one attempt to replicate Becker’s results failed to show any evidence of impact. By 2005 the public and science journalists were the major supporters of the Becker hypothesis, while the working stiff scientists objected—some harshly, which was a response to the tenor of the Becker group and their dogmatic belief in their findings.

  1. Carbon dioxide catastrophe. This idea, put forth in the late 1990s by Harvard University’s Andy Knoll and his colleagues, advocated a rapid and massive release of carbon dioxide that was previously locked up in deep-water Permian sediments. Carbon dioxide poisoning, accompanied by high heat brought about by the greenhouse heating effect of the newly released carbon dioxide, was the proposed kill mechanism. This hypothesis, while attractive, was soon shot down by oceanographers who pointed out that the release of carbon dioxide necessary for this scenario was physically impossible.

  2. Methane catastrophe. This idea was largely the brainchild of the same Greg Retallack who had mistakenly reported the presence of shocked quartz grains from latest Permian rocks. Retallack noted that carbon isotope values found from most Permian-Triassic boundaries were isotopically so “light” that they could not have been caused by the extinction alone (if all plant life is killed off on a planet, the carbon isotope values go light). But the gas methane is “light,” and a massive release of the stuff would produce the observed isotopic signature. Methane is an even better greenhouse gas than carbon dioxide, so Retallack invoked a sudden heating. At the same time, Retallack suggested that the drop of oxygen already thought to have occurred over the time interval of the Permian extinction happened so fast that land animals died of asphyxia. But Bob Berner dismissed this idea of sudden oxygen drop on scientific grounds. And finally, isotope geochemists even began to question the methane idea, recovering many measures that did not yield the methane signature. They ascribed the very light findings to rocks whose carbon isotope values had been perturbed by later heating and pressure.

  3. Heat spike and low oxygen caused by Siberian traps. With the falsification of the methane idea, attention shifted to the simultaneity of the largest flood basalt of the past 600 million years, the Siberian traps, with the Permian extinction. Because the flow from Earth of



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