Thursday, February 20, 2014

Nailing down the Permian Extinction

A pair of ammonites in a limestone bed near the Permian-Triassic boundary.
Credit: Seth Burgess.  Image source.
Though the extinction of the dinosaurs at the end of the Cretaceous period is the most well-known major extinction event on the old-earth timeline, a much larger one is thought to have occurred 252 million years ago at the end of the Permian, paving the way for the first dinosaurs to show up in the Triassic.  Based on patterns in the fossil record, paleontologists estimate that over 90% of marine life and a majority of land creatures died out at that time.  Numerous explanations for this event have been proposed, frequently attributing it to different forms of volcanism, but no consensus has been reached.

In order to better understand the nature of the Permian extinction, a team of geologists from MIT and the Chinese Academy of Sciences investigated how long the extinctions took.  Their results were published online ahead of print by the Proceedings of the National Academy of Sciences last week.  The layers containing fossils from the mass extinction are almost entirely composed of limestone, which cannot be dated using radiometric techniques.  Instead, zircon crystals in associated ash layers in China were dated using uranium and lead (U-Pb), which are frequently used to date specimens thought to be several million to billions of years old.  The ash layers yielded dates from 251.941 ± 0.037 to 251.880 ± 0.031 million years ago, meaning that the extinction took 60 ± 48 thousand years (note that directly subtracting the dates actually gives 61 ± 68 thousand years, but the nature of statistical confidence intervals permits this number to be narrowed down).  Though an error of 48,000 years may seem like a lot, it is only 0.02% of the claimed age of the rock.  Such precision is equivalent to measuring a mile of road to within a foot.

Does this prove, then, that radiometric dating is an accurate technique for determining age?  Not exactly.  This study demonstrated the precision of U-Pb dating, not its accuracy.  An extremely precise rifle may land twenty bullets in the exact same spot, but if that spot is on the side of the barn fifty feet away from the target, then the rifle is useless until it is recalibrated.  Likewise, U-Pb dating may be able to determine the age of a specimen to within .02%, but the results will be useless if the method is not properly calibrated.  As mentioned in a previous article, the RATE project by the Institute of Creation Research has collected data suggesting that the decay of uranium was accelerated in the past, which would certainly throw off the accuracy of U-Pb dating.  The precision, meanwhile, would not necessarily be affected if the acceleration was universal.

From a creationist standpoint, the Permian extinction may initially resemble the great die-off that occurred during Noah's Flood.  However, the creationist model proposes that the layers under and above the Permian-Triassic boundary were also deposited during the Flood.  So, while the creatures associated with the Permian extinction did die during the Flood, their absence from higher layers may be better attributed to geographic distribution.  It may be that Permian rocks were deposited by waters from a particular sea, while Trassic rocks were deposited by water from another sea that traveled over land.  Alternatively, the pre-Flood oceans may have had a distinct horizontal marine boundary, similar to the modern thermocline, that separated two marine ecosystems and was preserved as the Permian-Triassic boundary.  I again emphasize that such speculations are unproven and may not even be provable, but the fact remains that the creation model can easily account for the latest findings in the fossil record.

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