In the March 5, 2010 edition of the journal Science, an international panel of 41 experts in geology, paleontology and other related fields, after an exhaustive review of the data, declared an end to a 30 year controversy over what triggered the extinction of the dinosaurs – an asteroid or volcanoes. The panel ruled in favor of the asteroid, a theory first put forth in 1980 by one of Berkeley Lab’s greatest scientists, the late Nobel laureate Luis Alvarez, and his son Walter, a geologist with UC Berkeley.
The theory holds that an asteroid the size of San Francisco, traveling faster than a bullet, slammed into Earth 65 million years ago. The impact delivered a destructive blast thousands of times more powerful than the combined yield of all the world’s nuclear weapons, setting off earthquakes greater than 11 in magnitude and widespread tsunamis, and shrouding the globe for years in a thick cocoon of sky-blackening dust and debris. This cataclysm effectively ended the reign of the dinosaurs and opened the door for the ascension of mammals. With the panel’s report in Science having generated a flurry of stories in the media, it seems like a good time to take a look back at how the Alvarez asteroid theory came to be.
The Iridium Anomaly
The story begins in 1977 in Gubbio, Italy, a tiny village halfway between Rome and Florence, where geologist Walter Alvarez was collecting samples of limestone rock for a paleomagnetism study. The limestone rock outside of Gubbio, which was once below the sea, provides a complete geological record of the end of the Cretaceous period and the beginning of the Tertiary period. This time span is sometimes referred to as “the Great Dying,” because a massive extinction claimed nearly 75 percent of all the species of life on our planet, including in addition to the dinosaurs, most types of plants and many types of microscopic organisms. Walter Alvarez found that forming a distinct boundary between the limestone of the two periods was a thin layer of red clay. Immediately below this clay boundary, the Cretaceous limestone was heavily populated with a wide mix of the fossils of tiny marine creatures called foraminifera, or “foram” for short. Above the clay layer, in the Tertiary limestone, however, only the fossils of a single species of foram could be seen. The clay layer itself contained no foram fossils at all.
Walter brought samples of the Gubbio finding back to Berkeley and showed them to his father, winner of the 1968 Nobel Prize in Physics for his contributions to elementary particle physics. Even a Nobel Prize does not begin to do justice to the full breadth of the astonishing career of Luis Alvarez, who died of cancer in 1988 at the age of 77.
In addition to his development of the “dirty” bubble chamber and a proton linear accelerator that served as the prototype for today’s “linacs,” Luis Alvarez invented three important types of radar systems, including the Ground-Controlled Approach used by air-traffic control systems today. He flew as a scientific observer in an aircraft accompanying the Enola Gay when the “Little Boy” atomic bomb was dropped on Hiroshima, and designed the complex detonator for the “Fat Man” plutonium bomb. He co-discovered tritium and the “East-West effect” in cosmic rays, and later used cosmic rays to search for hidden chambers within the Chephren pyramid in Egypt.
Upon studying the Gubbio samples, Luis Alvarez wanted to know how long the foram extinction lasted before the species began to re-establish itself. He suggested the samples be handed over for analysis to another pair of Berkeley Lab scientists, nuclear chemists Frank Asaro and Helen Michel, who had developed a technique, called neutron activation analysis, that enabled precise measurements of very low concentrations of elements. Luis believed that neutron activation analysis would help determine how long it took for the clay layer to form. To the surprise of everyone involved, the measurements by Asaro and Michel showed that the clay layer was about 600 times richer in iridium than the surrounding limestone. Iridium, a silvery-white metal related to platinum, is virtually absent from the Earth’s crust, but high concentrations are common in extraterrestrial objects, such as asteroids.
The plot thickened when this same “iridium anomaly” was subsequently discovered in clay layers at locations in Denmark and New Zealand, and later dozens of other sites around the world where the geological record of the Cretaceous-Tertiary boundaries are also complete. These iridium-spiked layers of clay also contained an abundance of soot. Comparisons of ratios between iridium and several other key elements in the clay layers indicated that the widely scattered iridium anomalies all came from the same source – one that was not of this earth.
The Chicxulub crater
The Alvarezes along with Asaro and Michel published their seminal 1980 paper in Science: “Extraterrestrial Cause for the Cretaceous-Tertiary Extinction.” This paper was immediately resisted by scientific critics who argued that volcanic eruptions were behind the demise of the dinosaurs and cited as evidence the thousands of miles of volcanic rock in an area of India known as the Deccan Traps. However, that argument was weakened by two subsequent findings. First, there was the discovery of shocked quartz and microtektites along with the iridium and soot in the clay layer samples from around the world, which could only have been produced in the heat and violence of a titanic impact. Second came the discovery in 1991 of the scene of the crime – the Chicxulub crater, a 180-kilometer-wide, 20-kilometer-deep impact crater off the northern coast of the Yucatan peninsula in the Gulf of Mexico that is buried beneath a kilometer of Tertiary carbonates. The discovery of this impact site answered critics who’d been demanding to know: If an asteroid impact killed the dinosaurs, where’s the crater?
The lead author on the new paper in Science that ruled in favor of the Alvarez asteroid theory was Peter Schulte, a geophysicist at the University of Erlangen-Nurnberg. His panel concluded that the asteroid struck a sulfate-rich area that released deadly sulfur aerosols, making the global cocoon of dust even more toxic.
“We conclude that the Chicxulub impact was the ultimate cause for the mass extinctions of the dinosaurs,” Schulte told the media.
Said fellow panelist Kirk Johnson, a paleobotanist at the Denver Museum of Nature and Science, “Luis and Walter Alvarez and their team got it right; it was an inspired body of work.”
The new Science paper by Peter Schulte, et al, is titled “The Chicxulub Asteroid Impact and Mass Extinction at the Cretaceous-Paleogene Boundary.” It can be read (with subscription) here:
The original Science paper by Luis Alvarez, et al, can be read at http://www.sciencemag.org/cgi/content/abstract/208/4448/1095