Asteroid Impact

Gubbio, Italy, is one of several places in the world where the boundary between the Cretaceous and Tertiary is exposed. In 1978 a scientific team led by Walter Alvarez (professor of Geology at Berkeley) analyzed the clay and found it contained abnormal amounts of iridium and osmium. Both metals are classified as precious because they are rare on the earth's surface. The average concentration of iridium in the continental crust is 0.0001 ppm while osmium has an average of 0.0005 ppm. In our solar system iridium and osmium are about 1000 times more abundant than on the earth (Allaby and Lovelock). The clays from Gubbio were found to contain 30 times the amount of iridium than the strata below it.

A graph showing the anomalous amounts of irridium in the K/T clays

Now the question was: was the enrichment a local event or did it occur globally? The Alvarez team took samples at Copenhagen and New Zealand. Both samples contained more than the crustal average-160 and 20 times respectively. The team concluded that the enrichment was a global event. The team hypothesized that the origin of this extra iridium was extraterrestrial.

Places in the world where the iridium anomaly has been documented

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