Iridium spike not a comet strike?Iridium iridium (ĭrĭd`ēəm), metallic chemical element; symbol Ir; at. no. 77; at. wt. 192.22; m.p. about 2,410°C;; b.p. about 4,130°C;; sp. gr. 22.55 at 20°C;; valence +3 or +4. spike not a comet strike? When researchers discovered a high iridium level in sediments that had been deposited during the Cretaceous-Tertiary (K/T) mass extinctions, it stood out like a sore thumb. Iridium is rare on the earth's surface Noun 1. Earth's surface - the outermost level of the land or sea; "earthquakes originate far below the surface"; "three quarters of the Earth's surface is covered by water" surface , and since it's found in greater concentrations on comets or asteroids This is a list of numbered minor planets, nearly all of them asteroids, in sequential order. As of late September 2007 there are 164,612 numbered minor planets, and many more not yet numbered. Most asteroids are ordinary and not particularly noteworthy. , some scientists proposed that an impact of an extraterrestrial body caused the iridium spike, as well as the extinctions. And so began a fervent debate over what killed the dinosaurs and other life 65 million years ago (SN: 2/1/86, p.75). Recently, however, scientists at Exxon Production Research Co. in Houston concluded that the K/T iridium spikes may have nothing to do with the extinctions. Art Donovan Arthur Donovan, Jr. (born June 5 1925) is a former American football defensive tackle. He is the son of Arthur Donovan, a famed boxing referee, and the grandson of Professor Mike Donovan, the world middleweight boxing champion in the 1870's. and his co-workers have studied what they say is one of the most complete K/T sections known. They have found three iridium spikes created in a time span of about 1 million years at a site in the Clayton formation in central Alabama. Most significantly, each of these spikes corresponds to a period when sedimentation rates were very low. "So the concentrations of iridium at the boundary," says Donovan, "may be controlled by depositional patterns," and not the result of a comet hitting the earth at that time. Most K/T sections are very condensed con·dense v. con·densed, con·dens·ing, con·dens·es v.tr. 1. To reduce the volume or compass of. 2. To make more concise; abridge or shorten. 3. Physics a. , he adds, because the sea level rose at the end of the Cretaceous period, making sedimentation rates fall. "Impacts could still have been a source for the iridium in general," as could volcanic eruptions volcanic eruptions discharging of fumes, dust and lava from volcanoes. They have damaging potential in addition to those of being physically overpowering by the lava flow or the ash or dust fallout. , says Donovan, but the concentrated spikes of iridium seem to be due to changes in sedimentation rates. What scientists need to do now, he suggests, is look carefully at the layers deposited during other times of low sedimentation to determine the frequency of high levels of iridium and other elements. |
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