Dating the Permian-Triassic Extinction

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Excerpt One of the most controversial topics in the debate over origins is the age of the Earth. Geochronology, the science of dating the age of our planet and its major events, is less precise than most people might believe... Continue reading

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One of the most controversial topics in the debate over origins is the age of the Earth. Geochronology, the science of dating the age of our planet and its major events, is less precise than most people might believe. One of the most important geological and biological events in Earth’s history was the massive extinction that ended nearly all life at the end of the Permian Period and paved the way for the dinosaurs at the dawn of the Triassic Period. The timing of this event, known as P-T, has been a recent source of heated controversy among geochronologists.

According to the September 17 2004 issue of the journal Science, “A new, apparently improved, way to date the greatest mass extinction…fails to resolve geochronologists’ long-running differences” (Kerr 2004: 1705). The journal reported that

nailing down the time of the Permian-Triassic (P-T) extinction has revealed problems in the often competitive business of geochronology. P-T daters must draw their conclusions from vanishingly small isotopic remains of radioactive decay. For years, different laboratories using uranium-lead radiometric dating—the gold standard of geochronology—have been getting entirely different ages for the P-T extinction (Ibid).

Roland Mundil of the Berkeley Geochronology Center in California and his colleagues have used a new method of preparing samples for uranium-lead dating, and have arrived at a date older than previously thought, but Michael Villeneuve of the Geological Survey of Canada in Ottawa cautions that “all dates are interpretations….It [i.e., Mundil’s dating method] needs a bit more proving out” (Ibid).

Indeed, geochronologist Samuel Bowring of the Massachusetts Institute of Technology analyzed zircons from layers of volcanic ash that were laid down during the P-T event and got a more recent date. Mundil, however, rejects Bowring’s conclusion. According to the article in Science,

He thinks Bowring engaged in ‘arbitrary date culling’ by throwing out more than half his zircon ages before averaging the rest of them together. But Bowring says his choices were judicious, although ‘necessarily somewhat subjective.’ In some of his zircons, the two different uranium-lead ratios gave different ages, suggesting that lead had leaked out of those zircons during the past quarter-billion years. And other zircon ages looked distinctly old, as if those zircons had crystallized earlier than the rest and had later gotten mixed in with them…. Bowring believes he can confidently select the reliable zircon ages and discard the rest (Ibid).

Mundil dismissed this method as “picking and choosing” (Ibid). In response, Bowring pointed out that two independent studies of the P-T event, one in China and the other in Siberia, both came up with dates that matched not only each other, but Bowring’s date as well. According to Science, “Mundil hasn’t explained how subjective interpretation could have produced such a coincidence…” (Ibid).

These controversies demonstrate the inexact nature of the science of geochronology. Ages that are determined by a certain dating method cannot be held up as incontrovertibly, irrefutably reliable, at least not until a geochronological method is devised that can be proven beyond all reasonable doubt to provide scientists with accurate ages of the Earth and its most significant geological events.

Reference:
Kerr, R.A. 2004. “In Mass Extinction, Timing Is All.” Science 305, no. 5691.

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