Word: iridium
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...that pudding to be served before they commit themselves to the idea of periodicity, let alone a particular model. Says Cornell Astronomer Carl Sagan: "None of the explanations is anything like fully satisfying." Yet all but a few diehards acknowledge the brilliance of the Alvarez work. They believe the iridium layer and subsequent discoveries indicate that impacts of extraterrestrial objects may have played a significant role in certain extinctions, either directly or by delivering a final coup de grace to species already debilitated by climatic changes...
Analyzing clay collected from several parts of the world, they found samples containing 160 times as much iridium as is normally found in terrestrial rocks, Iridium, however, is a common component of many extraterrestrial objects, " such as comets, asteroids, and other planets...
...group then set about examining how the iridium spread throughout the world, says Helen Michael, one of the Berkeley team's principle researchers. They reasoned that iridium-rich asteroids bombarded the earth, kicking up so much terrestrial and asteroidal debris into the atmosphere that it blocked sunlight from the earth for as long as several months. As temperatures plummeted, plants and animals died, leaving the current fossil record...
...hypothesis was quickly put to the test as scientists from all over the world scrambled to test rocks for iridium, and to the surprise of many, they discovered abnormally high levels of material at layers corresponding to the most recent extinction...
...theory lost some ground when geologists failed to discover excesses of iridium corresponding to earlier extinctions...