Chapter 16, Problem 2DAA

Discovery of Iridium in the K–Pg Boundary Layer In the late 1970s, geologist Walter Alvarez was investigating the composition of the K–Pg boundary layer in different parts of the world. He asked his father, Nobel Prize–winning physicist Luis Alvarez, to help him analyze the elemental composition of the layer. The Alvarezes and their colleagues tested the K-Pg boundary layer in Italy and Denmark, and discovered that it contains a much higher iridium content than the surrounding rode layers (FIGURE 16.17). Iridium belongs to a group of elements that are much more abundant in asteroids and other solar system materials than they are in Earth’s crust. The Alvarez group concluded that the K–Pg boundary layer must have originated with extraterrestrial material.

Sample Depth	Average Abundance of Iridium (ppb)
+2.7m	<0.3
+1.2m	<0.3
+0.7m	0.36
boundary layer	41.6
–0.5 m	0.25
–5.4 m	0.30

FIGURE 16.17 Abundance of iridium in and near the K–Pg boundary layer.

Iridium content of rock samples above, below, and at the K–Pg boundary layer in Stevns Klint, Denmark. Sample depths are given as meters above or below the layer. ppb, parts per billion. An average Earth rock contains 0.4 ppb iridium; the average meteorite, 550 ppb. The photo shows Luis and Walter Alverez next to the K–Pg boundary layer in Stevns Klint.

How much higher was the iridium content of the boundary layer than the sample taken 0.7 meter above it?