Article: "Princeton geoscientist offers new evidence that meteorite did not wipe out dinosaurs"

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A Princeton University geoscientist who has stirred controversy with her studies challenging a popular theory that an asteroid wiped out the dinosaurs has compiled powerful new evidence asserting her position.

Gerta Keller, whose studies of rock formations at many sites in the United States, Mexico and India have led her to conclude that volcanoes, not a vast meteorite, were the more likely culprits in the demise of the Earth's giant reptiles, is producing new data supporting her claim.

Keller, a Princeton professor of geosciences, and several co-authors lay out the case in a paper published April 27 in the Journal of the Geological Society of London [KELLER, G., ADATTE, T., PARDO JUEZ, A. & LOPEZ-OLIVA, J.G. New evidence concerning the age and biotic effects of the Chicxulub impact in NE Mexico]. Examinations at several new sites have produced "biotic evidence" -- the fossilized traces of plants and animals tied to the period in question -- indicating that a massive die-off did not occur directly after the strike but much later.

Using these fossil remains to construct a timeline, she and her team were able to date the surrounding geologic features and begin to piece together proof that the impact occurred 300,000 years before the great extinction.

Over the years, Keller's group has amassed evidence for as many as four major events widely separated in time in that area of Mexico as well as in Texas. The oldest of the four events is the Chicxulub impact, seen by the fallout of glass beads. The second is about 150,000 years later and seen in a layer of sandstone with Chicxulub impact glass beads that were transported from shallow shore areas into deep waters during a sea level fall and was commonly interpreted as a tsunami generated by the Chicxulub impact. About 100,000 to 150,000 years later, the third event struck at the time of the K-T boundary with its iridium layer and mass extinction. This event may represent a second large impact or massive volcanism. The fourth event is possibly a smaller impact as evidenced by another iridium layer about 100,000 years after the mass extinction.

Advocates of the Chicxulub impact theory suggest that the impact crater and the mass extinction event only appear far apart in the sedimentary record because an earthquake or tsunami caused slumps and mixing of sediments surrounding the Gulf of Mexico. To date no evidence of major disturbance has been found in the sediments.

Keller says her team's newest research, however, confirms what she has found in earlier studies -- that the sandstone complex that overlays the impact layer was not deposited over hours or days by a tsunami but over a long time period. From El Peñon in Mexico and other sites listed in the new study, the scientists were able to calculate that between 13 and 30 feet of sediments were deposited at a rate of about an inch per thousand years after the impact. These sediments separating the impact layer from the sandstone complex and the overlying mass extinction were formed by normal processes. There is evidence of erosion and transportation of sediments in the sandstone layers, but no evidence of structural disturbance, Keller said.

Keller suggests that the massive volcanic eruptions at the Deccan Traps in India may be responsible for the extinction, releasing massive amounts of dust and gases that could have blocked sunlight, altered climate and caused acid rain. The fact that the Chicxulub impact seems to have had no effect on biota, she said, despite its 6-mile-in-diameter size, indicates that even large asteroid impacts may not be as deadly as imagined.

"Princeton geoscientist offers new evidence that meteorite did not wipe out dinosaurs"
Kitta MacPherson
Princeton University Press Release
04 May 2009 10:00 ET

Link: Article

Link: Journal of the Geological Society (Volume 166, Part 3, May 2009) - KELLER, G., ADATTE, T., PARDO JUEZ, A. & LOPEZ-OLIVA, J.G. New evidence concerning the age and biotic effects of the Chicxulub impact in NE Mexico, 393.
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