeletal spinel crystallites with an unusual mineralogy were found by Smit and Kyte (1984) in K/T spheroids which excluded the possibility of a volcanic origin. Today, these spinels are interpreted by some researchers as directly derived from the ablation in the atmosphere of the impacting bolide or the debris following it (i.e., comet tail or meteoritic splinters) (Robin et al., 1993).
The discovery of shocked quartz in the K/T clay by Bohor and Izett (1986) was perhaps the ultimate evidence convincing a wide range of Earth scientists that, in fact, the K/T boundary clay contained the fallout debris of an E.T. impact cloud: characteristic planar deformational features in quartz grains are the direct evidence of shock metamorphism which can be produced in nature only by extremely energetic impacts (i.e., 10 to > 50 GPa shock pressure).
By the mid 1990Õs, more than 3,000 scientific papers had been written on the issue of the K/T boundary. Most of these papers contributed to the mounting evidence for an impact, and the physical and biological consequences it had at a global scale (e.g. the discovery of soot by Wolbach et al., 1985, which led to the hypothesis of global fires triggered by the impact; geochemical anomalies in biogenic sediments reflecting suddenly stressed climatic conditions worldwide; more REE and siderophile anomalies, and mineralogical evidence for the suspect impactor). On the other hand, the paleontological record shows some obscure sides still awaiting for clear explanation. Why are turtles and crocodiles still crawling on this Planet? Why where there survivors of the K/T catastrophe in practically all the ecosystems of the Earth? It is from the fossils that we can fully understand how the worldÕs ecosystem was changed by the impact, and evolved after it. And this is still today the major issue to be resolved in the controversial general theory of impacts as cause of mass extinctions.
Debate over the nature of the K/T event took a new turn with the recognition of the K/T boundary tsunami, and proximal impact debris deposits in the Gulf of Mexico and surrounding areas (Smit and Romein, 1985; Burgeois et al., 1988; Hildebrand and Boiton, 1988; Alvarez et al., 1991; Smit et al., 1992), indicating that the suspect killer hit not far away from this region, leaving a unique set of footprints. Ultimately, these megawave deposits yielded direct geochemical and radioisotopic evidence for linking the Chicxulub impact structure hidden under the Yucatan peninsula (the largest impact crater known on Earth with a diameter of approximately 200 km; Hildebrand et al., 1991; Pope et al., 1991) with the K/T boundary layer: the time of mass killing was then settled at 65 Ma through 40Ar/39Ar dating of the impact melt glass from the crater itself, and the proximal tektites from Mexican and Haitian K/T sections (Swisher et al., 1992).
With the exception of crucial questions about how the enormous energy delivered to the Planet by the intruder effectively altered the global atmosphere and climate, and how the world biota handled the "day after" (these issues will keep physicists, modelers, paleontologists and palecologists busy for many years to come), in "geo-criminalogical" terms the case of the K/T boudary mass murder can be cautiously considered closed.
An intriguing question arises from this K/T verdict: is "E.T. impact" a serial killer of the Earth's biota? Investigation of the record of the past 150 Ma is producing circumstantial evidence that this may well be the case. However, in this span of geologic time, which is the best documented in EarthÕs history, the K/T is by far the largest impact event and the most severe extinction recorded in rocks. The second known largest impacts (Chesapeake and Popigai giant craters with diameters of 85 and 100 km, respectively) can be approximately correlated with global biologic crises at the end of the Eocene Epoch (about 35.5 Ma) and investigation of sediments of that age have already yielded Ir and other geochemical anomalies, spherules, shocked quartz, and spinels tightly associated with paleontologic signals of a suddenly stressed environment. Other less energetic impact events are feebly correlated with biologic crises, and their signatures in the stratigraphic record have yet to be carefully investigated at high resolution. Medium and small size impacts have not left any recognizable record in sediments worlwide but in some cases they seem to correlate with times of biologic radiation and speciation rather than mass extinction. It is worth noting that the effects of E.T. impacts on the biota may also depend on the actual state of the evolving global ecosystem. Slowly changing climates and environments controlled by global tectonics may produce (select) biologic populations which are more or less sensitive to the stress produced by small or medium size impacts, and some evolving population may actually be favored by such events. So, in the general issue of evolution vs. impact, we have to take into account the historical contingency of each individual event. As an example, the Tunguska event, which was so small that did not even produce a crater, may have caused a global mass extinction if it would have occurred a half a century or so later (a blink compared to the immensity of geologic time), and a few tens of degrees more to the west (an infinitesimal in the vastness of planetary distances): in this historical contingency, the Tunguska explosion would have destroyed Moscow during the Bay of the Pigs crisis, and I would probably not have written this abstract, and you would not be reading it.
REFERENCES: Alvarez et al., 1980, Science, 28:1095-1108; Alvarez et al., 1992, Geology 20:697-700; Bohor and Izett, 1986, LPSC 17:68-69; Bourgeois et al., 1988 Science 241:567-570 Hildebrand & Boynton, 1990, Science 248:843-846; Hildebrand et al., 1991, Geology, 19:867-871; Montanari et al., 1983, Geology 11:668-671; Pope et al., 1991, Nature 351:105; Robin et al., 1993, Nature, 363:611; Smit and Klaver, 1981, Nature, 292:47-49; Smit and Kyte, 1984, Nature, 310:403-405; Smit and Romein, 1985, EPSL, 74:155-170; Smit et al., 1992, Geology 20:99-103 Swisher et al., 1992, Science, 257:954-958; Wolbach et al., 1985, Science, 230:665-669.
Click here to see a Mesozoic-Cenozoic Cycle Chart.

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		A Chronology of the Scientific Theories and Discoveries that Brought Us to Italy.
	1980 Luis and Walter Alvarez, Frank Asaro, and Helen Michel at UC Berkeley advance the hypothesis that the mass extinction at the end of the Cretaceous era was caused by an asteroid or comet impact. Their chief evidence was the enhanced abundance of iridium in the clay layer that, in various parts of the world, marks the boundary between the last rocks of the Creatceous period, rich in dinosaur fossils, and the earliest Tertiary rocks, devoid of dinosaur fossils. The K/T boundary layer was formed 65 million years ago. Samples from the K/T layer in Italy showed a 30x normal amount of iridium. They speculated that the iridium was of extraterrestial origin, probably the result of the impact of a large (6-14 km diameter) asteroid or comet, which would have injected 60 times its mass in dust into the atmosphere. The resulting darkness would surpress photosynthesis for many years, leading to the extinction of many life forms, including the dinosaurs. The authors suggested that there be a search for a large impact crater which could then be tied to the extinction. Published in Science magazine. 1981 Glen Penfield and Antonio Camargo, petroleum geophysicists, discover a large crater at Chicxulub ("the devil's tail, in the local Mayan dialect) (pronounced "shick-zoo-loob"), located in the northwestern Yucatan peninsula in Mexico, and suggest that it is the one left by the dinosaur-killing impact, but few heard of this discovery and it was forgotten for nearly a decade. Published in Sky & Telescope magazine, March 1982, p. 249. 1990 Jay Melosh of the University of Arizona calculated that the heating of the atmosphere that would have been due to the sudden rise and fall of the meterotic debris could have broiled above-ground animals, except those protected by dense rainstorms or snowstorms. Afterward, the dust deposited in the stratosphere would have blocked sunlight for months ... similar to a nuclear winter. 1990 Alan Hildebrand and William Boynton of the University of Arizona used seismic imaging to identify a 300 km (180 mi) wide undersea depression that may have been the impact site. Thick layers of mud and debris on Cuba and Haiti indicate that a comet struck somewhere in the Columbian Basin of the Carribbean Sea, between North and South America. Reported in Science. 1991 Two different groups tie the Chicxulub crater to the K/T layer. In May, Adriana Ocampo of NASA JPL and Kevin Pope of Geo Eco Arch Research correlated a LanSat image mosaic showing a ring of sinkholes around Chicxulub with magnetic and stratigraphical information from the Pemax (petroleum company) data. In September, Alan Hildebrand of the University of Arizona looked at the thickness of the iridium layer within the K/T clay and found that it increased in proximity to Chicxulub. 1994 Carl Swisher III of the Institute of Human Origins in Berkeley used the radiometric technique that relies on the decay of potassium-40 to argon-40 and found that rocks from inside the Chicxulub circle formed exactly 65 million years ago and are the same age as the impact debris found around the Caribbean/Gulf of Mexico region. Reported in Science in August. 1995 Adriana Ocampo and Kevin Pope lead an expedition of Planetary Society members to Belize, about 364 km (226 mi) from Chicxulub, and examine an outcrop of Chicxulub ejecta for 12 days. Walter Alvarez was part of the expedition. Analysis of the relative amounts of heavy and light isotopes of carbon and oxygen in the carbonates will help to determine if fractionalization due to vaporization occured.
		A Summary of Impact Theory An Abstract by Sandro Montanari August 1996
	INVESTIGATING A SERIAL KILLER: GEOLOGIC TESTIMONY OF EXTRATERRESTIAL (E.T.) IMPACTS AND THEIR VICTIMS Alessandro Montanari Osservatorio Geologico di Coldigioco, 62020 Frontale, Italia, and Ecole des Mines de Paris When the discovery of an iridium anomaly in an inconspicuous clay layer marking the Cretaceous/Tertiary (K/T) boundary at Gubbio (Italy) was first published by the Berkeley Group (Alvarez et al., 1980), few imagined that it would have opened a new exciting field of scientific research and debate, leading to a significant discontinuity in the vision of how life may evolve on Earth. The 5 ppb Ir anomaly (a couple of orders of magnitude higher than background in deep water marine sediments) was considered as a strong hint that an extraterrestrial object collided against the Earth at the end of the Cretaceous Period, causing a global catastrophe and the extinction, on land and in the oceans, of numerous life forms, from unicellular marine plankton to the mighty dinosaurs. This chemical anomaly, subsequently found in numerous other K/T sites around the world exactly where the fossil record changes dramatically, permitted the Berkeley group to make a first, yet remarkably precise identikit of the suspect E.T. killer: a comet or asteroid with a chondritic composition and a diameter in the order 10 km which would have excavated a crater with a diameter in the order of 200 km releasing an explosion energy equivalent to 108 megatons of TNT. This subtle geochemical anomaly was, at the time, the only hint supporting a cause and effect relationship between an E.T. event and the terminal Cretaceous mass extinction; it had to be backed by more tangible evidence that could be understood and verified by the Earth science community as a whole. It was so that an ever increasing, international community of Earth scientists started an incredible investigation campaign about the most disparate, interdisciplinary, and integrated aspects of the K/T boundary (i.e., stratigraphy, petrography, mineralogy, paleontology, geochemistry, geophysics, ...you name it), leading to an unprecedented precise and detailed definition of this boundary compared to any other boundary event recorded in the stratigraphic record. As early as 1981, Smit and Klever linked some microscopic mineral spherules contained in the K/T boundary clay in Spain to the distal fallout of an extraterrestrial impact cloud. These spherules, eventually found in every K/T clay fingerprinted by the Ir anomaly, were further studied and interpreted as quenched melt droplets produced in an impact cloud (Montanari et al., 1983), and then altered by diagenetic processes. Other discoveries followed at a fast pace supporting the hypothesis of the killer impact. Skeletal spinel crystallites with an unusual mineralogy were found by Smit and Kyte (1984) in K/T spheroids which excluded the possibility of a volcanic origin. Today, these spinels are interpreted by some researchers as directly derived from the ablation in the atmosphere of the impacting bolide or the debris following it (i.e., comet tail or meteoritic splinters) (Robin et al., 1993). The discovery of shocked quartz in the K/T clay by Bohor and Izett (1986) was perhaps the ultimate evidence convincing a wide range of Earth scientists that, in fact, the K/T boundary clay contained the fallout debris of an E.T. impact cloud: characteristic planar deformational features in quartz grains are the direct evidence of shock metamorphism which can be produced in nature only by extremely energetic impacts (i.e., 10 to > 50 GPa shock pressure). By the mid 1990Õs, more than 3,000 scientific papers had been written on the issue of the K/T boundary. Most of these papers contributed to the mounting evidence for an impact, and the physical and biological consequences it had at a global scale (e.g. the discovery of soot by Wolbach et al., 1985, which led to the hypothesis of global fires triggered by the impact; geochemical anomalies in biogenic sediments reflecting suddenly stressed climatic conditions worldwide; more REE and siderophile anomalies, and mineralogical evidence for the suspect impactor). On the other hand, the paleontological record shows some obscure sides still awaiting for clear explanation. Why are turtles and crocodiles still crawling on this Planet? Why where there survivors of the K/T catastrophe in practically all the ecosystems of the Earth? It is from the fossils that we can fully understand how the worldÕs ecosystem was changed by the impact, and evolved after it. And this is still today the major issue to be resolved in the controversial general theory of impacts as cause of mass extinctions. Debate over the nature of the K/T event took a new turn with the recognition of the K/T boundary tsunami, and proximal impact debris deposits in the Gulf of Mexico and surrounding areas (Smit and Romein, 1985; Burgeois et al., 1988; Hildebrand and Boiton, 1988; Alvarez et al., 1991; Smit et al., 1992), indicating that the suspect killer hit not far away from this region, leaving a unique set of footprints. Ultimately, these megawave deposits yielded direct geochemical and radioisotopic evidence for linking the Chicxulub impact structure hidden under the Yucatan peninsula (the largest impact crater known on Earth with a diameter of approximately 200 km; Hildebrand et al., 1991; Pope et al., 1991) with the K/T boundary layer: the time of mass killing was then settled at 65 Ma through 40Ar/39Ar dating of the impact melt glass from the crater itself, and the proximal tektites from Mexican and Haitian K/T sections (Swisher et al., 1992). With the exception of crucial questions about how the enormous energy delivered to the Planet by the intruder effectively altered the global atmosphere and climate, and how the world biota handled the "day after" (these issues will keep physicists, modelers, paleontologists and palecologists busy for many years to come), in "geo-criminalogical" terms the case of the K/T boudary mass murder can be cautiously considered closed. An intriguing question arises from this K/T verdict: is "E.T. impact" a serial killer of the Earth's biota? Investigation of the record of the past 150 Ma is producing circumstantial evidence that this may well be the case. However, in this span of geologic time, which is the best documented in EarthÕs history, the K/T is by far the largest impact event and the most severe extinction recorded in rocks. The second known largest impacts (Chesapeake and Popigai giant craters with diameters of 85 and 100 km, respectively) can be approximately correlated with global biologic crises at the end of the Eocene Epoch (about 35.5 Ma) and investigation of sediments of that age have already yielded Ir and other geochemical anomalies, spherules, shocked quartz, and spinels tightly associated with paleontologic signals of a suddenly stressed environment. Other less energetic impact events are feebly correlated with biologic crises, and their signatures in the stratigraphic record have yet to be carefully investigated at high resolution. Medium and small size impacts have not left any recognizable record in sediments worlwide but in some cases they seem to correlate with times of biologic radiation and speciation rather than mass extinction. It is worth noting that the effects of E.T. impacts on the biota may also depend on the actual state of the evolving global ecosystem. Slowly changing climates and environments controlled by global tectonics may produce (select) biologic populations which are more or less sensitive to the stress produced by small or medium size impacts, and some evolving population may actually be favored by such events. So, in the general issue of evolution vs. impact, we have to take into account the historical contingency of each individual event. As an example, the Tunguska event, which was so small that did not even produce a crater, may have caused a global mass extinction if it would have occurred a half a century or so later (a blink compared to the immensity of geologic time), and a few tens of degrees more to the west (an infinitesimal in the vastness of planetary distances): in this historical contingency, the Tunguska explosion would have destroyed Moscow during the Bay of the Pigs crisis, and I would probably not have written this abstract, and you would not be reading it. REFERENCES: Alvarez et al., 1980, Science, 28:1095-1108; Alvarez et al., 1992, Geology 20:697-700; Bohor and Izett, 1986, LPSC 17:68-69; Bourgeois et al., 1988 Science 241:567-570 Hildebrand & Boynton, 1990, Science 248:843-846; Hildebrand et al., 1991, Geology, 19:867-871; Montanari et al., 1983, Geology 11:668-671; Pope et al., 1991, Nature 351:105; Robin et al., 1993, Nature, 363:611; Smit and Klaver, 1981, Nature, 292:47-49; Smit and Kyte, 1984, Nature, 310:403-405; Smit and Romein, 1985, EPSL, 74:155-170; Smit et al., 1992, Geology 20:99-103 Swisher et al., 1992, Science, 257:954-958; Wolbach et al., 1985, Science, 230:665-669. Click here to see a Mesozoic-Cenozoic Cycle Chart.