The Cretaceous–Palaeogene iridium anomaly

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A thin clay layer at the Cretaceous–Palaeogene boundary containing iridium at concentrations far above the surrounding sediment — the observation from which the entire impact hypothesis was built. The reasoning is elegantly indirect. Iridium is a siderophile element, so almost all of Earth's original inventory sank into the core during differentiation, leaving the crust strongly depleted. Chondritic meteorites are not depleted. A sudden crustal enrichment therefore points to an extraterrestrial source, and its thickness and global distribution give an estimate of the impactor size — the Alvarez team in 1980 inferred a body about ten kilometres across. The layer was first measured in the Bottaccione Gorge at Gubbio in Italy and at Stevns Klint in Denmark, and has since been found in well over a hundred sections worldwide, on land and in ocean cores. Associated in many sections are shocked quartz grains, spherules of melt droplets and, near the crater, thick chaotic deposits interpreted as tsunami and ejecta debris.