In the late 1970s, geologist Walter Alvarez discovered a layer of red clay between fossil-containing limestone deposits in Italy. His father, Nobel laureate Luis Alvarez, suggested how to analyze it. In 1980, neutron activation analysis at Lawrence Berkeley National Laboratory revealed that the clay was rich in iridium, a metal that is scarce in the Earth?s crust but common in extraterrestrial objects. Luis Alvarez then determined the global extent of the iridium, its origin, and the implications. The clay was found at a geological boundary formed 65 million years ago, at a time of mass extinction of many living species, including the dinosaurs. Alvarez suggested that the extinction was initiated by the impact of an asteroid or giant meteor, which caused a dust cloud that blocked much of the sunlight, leading to a drastic and devastating global climate change. This hypothesis, supported by a huge buried crater where a meteorite may have struck the Yucatán Peninsula of Mexico, is considered by many scientists to be the most likely explanation of the dinosaur extinction.

Scientific Impact: This work illustrated the value of the study of isotopes (different forms of the same element) as a tool to better understand the environment; the iridium anomaly has now been identified at more than 75 sites worldwide. This finding also led to development of the controversial Nemesis theory, which postulates that every 26 to 30 million years, a companion star to the sun unleashes a storm of comets, some of which strike the Earth.

Social Impact: The public is fascinated by dinosaurs. This hypothesis answers a long-standing question of great interest to many people, from scientists to schoolchildren.

Reference: "Extraterrestrial cause for the Cretaceous-Tertiary extinction," Science (1980).

URL: http://www.lbl.gov/Science-Articles/Archive/dinosaur-extinction.html

Technical Contact:

Press Contact: Jeff Sherwood, DOE Office of Public Affairs, 202-586-5806

SC-Funding Office: Office of High Energy and Nuclear Physics