The Sixth Extinction: An Unnatural History

While Walter had been studying plate tectonics, Luis had won a Nobel
Prize. He’d also developed the first linear proton accelerator, invented a
new kind of bubble chamber, designed several innovative radar systems,
and codiscovered tritium. Around Berkeley, Luis had become known as
the “wild idea man.” Intrigued by a debate over whether there were
treasure-filled chambers inside Egypt’s second-largest pyramid, he’d at
one point designed a test that required installing a muon detector in the
desert. (The detector showed that the pyramid was, in fact, solid rock.) At
another point, he’d become interested in the Kennedy assassination and
had performed an experiment that involved wrapping cantaloupes in
shipping tape and shooting them with a rifle. (The experiment
demonstrated that the movement of the president’s head after he was hit
was consistent with the findings of the Warren Commission.) When
Walter told his father about the puzzle from Gubbio, Luis was fascinated.
It was Luis who came up with the wild idea of clocking the clay using the
element iridium.
Iridium is extremely rare on the surface of the earth but much more
common in meteorites. In the form of microscopic grains of cosmic dust,
bits of meteorites 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. He contacted a Berkeley colleague, Frank Asaro, whose lab was
one of the few with the right kind of equipment for this sort of analysis.
Asaro agreed to run tests on a dozen samples, though he said he very
much doubted anything would come of it. Walter gave him some
limestone from above the clay layer, some from below it, and some of the
clay itself. Then he waited. Nine months later, he got a call. There was
something seriously wrong with the samples from the clay layer. The
amount of iridium in them was off the charts.
No one knew what to make of this. Was it a weird anomaly, or
something more significant? Walter flew to Denmark, to collect some late-
Cretaceous sediments from a set of limestone cliffs known as Stevns Klint.