What Killed the Dinos? Part 1: Iridium Level Spike

I was very surprised by these lines from Peter Brannen’s The Ends of the World:

“Dinosaurs roamed the earth for a long time. Tyrannosaurus Rex is closer in time to humans than to Stegosaurus.”

Turns out Calvin, the dino expert, knew this very well:

But Calvin couldn’t have known how dinosaurs went extinct. After all, back when Watterson was still drawing, there were multiple theories on the topic, but none was accepted as the answer.

Lisa Randall in her book, Dark Matter and the Dinosaurs, tells the story of how the answer was finally arrived at. In the 1970’s, geologist Walter Alvarez was studying the K-Pg (or K-T) boundary, the boundary between two eras in geological lingo. He found the iridium levels in the boundary layer to be abnormally high (30 times higher, later corrected to 90 times). His father, Luis, was a physicist who knew that earth’s iridium had dissolved into molten iron and sank to the earth’s core. Meteors, on the other hand, did have much higher levels of iridium in them.

“So any iridium on the surface should have an extraterrestrial origin.”

Correction: so said the physicist dad. To others, that was just one possibility. The Alvarezes assumed that over a period of time, meteorites burnt up in the atmosphere and the dust (including the iridium) rained down at a steady rate. But then they realized the iridium spike would have taken 3 million years to rain down. And the K-T layer boundary layer was too thin and had been formed in far less time than 3 million years. Things didn’t add up.

Could the source of the iridium spike have been a supernova explosion instead? Sure, but that would have also caused a spike in plutonium-244 levels in the soil. Tests revealed no such spike. So it felt like the source was a “large impact by an extraterrestrial object”:

-          It must have happened around 65 million years ago (estimated based on the age of the soil layers above and below the K-T boundary layer);

-          Based on the average iridium content of meteorites, they estimated the size of the object that struck to be “an incredible 10-15 km in diameter”.