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No. 37: Jan-Feb 1985

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More Doubts About Asteroids

In an apparent reaction to the stampede to climb aboard the extinction-by-asteroid bandwagon, dissenting papers have begun to appear in the scientific literature. For example, Van Valen's list of objections to the hypothesis of asteroid impact at the Cretaceous-Tertiary boundary was reproduced in the last issue of Science Frontiers. Now, in a recent issue of New Scientist, T. Hallam raises still more objections:

  1. Tropical plants, mammals, crocodiles, birds, and benthic invertebrates were little affected by whatever happened at the Cretaceous-Tertiary interface. Furthermore, many groups that were extinguished were already well into a decline.
  2. Some geologists insist that some of the supposedly synchronous extinctions were probably separated by several hundred thousand years; viz., plankton and dinosaurs.
  3. The vaunted iridium anomaly in deep-sea cores is spread through a considerable thickness of sediment. Even after allowing for the mixing of sediments, the iridium-rich layer is thousands of years thick.
  4. According to the asteroid scenario, the clay layer separating the Cretaceous from the Tertiary should represent the fallout from impact-raised dust, which would include asteroidal material and a mixed sample of earth rocks. However, in Denmark, the boundary is marked by the so-called Fish Clay, which is almost pure smectite -- a single mineral and not a mixture of terrestrial rock flour.

If it wasn't an asteroid impact, why the iridium concentration? At least three hypotheses have been proposed to circumvent the asteroid debacle: (1) volcanic activity; (2) a concentration of micrometeorites, thousands of tons of which fall each day, through extreme reduction of sedimentation; and (3) selective enrichment of iridium by an anoxic environment acting upon kerogenand pyrite-rich clay.

In short, some geologists at least do not find the asteroid hypothesis compelling at the moment.

(Hallam, Tony; "Asteroids and Extinction -- No Cause for Concern," New Scientist, p. 30, November 8, 1984.)

From Science Frontiers #37, JAN-FEB 1985. 1985-2000 William R. Corliss