Chicxulub Drilling debates, Is the Chicxulub crater the KT boundary killer or Not?
IN SCIENCE, as in most fields of human endeavour, fashion plays a role. Two decades ago, evidence was discovered that the dinosaurs (and a great many other, less well-known, creatures) were exterminated by a collision between the Earth and an extra-terrestrial rock. The evidence came in the form of a layer of clay rich in iridium that appeared to be the result of such a collision. One decade ago, a crater that seemed to be the same age as this layer was identified in southern Mexico. Since then, it has become fashionable to look for evidence of impacts at the time of the other four so-called mass extinctions that the record suggests have happened since fossils became abundant 545m years ago. Conversely, alternative explanations for mass extinctions, such as the massive volcanic eruptions that often seem to coincide with them, have fallen out of fashion.
Fashion, however, is fickle, and those other explanations are once again jostling on the catwalk with the impact theory. Some of them were aired at the annual meeting of the Geological Society of America (GSA) held in Seattle during the first few days of November. Meanwhile, the evidence for a single huge impact which wiped out the dinosaurs is itself under attack. Those sniping at it are notóat least not yetóarguing that the impact theory is completely wrong. But they are arguing that the Mexican crater is not part of the story because, they say, it was made some 300,000 years before the dinosaurs disappeared.
The chief heretics are Gerta Keller of Princeton University in America, Thierry Adatte of the University of Neuch*tel in Switzerland, and Wolfgang Stinnesbeck of the University of Karlsruhe in Germany. In April, they announced preliminary data to support their dissent at a conference in Nice. They have now published it in the Journal of Geological Society (the society in question being the British, rather than the American one).
The moment most people were persuaded that the dinosaurs were killed by an impact was when the crater in Mexico was shown to have been created 65m years ago, at the end of the Cretaceous period. This was when the iridium layer was formed (many extra-terrestrial rocks are far richer in iridium than those found on Earth, so a large impact that scattered the iridium seemed a reasonable conclusion to draw), and when the dinosaurs disappeared. But dating things as old as this, which is done by studying the products of radioactive decay, is not a precise science. An error of 300,000 years is not out of the question. This is where Dr Keller and her collaborators come in. They have convinced themselves that, wherever the iridium came from, it was not ejected by the Mexican impact.
Their evidence comes from the rocks of the Gulf of Mexico and the Caribbean that surround the crater. These contain small glass globules. No one disputes that these globules were formed from stuff melted and thrown into the air by the impact, because their chemical composition matches rock from the crater itself. Above the globules are several metres of sandstone, shale and limestone. Then comes the iridium.
The conventional explanation for this arrangement is that the glass fell to Earth first, then giant waves caused by the impact covered them with sediment, then iridium-containing dust settled out of the atmosphere over the course of a few weeks and formed the clay.
Dr Keller, however, contends that the sandstone, shale and limestone layers were deposited over a long period of time. Her evidence is that many of these layers contain animal burrows that seem to start at the surface of the layer, suggesting that the layer in question had been buried subsequently. She has also found several layers of globules. She is not suggesting that these came from different impacts (they are all chemically similar to one another), but rather that the sediments have been "reworked", perhaps by subsequent mudslides. That, again, would have taken time.
Most tellingly, she says that rock cores taken recently from the crater itself show a band of sediment above the impact that contains fossils of tiny creatures that became extinct only at the end of the Cretaceous. This band also contain several layers of a mineral called glauconite, each of which would have taken tens of thousands of years to form.
Putting all this together, she suggests the Mexican impact happened 300,000 years before the end of the Cretaceous. The iridium, and the end of the dinosaurs, she believes, were caused by another impact whose crater has yet to be located.
Naturally, not everyone agrees with this interpretation of the data. Jan Smit, of the Free University in Amsterdam, is particularly critical. It was he who first came up with the giant-wave explanation for the layers of sediment between the glass globules and the iridium.
According to Dr Smit, the multiple layers were the result of waves from the impact sloshing around in the primitive Caribbean and passing over individual sites several times. And the microfossils in the sediment over the crater are either misinterpretations of material that has recrystallised over time, or were washed in from nearby rocks just after the crater was formed. He points out that rocks from contemporary swamps in North America show little separation between the glass globules and the iridium. It is also unlikely that two impacts as big as the one that caused the Mexican crater and the one that spread iridium around the world would occur within 300,000 years of each other. But, of course, it is not impossible.
Look here for the Yaxcopoil-1 core segment 793.85 to 794.60 m: the transition of the impact to post-impact crater infill,