Sample of Cretaceous–Paleogene (K-Pg) Boundary

Sample of Cretaceous–Paleogene (K-Pg) Boundary

A sample of the Cretaceous–Paleogene (K-Pg) boundary, recording in the geological record the asteroid impact that wiped out the dinosaurs. This sample was collected from the site where the theory was confirmed that an asteroid crashing to Earth caused the mass extinction that marked the end of the Cretaceous Period. This extinction event was responsible for the end of the dinosaur era, 66 million years ago.

The sample was collected from the cliff site in Denmark known as Stevns Klint, where the K-Pg boundary is particularly well represented. Researchers Luis Alvarez, Walter Alvarez, Frank Asaro, and Helen Vaughn Michel had developed a theory based on discoveries in Italy that a larger than expected concentration of iridium existed in the K-Pg boundary layer. Iridium is not found in such concentrations at the Earth's surface, so this pointed to extraterrestrial origins - as iridium is much more common in meteorites, and by extension asteroids. When the research team discovered similarly high levels in samples at Stevns Klint it proved that they had discovered a worldwide event recorded in the geological record.

The later discovery of the Chicxulub impact crater in Mexico, caused by an asteroid about ten kilometres in diameter striking Earth around 66 million years ago, proved to be the missing piece in the story. The theory that all the dinosaurs, not including those species that evolved into birds, were wiped out due to an asteroid impact is now widely accepted - although other contributing factors (such as the volcanic activity that created the Deccan Traps in what is now India) also played a part.

Geologists study stratigraphy - looking at how rock layers build up over millions of years. This makes it possible to look back in time through Earth's evolution. Around 66 million years ago is a thin layer of rock – the K-Pg boundary. Below this line, and further back in time, the fossil record is dominated by reptiles. Above it, we see the dominance of mammals and birds. The fact that the K-Pg boundary layer and its iridium content has been found all around the world shows that it was a major asteroid impact that wiped out the dinosaurs. The asteroid struck the Yucatán Peninsula (modern day Mexico) creating a huge crater. Material was blasted out into the atmosphere, raining back down all around the world – creating a layer preserved in rock, which we know as the K-Pg boundary. At Stevns Klint this clay layer is also known as the 'fish clay' layer, which is clear to see in this sample. It is called this because the only fossils that exist in this layer are fish scales and bones, revealing how much life was wiped out by the extinction event – around 75% of all plants and animals.