Prior to the time when Luis and Walter Alvarez proposed that an impact led to the extinction of the dinosaurs, it was generally assumed that the dinosaurs had died out over a period of at least thousands to millions of years. Consider each piece of evidence listed below and sort it into the correct bin based on whether it supports a sudden cause for the demise of the dinosaurs or whether it could also be consistent with a gradual cause leading to their demise over many thousands or more years. Note: The "K-T sediment layer" refers to the geological layer dating to about 65 million years ago that separates sediments that contain dinosaur fossils from those that do not.

Explanation:pon studying the Gubbio samples, Luis Alvarez wanted to know how long the foram extinction lasted before the species began to re-establish itself. He suggested the samples be handed over for analysis to another pair of Berkeley Lab scientists, nuclear chemists Frank Asaro and Helen Michel, who had developed a technique, called neutron activation analysis, that enabled precise measurements of very low concentrations of elements. Luis believed that neutron activation analysis would help determine how long it took for the clay layer to form. To the surprise of everyone involved, the measurements by Asaro and Michel showed that the clay layer was about 600 times richer in iridium than the surrounding limestone. Iridium, a silvery-white metal related to platinum, is virtually absent from the Earth’s crust, but high concentrations are common in extraterrestrial objects, such as asteroids.

The plot thickened when this same “iridium anomaly” was subsequently discovered in clay layers at locations in Denmark and New Zealand, and later dozens of other sites around the world where the geological record of the Cretaceous-Tertiary boundaries are also complete. These iridium-spiked layers of clay also contained an abundance of soot. Comparisons of ratios between iridium and several other key elements in the clay layers indicated that the widely scattered iridium anomalies all came from the same source – one that was not of this earth.