A chemical fingerprint of extinction
The white chalk cliffs of Stevns Klint, stretching 17 kilometers along the coast, hold a dark, thin line that records a violent event. This layer, just 5 to 10 centimeters thick, is known as the Fish Clay ('Fiskeler' in Danish). It marks the geological moment 66 million years ago when an asteroid struck the Earth, leading to the extinction of over 50% of all species, including the non-avian dinosaurs. The cliff exposes the Cretaceous-Paleogene (K-Pg) boundary, one of the best-preserved sections of this event on the planet.
The discovery that made this location famous came in the 1970s. A team of scientists led by Luis and Walter Alvarez was investigating a similar layer in Italy. They decided to measure the concentration of the element iridium, a metal that is very rare in the Earth's crust but much more common in asteroids. Their analysis revealed a massive spike in iridium. When they tested samples from Stevns Klint, they found the same anomaly, with iridium levels up to 160 times higher than in the surrounding rock. One measurement showed a peak concentration of 29.9 parts per billion. This global iridium layer became the evidence for the Alvarez hypothesis: that the dust from a colossal asteroid impact blocked the sun, collapsed food chains, and caused a mass extinction.