Iridium-Rich Layer

The element iridium was brought into the public view with the discovery of a subsurface layer which was greatly enriched in iridium compared with its normal abundance. This layer was found many places around the globe and came to be associated with the boundary between the Cretaceous and Tertiary Periods, referred to as the K-T Boundary on the geological age scale.

The fact that a layer like this has been found at several locations scattered around the world suggests a large-scale atmospheric suspension of the material, such as would occur upon the impact of a sizable asteroid. Coupled with the presence of dinosaur fossils below this layer, but not above, this evidence has led to the asteroid model for the extinction of the dinosaurs. 这种层在世界多个地点被发现的事实，表明该物质可能在全球范围内悬浮，这可能是在大型小行星撞击时发生的。结合该层下方存在恐龙化石而上方没有这一现象，这些证据支持了小行星撞击导致恐龙灭绝的模型。 This cross-section of the strata containing the iridium-rich layer is on display at the Smithsonian Museum of Natural History. The thickness of the layer appeared to be 3-4 cm. 这一含铱层的岩层横断面在史密森尼自然历史博物馆展出。该层的厚度似乎为3-4厘米。

The iridium-rich layer at the K-T Boundary has been associated with the Chicxulub Crater centered off the Yucatan Peninsula of Mexico, for which a strong case has been made for an asteroid impact that contributed to the observed mass extinction. The K-T Boundary had been placed at 65 million years in the past by potassium-argon and other geologic dating processes. The association of the Chicxulub impact with that time period was strengthened by ⁴⁰Ar/³⁹Ar dating.