In 1980, a physicist studying a thin clay layer found an iridium spike, and it wasn’t just trace metal: It became one of the biggest clues behind the dinosaurs’ extinction

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.

The iridium clue pushed scientists toward an asteroid-impact explanation

Although the discovery of iridium did not directly indicate that an asteroid caused the dinosaurs' extinction, it drastically changed the kinds of questions scientists had been asking. Iridium is relatively rare on Earth’s surface since most of the iridium in the planet settled toward its core when Earth was formed, and in contrast, meteorites frequently contain high levels of iridium.

As explained by the US Geological Survey, the increased levels of iridium and osmium in the boundary-layer clay became important indicators of the asteroid extinction theory at the end of the Cretaceous period. For years, extinctions have been theorized to result from climatic variations, volcanism, sea-level changes, and ecological deterioration.

As noted in the Lawrence Berkeley National Laboratory’s archives, the group’s celebrated work on the iridium anomaly was directly linked to a large asteroid impact that could have resulted in the global dispersal of debris. Consequently, it was not the chemistry alone but rather its ability to transform a regional problem into a global one that made it relevant to the study of mass extinctions.

Scientists began looking for the same phenomenon in other locations around the globe, which was important because it would show that the signal was not just local. One such review, available through PubMed Central, showed that iridium and other impact products were present in ejecta spherules around the globe.

The clay layer eventually connected to the Chicxulub impact crater

While the iridium anomaly served as the means to the end of a discovery that would come much later, it would be the recognition of the Chicxulub crater as the site where the asteroid struck Earth and led to the extinction event. As noted by Nature Reviews Earth & Environment, current evidence suggests that the Chicxulub asteroid impact is very likely responsible for the mass extinction that wiped out the non-avian dinosaurs and drastically altered life on Earth.

While this later confirmation did nothing to detract from the importance of the initial discovery of the clay layer, it certainly reinforced its significance. After all, the detection of the iridium spike at Gubbio marked one of the first important steps toward the hypothesis that an asteroid had collided with Earth.

What made the Alvarez family's story even more interesting was the way the initial evidence was presented. It was not a giant hole in the ground discovered by satellites orbiting our planet, nor were there any fossil deposits full of dinosaur bones; the decisive piece of evidence for the asteroid theory was merely a strip of clay containing unusual concentrations of metal.

Such insignificant evidence led scientists to think about the scale of planetary processes. All these sciences were united into a single theory: geology, chemistry, astronomy, and extinction studies. A strip of clay within the layers of Italian rock helped prove that the dinosaurs' extinction was caused by one of the most devastating impacts of all time on our planet.