Iridium 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, and since it's found in greater concentrations on comets or asteroids, 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 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, 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, 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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|Title Annotation:||high iridium levels deposited during mass extinctions|
|Date:||Dec 6, 1986|
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