RE: [escepticos] RE: Dinosaurios de moda

["C. Mario del Río" <cdelrioc@...>]

    Por favor lean el artículo que el señor Navarro incluye entre los "que
exponen teorias contrarias a la desaparición de los dinosaurios por culpa de
los aerolitos."

    Como se habrán dado cuenta muchos de los colisteros, a Héctor le importa
un pimiento si el impacto del K-T mató a los dinosaurios o al pájaro dodo,
lo que le interesa es discutir. Por favor, ruego a los que están
argumentando con él, especialmente a Gámez y Ernesto, que no desaprovechen
su calidad divulgativa en una trampa tan burda. Por mi parte, rompo mi
compromiso de no participar en esta discusión para probar la teoría popular
que afirma que se pilla antes a un mentiroso que a un cojo.

    Saludos

    Mario


----- Mensaje original -----
De: Héctor Walter Navarro <hwnavarro en abogados-rosario.org.ar>
Para: <escepticos en ccdis.dis.ulpgc.es>
Enviado: martes, 20 de junio de 2000 18:33
Asunto: Re: [escepticos] RE: Dinosaurios de moda



> He encontrado, apenas empecé a buscar, cerca de cincuenta
> sitios en Internet que exponen teorias contrarias a la desaparición
> de los dinosaurios por culpa de los aerolitos.

>           Chicxulub Impact Crater Provides Clues to Earth's History by
Virgil L.
> Sharpton dicusses
>           the most likely site for the main impact at the end of the
Cretaceous,
> Chicxulub in the
>           Yucatan, Mexico


EARTH'S HISTORY
Chicxulub Impact Crater Provides Clues to Earth's History

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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.
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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.


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.


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Suggested Reading
Alvarez, L. W., W. Alvarez, F. Asaro, and H. V. Michel, Extraterrestrial
cause for the Cretaceous/Tertiary extinction, Science, 208, pages 1095-1008,
1980.
Dressler, B. O., R. A. F. Grieve, and V. L. Sharpton, (Eds.), Large
meteorite impacts and planetary evolution, Geological Society of America
Special Paper 293, 348 pp., 1994.

French, B. M., and N. M. Short, editors, Shock Metamorphism of Natural
Materials, Mono Book Corp., Baltimore, Md., 644 pp., 1968.

Grieve, R. A. F., Terrestrial Impact Structures, Annual Reviews of Earth and
Planetary Science, 15, pp. 245-270, 1987.

Melosh, H. J., Impact Cratering: A Geological Process, Oxford University
Press, New York, 245 pp., 1989.

Roddy, D. J., R. O. Pepin, and R. B. Merrill, (Eds.), Impact and Explosion
Cratering, Pergamon Press, New York, 1301 pp., 1977.

Sharpton, V. L., and P. D. Ward, editors, Global catastrophes in Earth
history, Geological Society of America Special Paper 247, 631 pp., 1990.

Sharpton, V. L., G. B. Dalrymple, L. E. Marín, G. Ryder, B. C. Schuraytz,
and J. Urrutia-Fucugauchi, New links between the Chicxulub impact structure
and the Cretaceous/Tertiary boundary, Nature, 359, 819-821, 1992.

Sharpton, V. L., et al., Chicxulub Multiring Impact Basin: Size and other
characteristics derived from gravity analysis, Science, 261, pp. 1564-1567,
1993.

Silver, L. T., and P. H. Schultz, editors, Geological implications of
impacts of large asteroids and comets on the Earth, Geological Society of
America Special Paper 190, 128 pp., 1982.