Mass extinction is an extremely difficult subject to study. It is one thing to identify a mass extinction in the fossil record, but it is quite another to be able to fully explain its cause. It is not surprising, then, that the triggers for the great mass extinctions in earth's history are hotly debated. The end-Cretaceous extinction that wiped out the non-avian dinosaurs (among other creatures) is no exception.
A new paper published in the Journal of the Geological Society has once again stirred debate over whether the impact of an asteroid about 65 million years ago caused the end-Cretaceous mass extinction. Written by Gerta Keller and Thierry Adatte, the paper suggests that the asteroid that impacted at the site of Chicxulub came 300,000 years before the mass extinction, thus making the asteroid a poor candidate for the extinction's trigger. The key to the hypothesis presented in the paper is a 30-foot layer of rock near the impact site that sits right above the impact layer. Keller and Adatte argue that this layer accumulated relatively slowly, over 300,000 years, and no species go extinct within it. It is not until the upper limit of the layer that species go extinct.
Keller has long been a critic of the hypothesis that the end-Cretaceous extinction was sparked by the asteroid strike at Chicxulub. In the past she has favored multiple asteroid impacts as an explanation, although more recently she has preferred the activity of volcanoes that formed the Deccan Traps rock formation in India. The volcanoes erupted at the end of the Cretaceous, between about 68 and 60 million years ago, and they were so violent that some scientists think that they were the primary agents of mass extinction. Either way, though, over the past several years Keller has sampled rock in regions close to the Chicxulub impact crater and since at least 2003 has been saying that the asteroid struck 300,000 years prior to the end-Cretaceous mass extinction.
The problem with many of Keller's papers, however, is that she has often sampled the area closest to the impact crater. This is the area that was the most affected by the immediate after-effects of the strike. Huge waves swept towards the coast, shock waves ran through the rock, and earthquakes were triggered by the impact. All of this makes the area in and around the crater very geologically complex. As paleontologist J. Smit has pointed out, for instance, fossils that Keller had previously identified as being Cretaceous in age really came from the Paleocene, the epoch right after the Cretaceous. Smit's observations are more consistent with what is seen at end-Cretaceous boundary sites elsewhere.
While it is important to study the Chicxulub impact crater and the surrounding area, the best evidence for the timing of the impact and the end-Cretaceous mass extinction is found farther afield. The correlation of sites around the world shows that many of the groups that went extinct at the end of the Cretaceous went extinct at or shortly before the impact layer. There still many places in the world, primarily in the southern hemisphere, where the end-Cretaceous mass extinction has yet to be studied in detail, but the asteroid remains a major contender for the cause of extinction. But debate will continue and Keller's hypothesis will stand or fall according to the evidence.