"At Last, the Smoking Gun?"

Magazine article

By: Leon Jaroff

Date: July 1, 1991

Source: Leon Jaroff, "At Last, the Smoking Gun?" Time. 137 (July 1, 1991): 26.

About the Author: Since 1951, Leon Jaroff has been an editor, writer and contributor to both Time and LIFE magazines. In 1980 Jaroff became the founding editor of DISCOVER magazine. His writing has earned a variety of awards from both scientific and medical associations.

INTRODUCTION

The KT boundary is an actual boundary between layers of rock that refers to the end of the period of Earth's history when dinosaurs dominated the planet and the beginning of the rise of mammals. Geologists find rocks containing dinosaur fossils below the KT boundary. In the rock layers above the KT boundary, scientists find rocks containing fossils of mammals and other more modern animals and plants. The change in the types of fossils found above and below the KT boundary is very abrupt and very dramatic.

The term KT refers to two periods in geologic history, which are separated by the boundary. The older period is the Cretaceous, which is part of the Mesozoic Era and extended between 135 million years ago and 65 million years ago. (In German, Cretaceous begins with the letter K, which is the origin of the K in KT boundary). The geologic era following the Mesozoic is the Cenozoic, which extends between 65 million years ago and today. The first part of the Cenozoic is the Tertiary, which spanned the time between 65 million years ago and 1.75 million years ago. The T in KT refers to the Tertiary period.

Evidence of the KT boundary was first discovered by French naturalist George Cuvier, who was studying rock layers in the early part of the nineteenth century in the Paris basin. He found that the types of fossils found below the layer were extremely different from those found above it. In particular, Cuvier and other scientists who followed his work found that the boundary represented a period of mass extinctions of many different species. This led Cuvier to propose the idea that catastrophic events play an important role in the Earth's history. Cuvier's idea was not extremely popular in the scientific community, giving way to the theory of gradualism that stated that geologic change occurs very slowly.

In 1980, Luis and Walter Alvarez analyzed the chemistry of the gray layer of clay that comprises the KT boundary. They found extremely high concentrations of iridium in the layer. Iridium is rare on Earth but common in extra-terrestrial rocks. The Alvarezes proposed that the iridium indicated that a giant meteor hit the Earth 65 million years ago, vaporized and spread iridium-laden dust throughout the atmosphere. When it settled, it became the KT boundary.

The Alvarezes' idea was met with intense criticism from the scientific community. However, in 1991, as reported in the article below, the Alvarezes found the impact crater from a massive meteor that dated to 65 million years ago in the Yucatan Penninsula in Mexico.

PRIMARY SOURCE

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[This text has been suppressed due to author restrictions]

SIGNIFICANCE

Since the time of the article, scientists from around the world have measured high concentrations in iridium in a clay band from about eighteen locations throughout the world. In all cases the band dates from around 65 million years ago. In places closer to the Yucatan, like Central America, the band is approximately 1.2 inches (3 centimeters) thick. In Europe the band is only about 0.4 inches (1 centimeter) thick. The band is unique in geological studies of the Earth because it was laid down at exactly the same time throughout the world. This evidence gives additional support to the theory that the massive meteor that caused the crater in Chicxulub produced dust that surrounded the entire Earth.

After finding the impact crater in Chicxulub, most of the scientific community accepted the existence of a major meteor impact 65 million years ago, however controversy still remains as to the effect that the impact had on life on Earth. Not all scientists believe that the collision with the meteor was responsible for the mass extinctions at the KT boundary. An alternative theory involves the Deccan Traps, which are a series of volcanoes in the west central part of India. About 68 to 66 million years ago, these volcanoes underwent a period of massive eruption. They released enormous quantities of sulfur dioxide, which is toxic to air-breathing vertebrates, and carbon dioxide, which would have increased the temperature of the planet. Some paleontologists believe that these events could have led to the extinction of land-dwelling animals, or at least have had a major impact on them.

The mass extinctions of the KT boundary had a significant impact on life on Earth. Given the physical events that would have occurred in the wake of the meteor's collision with Earth, paleontologists have made predictions about the type of animals that would have been able to survive the environmental damage. These predictions suggest that the animals that persisted through the KT boundary were probably small creatures that could burrow underground during the heat wave that followed the meteor collision. In addition, these animals would have to be able to survive on insects, rather than plants, for extended periods of time. Such animals were primarily the small mammals that lived at the same time as the dinosaurs. Because approximately 75 percent of all animals and plants went extinct at the time of the collision, those that did survive the events were able to take advantage of environmental niches that had been previously occupied by other species. As a result, the mammals that survived the KT boundary underwent an enormous diversification, producing the wide variety of land-dwelling mammals, including humans, that inhabit the Earth today. In addition to the mammals that persisted following the meteor collision, some of the of the smaller dinosaurs probably also survived. Their descendents are represented by the modern birds.

FURTHER RESOURCES

Web sites

BBC Radio. "In Our Time: The KT Boundary." 〈http://www.bbc.co.uk/radio4/history/inourtime/inourtime_20050623.shtml〉 (accessed January 6, 2006).

NASA/UA Space Imagery Center. "Chicxulub Impact Event." 〈http://www.lpl.arizona.edu/SIC/impact_cratering/Chicxulub/Chicx_title.html〉 (accessed January 6, 2006).

Smithsonian National Museum of Natural History. "A Blast from the Past!" 〈http://www.nmnh.si.edu/paleo/blast/index.html〉 (accessed January 6, 2006).