In 1980, while examining a thin layer of clay near Gubbio, Walter Alvarez and a team of researchers found themselves at the center of what would become one of the greatest discoveries about species extinction in recent times. The layer in question was placed right between two different types of rock, Cretaceous limestone and Paleogene sediments, providing an excellent geological boundary that could be used to determine the end of the dinosaurs' age, some 66 million years ago.
The thin layer contained not the clay itself but a chemical anomaly: a high level of iridium, which is rather scarce on Earth but abundant in meteorites. In this regard, according to information obtained from the Lawrence Berkeley National Laboratory, the researchers' main goal was to calculate the time required to form the thin layer in question. To do so, they examined various trace elements in the sediment, with iridium standing out due to its high abundance.
However, another study indexed in PubMed reported that the iridium anomaly at Gubbio was confined to a layer about 1 centimeter thick, supporting the theory that the element occurred abruptly rather than gradually. This is significant because if the element accumulated over many years, there would be more layers of rock showing the same iridium anomaly. But since the anomaly occurred abruptly and with sharp boundaries, this suggested an external event that caused the element's abnormal presence on Earth's surface.
