The Alvarez Hypothesis

In 1977, geologist Walter Alvarez, while studying limestone rock in Gubbio, Italy, discovered a distinct boundary marked by a thin layer of red clay between the Cretaceous and Tertiary periods. This clay layer, revealed to be rich in iridium, provided evidence supporting the Alvarez Hypothesis that a massive asteroid impact caused the mass extinction event, known as "the Great Dying," around 66 million years ago.

"Platinum metals are depleted in the earth's crust relative to their cosmic abundance; concentrations of these elements in deep-sea sediments may thus indicate influxes of extraterrestrial material. Deep-sea limestones exposed in Italy, Denmark, and New Zealand show iridium increases of about 30, 160, and 20 times, respectively, above the background level at precisely the time of the Cretaceous-Tertiary extinctions, 65 million years ago. Reasons are given to indicate that this iridium is of extraterrestrial origin, but did not come from a nearby supernova. A hypothesis is suggested which accounts for the extinctions and the iridium observations. Impact of a large earth-crossing asteroid would inject about 60 times the object's mass into the atmosphere as pulverized rock; a fraction of this dust would stay in the stratosphere for several years and be distributed worldwide. The resulting darkness would suppress photosynthesis, and the expected biological consequences match quite closely the extinctions observed in the paleontological record."