Iridium is depleted in the earth’s crust relative to its normal solar system abundance. Several hundred measurements by at least seven laboratories have disclosed an iridium abundance anomaly at the Cretaceous/Tertiary (C/T) boundary in 36 sites worldwide. Discovery of the first iridium anomaly in nonmarine sediments, by Charles Orth and his colleagues, shows that the iridium was not extracted from sea water. Sediment starvation and a nearby supernova have also been eliminated as possible sources. Impact of a large extraterrestrial object is now widely accepted as the best explanation of the iridium anomaly. Paleomagnetic reversal stratigraphy of four marine and five non-marine C/T boundary sections is consistent with simultaneous extinction worldwide, but does not prove it. Ultra-high-resolution stratigraphic studies at Caravaca, in southern Spain, by Jan Smit, gave an unparalleled record of the extinction of the planktonic foraminifera and the associated geochemical patterns. Au/Ir and Pt/Ir ratios from two C/T boundary clays indicate a type I carbonaceous chondrite composition for the impacting object. Iridium anomalies are known from two other stratigraphic horizons, in each case associated with direct evidence for an extraterrestrial impact: in the Pliocene, with chondritic ablation debris, and in the late Eocene with microtektites. The C/T impact site has not been located. Two interesting candidate sites are the circular sea-floor features west of Portugal and the Deccan Traps of India. There is a 20% probability that impact occurred on sea floor that has subsequently been subducted. Recent computer modeling of impact processes is yielding important information. The killing mechanism has not yet been established, but both temperature changes and darkness due to atmospheric dust are probable contributors. Darkness would have lasted a few months, rather than our originally suggested few years; this is indicated by (1) calculated rapid dispersal of dust in ballistic trajectories, (2) more rapid settling of heavier, coagulated dust particles, (3) calculated effects of darkness on phytoplankton, and (4) compatibility of the plant record with a few months—but not with a few years—of darkness.

As a test of the asteroid-impact theory, which predicts that extinctions of taxa and geochemical anomalies similar to those found near the Cretaceous/Tertiary boundary should occur with a frequency of about 100 million years (m.y.), geochemical studies have been made near the Permian/Triassic and Eocene/Oligocene boundaries. The Permian/Triassic (P/T) boundary region (∼230 m.y. old) was chosen for study because it is associated with the most massive extinctions in the geologic record. The Eocene/Oligocene boundary region (~34 m.y. old) was chosen because microtektites, which are usually considered to be products of impacts, had been previously found somewhat below the boundary and synchronous with extinction events. An extensive clay layer, which had previously been assigned to the P/T boundary, was found to be chemically and mineralogically very different from the clays above and below, and it probably originated as an ash. As no iridium (Ir) anomaly (< 0.055 ppb) was detected in the layer, it probably had a volcanic rather than an impact origin. The latter possibility, however, cannot be ruled out, as high-speed comets could have the necessary explosive force and still have very little Ir. An Ir anomaly (0.4 ppb) was found near the Eocene/Oligocene boundary in a deep-sea core from the Caribbean Sea (DSDP Site 149, Core 31, Section 1, Intervals 1–2 and 3–4 cm) at exactly the same position that microtektites and extinctions of five species of radiolaria had been previously detected. Many other geochemical anomalies were detected between cores 30 and 31, but the most prominent could be related to simply the variation in the CaCO 3 deposition rate. The relative abundances of Cr, Ni, and Ir in the samples suggest that the Ir anomaly has an extraterrestrial origin rather than terrestrial, and the abundance patterns for many elements in the top of Core 31 indicate that the Ir was probably deposited too rapidly to be due to normal meteoritic dust. Thus, the Ir anomaly, the microtektite data, and the radiolarian extinctions are all supportive of a major bolide impact 34 m.y. ago. A worldwide distribution of the Ir anomaly is strongly suggested by very recent studies made in collaboration with Billy P. Glass in which Ir anomalies associated with microtektites in late Eocene sediments have been found in the Gulf of Mexico (DSDP Site 94), the Central Pacific Ocean, (DSDP Hole 69A and DSDP Site 166), and the Indian Ocean (DSDP Site 216).