The impact hypothesis - II

Enter Walter Alvarez. He came from a long line of distinguished scientists. His great-grandfather and grandfather were both noted physicians, and his father, Luis, was a physicist at the University of California-Berkeley. Walter attended graduate school at Princeton and took up geology. In the early 70s, he got a research post at the Lamont-Doherty Earth Observatory. Alvarez decided to try to figure out, on the basis of plate tectonics, how the Italian peninsula had formed.

In this quest, he found himself working in a hill town of Gubbio, about a hundred miles north of Rome, with an Italian geologist who was an expert on foraminifera or “forams” for short. They are the tiny marine creatures that create little calcite shells which drift down to the ocean floor once the animal inside has died. They can only be seen with microscopes. The geologist drew Alvarez's attention to a curious sequence. 

In one centimeter of clay separating two limestone layers, there were no fossils at all. In the older layer that lay below the clay, the forams were much larger than in the younger layer above the clay. The same of distribution of forams above and below the clay layer was present everywhere he looked. What had caused such a change in the forams? How fast did it happen? These were the questions that puzzled Walter. The pursuit of these questions led him to one of the biggest discoveries about one of the most important days in the history of life.

First a brief description of Deep Time. The history of life is divided into three chapters called "eras". The first is called the Paleozoic (“ancient life”), the second the Mesozoic (“middle life”), and the third the Cenozoic (“new life”). Each era comprises several “periods”; the Mesozoic, for example, spans the Triassic, the Jurassic, and the Cretaceous. The next period is the Tertiary (now renamed the Paleogene).

               Geological Time Scale 

The boundary between the Cretaceous and Tertiary layers, where the clay layer is found is called the K-T boundary. K is used as the abbreviation for Cretaceous because C was already taken by an earlier geological period known as the Carboniferous; today, the border is formally known as the Cretaceous-Paleogene, or K-Pg, boundary. It is a line that definitively marks the end-Cretaceous mass extinction everywhere in the world where the right aged rocks are preserved. It happened 66 mya (million years ago). 

                                                             KT (or KPg) boundary 

 Alvarez had been used to believing in uniformitarianism. He had learned that the disappearance of any group of organisms had to be a gradual process, with one species slowly dying out, then another, then a third, and so on. But the sequence in the Gubbio limestone gave him a different picture. The many species of forams in the lower layer seemed to disappear suddenly and all more or less at the same time. He also realized another thing. These forams appeared to vanish right around the point the last of the dinosaurs were known to have disappeared.

In 1977, Alvarez got a job at Berkeley, where his father, Luis, was working. He brought with him to California his samples from Gubbio. While Walter had been studying plate tectonics, Luis was busy winning the Nobel Prize in Physics in 1968. He’d also developed the first linear proton accelerator, invented a new kind of bubble chamber, designed several innovative radar systems, and codiscovered tritium.  In 2007 the American Journal of Physics commented, "Luis Alvarez was one of the most brilliant and productive experimental physicists of the twentieth century."

Luis Alvarez was interested in all sorts of riddles. An example was whether there were treasure-filled chambers inside Egypt’s second-largest pyramid. He would often come up with innovative ideas to approach the problem. When Walter told his father about the puzzling fossil distribution in Gubbio, Luis was fascinated. He came up with the wild idea of clocking the clay using the element iridium which is extremely rare on the surface of the earth. But Luis knew that it was much more common in meteorites. 

On earth, the tiny amounts of iridium come from bits of meteorites that are constantly raining down on the planet. Luis reasoned that the longer it had taken the clay layer to accumulate, the more cosmic dust would have fallen; thus the more iridium it would contain. By this technique he would be able to find out what length of time the clay layer represented. Walter gave him some limestone from above the clay layer, some from below it, and some of the clay itself. 

When the results came from the lab, it was puzzling. The amount of iridium that was present in the layers above and below the clay layer was what was normally present on earth. But the amount of iridium in the clay layer in the middle was 30 times higher. No one knew what to make of this. Was it a weird anomaly, or something more significant? Something very unusual, and very bad, had happened at the K-T boundary. The forams, the clay, the iridium, the dinosaurs, were all signs — but of what?

Two other sites having sediments dated to 66 mya when the Dinosaurs disappeared were found - one in  Denmark and another in New Zealand. They had the same pattern as the ones at Gubbio - a thin clay layer between earlier and later layers. They too showed an iridium “spike” in the clay layer. The Alvarezes knew they were onto something and started thinking up theories that would fit the available data. Finally, after almost a year’s worth of dead ends, they arrived at the impact hypothesis.