How important were the deaths of a few dinosaurs?
The mass extinction to which the non-avian dinosaurs finally succumbed after thriving for about 164 million years on Earth is called the Cretaceous–Paleogene extinction, commonly abbreviated K-Pg. The K-Pg mass extinction (Box 16.1) involved much more than just dinosaurs. Among the “highlights” were:
• a ~10 km asteroid collided with Earth;
• the great cycles of nutrients that formed the complex food webs in the world's oceans temporarily shut down;
• many marine and terrestrial animals, as well as many plants, went extinct;
• landscapes were deforested;
• epic tsunamis rolled across ocean basins, and
• wildfires likely raged.
By comparison with that, how important were the deaths of a few dinosaurs?
Whatever else is truth, the Cretaceous ended – and the Paleogene began – with a real bang. Here's how we learned about it. In the late 1970s, geologist Walter Alvarez and a team of co-workers (Figure 16.1) were studying K-Pg marine outcrops now exposed on land near a town called Gubbio, in Italy. They were struck by the fact that the lower half of the Gubbio rock exposure is composed of a rock made up entirely of thin beds of the microscopically sized shells of Cretaceous marine organisms. The upper half of the exposure was almost exclusively of thin beds of the microscopic shells of Paleogene marine organisms. Between the two was a thin (2–3 cm) layer of clay, obviously the K-Pg boundary.
Analyses showed that the clay layer contained unusually high concentrations of iridium, a rare, platinum-group metal. Instead of the expected amount at the Earth's surface, about 0.3 parts per billion (ppb), the iridium content was a whopping 10 ppb at Gubbio. So the iridium anomaly, as it came to be called, contained about 30 times as much iridium as Alvarez and his co-workers had expected to find (Figure 16.2).
Iridium is normally found at the Earth's surface in very low concentrations, but it is found in higher concentrations in the core of the Earth and from extraterrestrial sources; that is, from outer space. Given that, the Alvarez group determined that the source of the iridium had to be extraterrestrial. The deal was sealed when they found iridium anomalies at two other K-Pg sites, one in Denmark and the other in New Zealand.