ADU Web Site: Chicxulub Impact Crater Provides Clues to Earth's History

EARTH'S HISTORY

Chicxulub Impact Crater Provides Clues to Earth's History

Earth in Space Vol. 8, No. 4, December 1995, p. 7. © 1995 American Geophysical Union. Permission is hereby granted to journalists to use this material so long as credit is given, and to teachers to use this material in classrooms.

An extraterrestrial impact 65 million years ago changed the course of life on Earth. The crater it carved in the Earth's surface could now help scientists study past global change.

by Virgil L. Sharpton, Lunar and Planetary Institute, Houston, Texas

In 1980, Luis Alvarez and his geologist son, Walter, proposed that a giant asteroid or comet struck the Earth approximately 65 million years ago and caused the mass extinctions of the dinosaurs and over 70% of all life on Earth. This bold proposition resulted from their discovery, near the Medieval town of Gubbio, Italy, of a centimeter-thick clay layer among limestones deposited on the Earth's surface at the time of the extinction event, between two geologic time intervals, the Cretaceous (K) and Tertiary (T). The limestone directly beneath the clay layer abounds with planktic formaniferids of latest Cretaceous age, while the Tertiary limestone unit immediately above the clay layer showed only rare and poorly formed fossils. Thus the clay layer itself, the scientists reasoned, must hold clues to the duration and nature of the mysterious KT extinction event, one of the most dramatic calamities to afflict Earth's biosphere since the development of complex life over a billion years ago. They found that the clay contained high concentrations of the element iridium, extremely rare in Earth's crustal rocks but quite abundant in certain meteorites, and proposed that this clay was the altered remains of the dust cloud blasted around the world when a 10-km-wide asteroid or comet struck the Earth. Fifteen years of research has upheld this idea, and, now, all indications are that the source crater has been found.

The collision occurred on the Yucatán platform and is centered near the port city of Progreso, Mexico. The 200–300-kilometer wide crater lies buried beneath 1100 meters of limestone laid down in the intervening years and few clues of its presence remain at the surface. Yet prominent circular anomalies in geophysical data gained the interest of Petroleos Mexicanos and in the early 1950s, they began an exploration campaign that included deep drilling to recover samples of the subsurface rocks. The buried feature became known as the Chicxulub structure (see figure 1), named for the first well located near the Mayan village by the same name. Pemex drilling continued throughout the early 1970s, and by that time, Mexican scientists realized that the Chicxulub structure was quite unusual. Three wells near the center had recovered silicate rocks with igneous textures, initially mistaken for volcanic rocks, and others, located between 140 km and 210 km from ground zero recovered breccia deposits hundreds of meters thick, indicating catastrophic or explosive conditions. By 1980, at least one scientist at Pemex felt that the evidence pointed to impact, although a volcanic origin for the Chicxulub structure could not be ruled out.

Fig.1. Surface geology, ring locations from gravity data, and wells near the Chicxulub impact basin. The three wells that penetrated impact melt rocks and breccias beneath the carbonate cover rocks are C1 (Chicxulub 1), S1 (Sacapuc 1), and Y6 (Yucatan 6). Other well sites shown are Yucatan 1 (Y1), Yucatan 2 (Y2), Yucatan 5A (Y5A), and Ticul 1 (T1). The Yucatan 4 (Y4) well site is located off the map, ~65 km east of Y5A. Carbonate units at the surface are Q (Quaternary; <2 Ma), Tu (Upper Tertiary; ~2–35 Ma), To (Oligocene; ~25–35 Ma),Te (Eocene; ~35–55 Ma), and Tpal (Paleocene; ~55–65.0 Ma). The crater center is indicated by ×. Hatchured lines represent the Ticul fault system. Dashed lines indicate trend of ringlike zone of water-filled sinkholes.

Beginning in 1990, however, samples from the Pemex wells were located in Mexico City and teams of scientists from the United States and Mexico quickly developed an impressive case that the Chicxulub structure was indeed the KT "smoking gun." Mineral evidence of shock metamorphism, requiring pressures and strain rates considerably higher than those produced by terrestrial processes, indicated that the crystalline rocks within the basin were melt rocks formed by an impact event and not by volcanism. Biostratigraphic information indicates that the structure was formed in uppermost Cretaceous rocks, consistent with a KT age. Argon and uranium-lead age determinations reveal that the melt rocks and the associated breccias are the same age as the tiny spherules of impact glass found within KT boundary deposits in Haiti and Mexico and the unmelted granitic fragments found in KT boundary exposures throughout western North America. Isotopic analyses demonstrate that the Chicxulub melt rocks and the ejecta spherules originated from the same source rocks. Consequently, there is a clear chemical as well as temporal link between the Chicxulub structure and the KT boundary deposits.

Additional geological and geophysical evidence collected over the last few years now suggests that Chicxulub could be the largest impact basin to form on Earth in the last billion years or so. Over 200 thousand cubic kilometers of the Earth's crust was instantly vaporized, melted or ejected from the crater. Studies of this structure through additional scientific drill coring and seismic profiling will shed valuable new light on understanding the cratering process and its geological implications. Perhaps less clear but of more immediate concern is what additional studies of the Chicxulub basin-forming event could provide for research into modern global climate change. Concerns over global warming and acid rain production induced by human activities prompt important questions about the future health and economic security of the world's population. Experts readily admit, however, that modern climate change research is chock-full of uncertainties about how the Earth responds to the rapid and dramatic changes imposed by human activities. These uncertainties adversely affect predictions of sea level rise and changing climate patterns and impede the formulation of effective international climate change policies.

The KT impact event provides a case completely played out in the rock record where a localized geological process—the collision of large piece of space debris—induced a global environmental collapse culminating in biological devastation. Yet, much is not understood at present about how such an impact event could have changed the environment, but adverse changes in the composition of the atmosphere are strongly implied. The uppermost 2–3 kilometers of rocks involved in the Chicxulub collision are composed of calcium carbonate (limestone) and calcium sulfate (anhydrite). When such rocks are heated under the extreme temperatures of high-speed impact, the greenhouse gas carbon dioxide and sulfate aerosols, which both contribute to acid rain and smog, are released into the atmosphere. Thus in addition to the dust and ash that could interfere with sunlight reaching Earth's surface, the Chicxulub impact event probably changed the global atmospheric composition in ways not unlike those that result from modern human activities. By studying the record in the rocks at Chicxulub, the magnitude of the crisis and the Earth's response can be reconstructed. A better understanding of how Earth's complex system of atmospheric, hydrologic, and geologic processes reacted under the KT environmental crisis 65 million years ago might help the contemporary global change community forecast and develop remediation policies.

Source: Eos, Vol. 76, December 26, 1995.

GLOSSARY

Anomalies: Deviations from the common or normal type
Biostratigraphic: Pertaining to layers of the Earth identified by particular fossil content
Cretaceous: The span of time between 135 and 65 million years ago
Planktic formaniferids: Tiny shelled sea creatures
Spherules: Little spheres or spherical bodies
Tertiary: The span of time between 65 and 3 to 2 million years ago

Chicxulub Impact Crater Provides Clues to Earth's History

GLOSSARY

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