Chapter 16, Problem 1DAA

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.

What was the iridium content of the K–Pg boundary layer?

Summary Introduction

To determine: The iridium content of the K–Pg boundary layer.

Introduction: The iridium is a chemical element that is abundantly found in the Cretaceous–Paleogene (K–Pg) boundary layer. The iridium level in an average rock is about 0.4 ppb, and in an average meteorite, it is about 550 ppb. This allows scientists to conclude whether the origin of the rocks is from the Earth or from any other sources.

Explanation of Solution

As given in the problem statement, Researcher W examined the composition of the K–Pg boundary layer in various parts of the world. He took help from his father to examine the composition of elements in the layer. Researcher L and his colleagues tested the K–Pg boundary layer in Country D and Country I. They discovered that the iridium content was found to be in large quantity than the surrounding rock layers. They also concluded that the boundary layer may have originated with the extraterrestrial material.

Refer to Fig. 16.17, “Abundance of iridium in and near the K-Pg boundary layer”, in the textbook. The given table shows the average abundance of iridium (ppb), and the sample depth (in meters) in Country D. The chemical element iridium was found in abundance on an average of about 41.6 ppb in the K–Pg boundary layer.

Conclusion

The iridium content was 41.6 ppb in the K–Pg boundary layer.