Chapter 16, Problem 1DAA

Discovery of Iridium in the K-Pg Boundary Sequence In the late 1970s, geologist Walter Alvarez was investigating the composition of the K-Pg boundary sequence 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 sequence in Italy and Denmark. They discovered that it contains a much higher iridium content than the surrounding rock layers. Some of their results are shown in FIGURE 16.18.

Iridium belongs to a group of elements (Appendix I) 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 sequence must have originated with extraterrestrial material. They calculated that an asteroid 14 kilometers (8.7 miles) in diameter would contain enough iridium to account for the extra iridium in the K-Pg boundary sequence.

FIGURE 18.16 Abundance of iridium in and near the K-Pg boundary sequence in Stevns Klint, Denmark. Many rock samples taken from above, below, and at the boundary were tested for iridium content. Depths are given as meters above or below the boundary.

The iridium content of an average Earth rock is 0.4 parts per billion (ppd) of iridium. An average metecrite contains about 550 parts per billion of iridium.

The photo shows Luis and Walter Alvarez with a section of the boundary sequence.

l. What was the iridium content of the K-Pg boundary sequence?