ScienceWeek

EARTH SCIENCE: A POSSIBLE END-PERMIAN IMPACT CRATER

The following points are made by L. Becker et al (Science 2004 304:1469):

1) The cause of the catastrophic mass extinction at the end of the Permian has been the focus of considerable debate. Becker et al. (1-3) and others (4,5) have presented evidence that a major impact was associated with the extinction of > 90% of marine taxa. The evidence includes fullerenes with extraterrestrial helium and argon (1), meteorite fragments, Fe-Ni-Si "metamorphosed grains" of probable meteoritic origin (5), Fe-Ni metals with impact spherules, and shocked quartz (4).

2) Acceptance of the idea that an impact accompanied the Cretaceous-Tertiary (K-T) extinction increased dramatically with the discovery of the Chicxulub crater. The authors searched for a Permian-Triassic (P-T) boundary impact crater in parts of the Southern Hemisphere that once comprised the supercontinent of Gondwana, because the impact evidence is most abundant in continents from this region (such as Australia and Antarctica). Gorter, based on the study of a single seismic line, suggested that the Bedout ("Bedoo") High offshore of northwestern Australia might be the central uplift of a large end-Permian impact crater.

3) The Bedout High is part of the Roebuck basin, which forms the northwestern continental margin of Australia. Existing studies of the structure include two regional seismic surveys conducted by the Australian Geological Survey (AGSO) and the Japan National Oil Company (JNOC), and two exploratory wells drilled 9 km apart on the top and flank of the Bedout High (Bedout-1 and Lagrange-1) that extend to depths of 3052 m (9986 feet) and 3273 m (10,738 feet), respectively. Both wells were drilled through 3 km of marine and fluvial sediments consisting of carbonates with occasional interbeded siltstones and mudstones (Tertiary to Cretaceous) and sandstones interbeded with claystones, siltstones, and coal (Cretaceous to Triassic) before reaching a breccia (Late Permian).

4) In summary: The Bedout High, located on the northwestern continental margin of Australia, has emerged as a prime candidate for an end-Permian impact structure. Seismic imaging, gravity data, and the identification of melt rocks and impact breccias from drill cores located on top of Bedout are consistent with the presence of a buried impact crater. The impact breccias contain nearly pure silica glass (SiO2), fractured and shock-melted plagioclases, and spherulitic glass. The distribution of glass and shocked minerals over hundreds of meters of core material implies that a melt sheet is present. Available gravity and seismic data suggest that the Bedout High represents the central uplift of a crater similar in size to Chicxulub. A plagioclase separate from the Lagrange-1 exploration well has an Ar/Ar age of 250.1 +- 4.5 million years. The location, size, and age of the Bedout crater can account for reported occurrences of impact debris in Permian-Triassic boundary sediments worldwide.

References (abridged):

1. L. Becker et al., Science 291, 1530 (2001)

2. L. Becker, R. J. Poreda, T. E. Bunch, Proc. Natl. Acad. Sci. U.S.A. 97, 2979 (2000)

3. L. Becker, C. Nicholson, R. J. Poreda, American Geophysical Union (AGU) Abstract, December 12 to 17 2002, OS22C-0291 (2002)

4. G. J. Retallack et al., Geology 26, 979 (1998)

5. K. Kaiho et al., Geology 29, 815 (2001)

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Related Material:

EVIDENCE FOR AN IMPACT EVENT RELATED TO THE PERMIAN EXTINCTION

Notes by ScienceWeek:

The available evolutionary record on Earth provides evidence of recurrent mass extinctions of biological species. Apparently, environmental catastrophes, occurring for various reasons, have suddenly removed many groups with a resultant collapse of ecosystems. Eventually new forms appear and evolution continues, but the periods of mass extinction apparently are a major factor in the various patterns of evolution.

The geological period ranging approximately from 146 million years ago to 65 million years ago is called the "Cretaceous period", and the geological period comprising the approximate time-frame 65 million to 3 million years ago is called the "Tertiary period". The largest mass extinction of the past 200 million years apparently occurred 65 million years ago at the end of the Cretaceous period, when approximately half of the genera of multicellular organisms on Earth, including all of the dinosaurs, suddenly died off. The geological record indicates that a layer of impact-produced minerals and the element iridium (an element rare in the crust of the Earth but more abundant in primitive meteorites) was deposited at the time the dinosaurs vanished -- the so-called Cretaceous/Tertiary or K/T boundary. In addition to this, the largest known crater on Earth (Chicxulub, Yucatan, MX) to be dated at less than 1 billion years old was apparently formed at this time. Taken together, these data imply that the K/T mass extinction was caused by the impact into the Yucatan peninsula of an asteroid or comet of approximately 10 kilometers in radius.

The "Permian period" comprises the approximate time-frame 286 to 248 million years ago, and the "Triassic period" comprises the approximate time-frame 248 to 213 million years ago. At the end of the Permian period, many groups of animals and plants apparently vanished in the greatest known crisis in the history of life on Earth. This Permian/Triassic mass extinction has traditionally been considered to have had a slow time course, but recent evidence has suggested this event was more abrupt than previously recognized.

"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. The "Murchison meteorite" is a carbonaceous chondrite that fell in 1969 near Murchison, Australia.

"Fullerenes" are large molecules composed entirely of carbon, with the chemical formula C(n), where n is any even number from 20 to over 100. They apparently have the structure of a hollow spheroidal cage with a surface network of carbon atoms connected in hexagonal and pentagonal rings, and the cage large enough to trap atoms and small molecules. Fullerenes have been identified in the Murchison meteorite.

In this context, the term "planetary" refers to the primordial aggregates (primitive planets) surrounding the Sun from which the current planets were formed.

The following points are made by L. Becker et al (Science 2001 291:1530):

1) The authors point out that the extinction event that marks the Permian/Triassic boundary (251.4 +- 0.3 million years ago) was the most severe in the past 540 million years, killing off over 90 percent of all marine species, approximately 70 percent of terrestrial vertebrate genera, and most land plants. Several new studies have demonstrated that these extinctions were much more abrupt than previously thought, with estimates of the extinction interval ranging from less than 500,000 to approximately 8000 years. Proposed catastrophic hypotheses for the Permian/Triassic boundary extinction include an exploding meteor (bolide) (asteroidal or cometary) and or massive volcanic lava flows (flood basalt volcanism). Other extinctions mechanisms involving ocean anoxia, as well as changes in sea level and climate, have also been proposed.

2) The authors report that fullerenes [C(sub60) to C(sub200)] from sediments of the Permian/Triassic boundary contain trapped helium and argon with isotope ratios similar to the apparent planetary component of carbonaceous chondrites. The authors suggest these data imply that an impact event (asteroidal or cometary) accompanied the extinction, as was the case for the Cretaceous/Tertiary extinction event approximately 65 million years ago.

3) The authors conclude: "Based on the measured helium-3 content for the Permian/Triassic boundary and Murchison fullerenes, the estimated size of the bolide is 9 +- 3 kilometers or comparable to the K/T Chicxulub impactor. Such an event could have caused the severe end-Permian mass extinction. Our results are consistent with recent paleontological studies that now point to a very rapid extinction event. The unique planetary signature measured in fullerenes isolated from the Murchison carbonaceous chondrite and from the Permian/Triassic boundary sediments demonstrates that this distinctive noble gas carrier can survive major impact events and contribute to the unique gas signature of the terrestrial planetary atmospheres."

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