Alvarez et al. 1980
In 1980, Luis Alvarez and co-authors
published a work in Science entitled
"Extraterrestrial Cause for the Cretaceous-Tertiary
Extinction."
Their primary argument was an iridium
spike found exactly at the Cretaceous-Tertiary
boundary in the rock record
(remember your
geologic
timeline): The Cretaceous-Tertiary
boundary marked the end of the dinosaurs,
65 million years ago.
Alvarez et al. 1980
Iridium is an element
(see the
periodic table of the elements)
which, it turns out, is quite rare in the Earth's
crust
but somewhat less rare in meteorites.
Alvarez et al. saw a spike in iridium concentrations
65 million years ago.