Extraterrestrial Cause for the Cretaceous-Tertiary Extinction

TLDR

Platinum metals are depleted in Earth's crust relative to cosmic abundance, so their concentrations in deep‑sea sediments can signal extraterrestrial influxes. The study proposes a hypothesis linking the Cretaceous‑Tertiary extinctions to extraterrestrial iridium deposition. An asteroid impact would inject roughly 60 × its mass as pulverized rock into the atmosphere, with a fraction of the dust remaining in the stratosphere for years and spreading globally. Iridium spikes in deep‑sea limestones at the Cretaceous‑Tertiary boundary, the extraterrestrial origin of the iridium, the predicted global dust‑induced darkness suppressing photosynthesis, the distinct composition of boundary clay, and asteroid diameter estimates of 10 ± 4 km collectively support the asteroid impact hypothesis for the extinction.

Abstract

Platinum metals are depleted in the earth's crust relative to their cosmic abundance; concentrations of these elements in deep-sea sediments may thus indicate influxes of extraterrestrial material. Deep-sea limestones exposed in Italy, Denmark, and New Zealand show iridium increases of about 30, 160, and 20 times, respectively, above the background level at precisely the time of the Cretaceous-Tertiary extinctions, 65 million years ago. Reasons are given to indicate that this iridium is of extraterrestrial origin, but did not come from a nearby supernova. A hypothesis is suggested which accounts for the extinctions and the iridium observations. Impact of a large earth-crossing asteroid would inject about 60 times the object's mass into the atmosphere as pulverized rock; a fraction of this dust would stay in the stratosphere for several years and be distributed worldwide. The resulting darkness would suppress photosynthesis, and the expected biological consequences match quite closely the extinctions observed in the paleontological record. One prediction of this hypothesis has been verified: the chemical composition of the boundary clay, which is thought to come from the stratospheric dust, is markedly different from that of clay mixed with the Cretaceous and Tertiary limestones, which are chemically similar to each other. Four different independent estimates of the diameter of the asteroid give values that lie in the range 10 ± 4 kilometers.