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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.

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