@oristarA,
Yes, that is what it means (since no one else seems interested in answering your question). Specifically, it refers to the iridium layer. Iridium is a very rare element, but there is a layer of iridium found in extraordinarily high concentrations (about 100 times the normal distribution of iridium) in clay formations around the world. Geological markers (i.e., rock formations which can be reliably dated) put the age of the iridium layer (also known as the iridium anomaly) at about 65 million years before the present. This is known as the "K-T" boundary, meaning the boundary between the Cretaceous period and the Tertiary period (those are the names of geological periods). This was discovered by a team of physicists lead by Luis Alvarez and his son Walter, in about 1980. There was a very acrimonious debate in the geological and physics community about this, especially as the Alvarez' put this forward as evidence of a major meteor impact which was responsible for the extinction of the dinosaurs. In about 1990, after Luis Alvarez had died, a gigantic impact crater was identified in the sea near Chicxulub in the Yucatan peninsula of Mexico. Alvarez was a brilliant experimental physicist, who had access to the equipment, labs and other physicists necessary to test the hypothesis (actually proposed by his son). Iridium is much more common in the composition of some meteors than it is on earth, and the iridium layer also contains microscopic diamonds, stressed quartz, soot and many other pieces of physical evidence of great pressure and temperature.
The iridium anomaly, or the iridium layer, is what is referred to in the passage you quoted when it writes of fine ash.
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