NYTimes article on evidence (or alternatives) to the K-T impact. Selections from the article...
Scientists associate the iridium anomaly with the asteroid impact or impacts thought to have set off the extinctions. The thin layer, which has been detected worldwide, is also considered the marker for the end of the Cretaceous and beginning of the Tertiary period, known as the K-T boundary.
The discovery of thriving communities of survivors at the end of the Cretaceous is giving some scientists second thoughts about the extinction’s causes and effects. Some question the conventional explanation of a single large impact that enveloped Earth in a cloud of dust and almost instantaneously brought on a deadly global winter. They contend that this may be an oversimplification, and that the real story behind the dinosaur-ending disaster is more complicated and as yet unclear.
In other words, the world’s ecosystem was under widespread stress for an extended time. The extinctions might have had multiple causes, not the single asteroid impact and almost instant death as hypothesized in 1980 after the detection of the global iridium layer.
A few other paleontologists have also cast doubt on the timing and single-impact suddenness of the mass extinction. The idea of a killer impact that became the standard theory was proposed in 1980 on the basis of iridium traces; it gained wide acceptance after the discovery in 1991 of the impact crater in Mexico. But in some places, the fossil record for dinosaurs seems to disappear a little before the iridium is deposited. Geologists have found several other crater remnants that could have been gouged out by asteroids and also the suspect volcanoes of India.
"Rethinking What Caused the Last Mass Extinction"
John Noble Wilford
November 6, 2007
New York Times
Link: NYTimes Article
No comments:
Post a Comment