This ability to zoom in at very high resolution on a time window just 21 years wide and almost 13,000 years in the past comes to us courtesy of an amazing scientific resource consisting of ice cores from Greenland. Extracted with tubular drills that can reach depths of more than 3 kilometers, these cores preserve an unbroken 100,000-year record of any environmental and climatic events anywhere around the globe that affected the Greenland ice cap. What they show, and what Allen is referring to, is a mysterious spike in the metallic element platinum--'a 21-year interval with elevated platinum,' as he puts it now--'so we know that was the length of the impact event because there's very little way, once platinum falls on the ice sheet, that it can move around. It's pretty well locked in place.
To this day, scientists know of only two layers of sediment 'broadly distributed across several continents that exhibit coeval abundance peaks in a comprehensive assemblage of cosmic impact markers, including nanodiamonds, high-temperature quenched spherules, high-temperature melt-glass, carbon spherules, iridium and aciniform carbon.' These layers are found at the Cretaceous-Paleogene Boundary 65 million years ago, when it has long been agreed that a gigantic cosmic impact in the Gulf of Mexico caused the mass extinction of the dinosaurs, and at the Younger Dryas Boundary 12,800 years ago.
