Isotopic evidence for the Cretaceous-Tertiary impactor

The Cretaceous period is the geological period ranging approximately from 146 million years ago to 65 million years ago, and was apparently characterized towards its end by the rapid extinction of a number of species, including the dinosaurs. There have been five major extinctions according to the fossil record, the Cretaceous extinction one of them, and the consensus is that these extinctions were related to violent geophysical events, perhaps asteroid impacts. The Chicxulub impact crater in the Yucatan peninsula of Mexico is a large impact crater apparently caused by a 10 kilometer-diameter asteroid, the impact area extending at least 100 kilometers from the impact center. Using Argon(40)/Argon(39) isotope dating methods, this impact crater has been dated with high precision at 64.98 million years ago, which places the impact at the end of the Cretaceous, and the most popular current hypothesis to explain the Cretaceous extinction is the global effect of the Chicxulub impact on the extant life forms. This hypothesis was first proposed by Luis and Walter Alvarez in the 1970s on the basis of non-terrestrial dust of presumed cosmic origin in deposits at the K/T boundary, but the Yucatan crater was unknown at that time and was not discovered until the 1990s. But direct isotope evidence of an impactor is still missing, and some researchers have argued that high concentrations of iridium and other noble metals in K/T boundary sediments, the basis for the K/T impactor hypothesis, can be explained by enhanced volcanic activity that occurred near the end of the Cretaceous, bringing up noble metals from Earth's mantle, which similar to meteorites has high concentrations of noble metals. ... A. Shukolyukov and G.W. Lugmair now report a high-precision *mass spectrometric analysis of chromium in sediment samples from the K/T boundary confirms the cosmic origins of the K/T phenomenon. The authors report that the isotopic composition of chromium in K/T boundary samples from Stevns Klint, Denmark, and Caravaca, Spain, is different from that of Earth and indicates its extraterrestrial source. The authors suggest the chromium isotope signature is consistent with a *carbonaceous chondrite-type impactor, and that the observed differences in the chromium isotopic composition among the various meteorite classes can serve as a diagnostic tool for deciphering the nature of impactors that have collided with Earth during its history.

QY: A. Shukolyukov and G.W. Lugmair (Univ. of Calif. San Diego, US)
QY: A. Shukolyukov, Univ. of Calif. San Diego 619-534-2230.
Science 30 Oct 98, 282:927

Text Notes:
* mass spectrometric analysis: The mass spectrometer is a device in which molecules are ionized and the accelerated ions are separated according to their mass to charge ratio. The relative abundance of isotopes or other ionized species can thus be determined by measuring positive or negative ion currents.

* carbonaceous chondrite: "Stony" meteorites (aerolites) are meteorites formed solely of rock-forming silicates, and chondrites are a type of stony meteorite consisting of an agglomeration of millimeter-sized globules (chondrules) that are thought to be unchanged since the original condensation out of the nebula from which the sun and solar system formed. A carbonaceous chondrite is a chondritic meteorite that contains a relatively large amount of carbon, with a resultant dark appearance.

Summary & Notes by SCIENCE-WEEK 27Nov98
Related Background:

... In a short review of the meteorite impact hypothesis and the K/T extinction, K.O Pope et al make the following points: 1) Confirmation of the impact portion of the Alvarez hypothesis marks a turning point in the study of the K/T mass extinction, a turning point away from speculations about possible causes and toward linking the extinctions to a single catastrophic event. 2) Advances in computer modeling of the impact, coupled with knowledge of the target rocks and their behavior under the high-pressure shock, have shed light on what happened during the first few seconds after impact. A key aspect of the Yucatan site is that the upper 3 kilometers of rock were rich in water, carbonate, and sulfate, which upon impact produced about 200 gigatons each of SO(sub2) and H(sub2)O vapor and other gases that greatly altered the properties of the stratosphere. 3) Early work predicted that smoke and dust from the impact plunged the Earth into a freezing blackout. Recent computer simulations and atmospheric models indicate that within a few weeks to months temperatures and light levels would have begun to rebound due to the release of heat stored in the oceans and the coagulation and fall of the dust and soot. The major effects of the dust and soot would last about 1 year or less, but SO(sub2) and water vapors would remain in the stratosphere and ultimately produce sulfuric acid aerosols. Models indicate that a global aerosol cloud would be continuously produced for approximately 12 years, blocking out over 50 percent of the sunlight during the first 10 years. The authors conclude: "Now that we have a better understanding of the dynamics of the impact, gleaned from the discovery of the crater and the studies that followed, we can begin to address a wide range of complex global effects. There is much work ahead, but the course is clear."

K.O. Pope et al (3 authors at 3 installations, US)
Meteorite impact and the mass extinction of species at the Cretaceous/Tertiary boundary.
(Proc. Natl. Acad. Sci. US 15 Sep 98 95:11028)
QY: Kevin O. Pope, Geo Eco Arc Research, 2222 Foothill Blvd., La Canada, CA 91011 US.
Summary by SCIENCE-WEEK 23Oct98
Related Background:
... An important parameter of the [Chicxlulub] impact is the total area of the impact crater, since that area would be related to the amount of debris thrown into the atmosphere. Until now, the usual figure for the largest dimension of the impact crater has been approximately 300 kilometers. Morgan et al (20 authors at 8 installations, UK US MX CA) now report an analysis of seismic data of the Chicxulub impact, determining the diameter of the transient cavity at about 100 kilometers. The authors suggest this parameter is critical for constraining any proposed impact-related effects on the Cretaceous environment, and that the seismic data indicate the morphology of the crater is similar to large impact structures observed on other planets such as Venus.

QY: Mike Warner
(Nature 4 Dec 97) (Science-Week 26 Dec 97)

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