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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...

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