Alvarez had access to the nuclear chemists at the Lawrence Berkeley Laboratory and was able to work with Frank Asaro and Helen Michel. The chemists used a technique known as neutron activation analysis and were astounded to discover that exactly at the clay boundary the iridium content was enormous, but not in the limestone on either side. Whatever had caused the iridium content in the clay, it was far too high to have come from micrometeorites. Carefully checking their work, the next step was to determine if the clay from other locations contained the same level of iridium (the K-Pg clay is well known and is distributed world wide), which it did. Within a few years of the publication of their paper, more than 100 iridium-containing clay sites were found. The team, knowing of no terrestrial source which could produce and deliver so much iridium, concluded that the source had to be extraterrestrial.[8] In the years following their publication the clay was also found to contain soot, glassy spherules, shocked quartz crystals, microscopic diamonds, and other rare minerals formed only under conditions of great temperature and pressure.[1]
They considered a number of possible sources for the iridium anomaly; the passage of Earth through giant nebular clouds, a nearby supernova, and other low probability scenarios. With time, effort, and subsequent experimentation, all of these were eliminated, leaving a direct impact on the earth by a comet or an asteroid as the only hypothesis which could satisfy all of the conditions, the impact hypothesis, a significant challenge to current theory.
They published their paper in 1980; there was criticism from the geological community, and an often acrimonious scientific debate ensued. Ten years after this initial proposal, after Alvarez's death, evidence of a huge impact crater called Chicxulub off the coast of Mexico strongly supported their theory. Other researchers later found that the end-Cretaceous extinction event that wiped out the dinosaurs may have occurred over thousands of years, rather than millions of years as had previously been accepted. Others continue to study such theories as increased volcanism, particularly the massive Deccan Traps eruptions that occurred around the same time, and climate change, checking against the fossil record. Significantly, however, on March 4, 2010, a panel of 41 scientists agreed that the Chicxulub asteroid impact triggered the mass extinction.[9] ..."
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