The Cretaceous–Palaeogene iridium anomaly

Assembled from 20 blocks · 2 evidence · 14 related

1. Story
2. 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.
3. Knowledge
4. The Cretaceous–Palaeogene iridium anomaly
5. The Cretaceous–Palaeogene mass extinction
6. Connections
7. The Cretaceous–Palaeogene mass extinction
8. Bottaccione Gorge, Gubbio
9. The Cretaceous–Palaeogene mass extinction
10. The Alvarez impact hypothesis is published
11. Chicxulub is identified as the K–Pg impact crater
12. The Cretaceous–Palaeogene iridium anomaly
13. Chicxulub impact crater, Yucatán
14. Mass extinction
15. The Cretaceous–Palaeogene iridium anomaly
16. Bottaccione Gorge, Gubbio
17. The end-Triassic mass extinction
18. Evidence
19. Supports the iridium anomaly at Gubbio and Stevns Klint, the siderophile-depletion reasoning and the inference of an impactor about ten kilometres across. V55 verification basis: this session had no network access to any source — WebFetch was egress-blocked and the WebSearch budget was exhausted — so the cited work was not retrieved and its pagination was not re-checked. Volume and page range are quoted from standing knowledge and were not re-checked.
20. Supports the multi-author synthesis position that the Chicxulub impact was the proximate cause of the K–Pg extinction, and the global distribution of the boundary layer with its shocked quartz and spherules. The Deccan Traps alternative continued to be argued after this review; that literature is not cited here. V55 verification basis: this session had no network access to any source — WebFetch was egress-blocked and the WebSearch budget was exhausted — so the cited work was not retrieved and its pagination was not re-checked. Volume and page range are quoted from standing knowledge and were not re-checked.