No. 73: Jan-Feb 1991
The debate over the real cause of the terrestrial catastrophism that occurred at the Cretaceous-Tertiary boundary, some 65 million years ago, grinds on. Some physical scientists claim rather imperiously that the dinosaurs and many other species were done in by the impact of a huge asteroid/meteorite. The worldwide iridium spike is conclusive, they say. Many paleontologists and geologists, however, remain unconvinced and prefer widespread volcanism. We have already covered the various arguments in past issues of Science Frontiers; here, we want to advise our readers that a pair of excellent articles by principals in this debate have appeared in Scientific American. Generally speaking, it seems that the proponents of the impact theory are now listening to the other side. For example, multiple impacts are now proposed to account for evidence of the type introduced below. (Alvarez, Walter, and Asaro, Frank; "An Extraterrestrial Impact," Scientific American, 263:78, October 1990. Also: Courtillot, Vincent E.; "A Volcanic Eruption," Scientific American, 263:85, October 1990.)
A spike dulled. The case for a single asteroid/meteorite impact has been weakened by a recent reexamination of the classic exposure of the CretaceousTertiary boundary at Gubbio, Italy. Here, the discovery of an iridium "spike" at the boundary was thought to betoken a sudden, catastrophic, extraterrestrial event. On further study, though, the spike no longer seems so sharp. In fact, it is spread through about 3 meters of sediments at the boundary. Said sediments apparently took 500,000 years to accumulate! This broad smear of iridium no longer seems indicative of a single extraterrestrial impact. (Rocchia, R., et al; "The CretaceousTertiary Boundary at Gubbio Revisited: Vertical Extent of the Ir Anomaly," Earth and Planetary Science Letters, 99:206, 1990.)
Comment. Is it even indicative of multiple impacts? Recall that no one has yet (1990) found a single crater that everyone will accept as evidence.
Reference. Iridium spikes and the supposed Cretaceous-Tertiary impact are to be found in ESB1 in our catalog: Anomalies in Geology. For more on this volume, see: here.