Scientists find 'missing' 10 million years
7 May 2009
The disappearance of species 251 million years ago was a result of a major extinction event, not just a gap in the geological record, according to new research.
Graeme Taylor measuring a sample's orientation in Russia.
The findings, published April in Earth and Planetary Science Letters, show that rocks from Russia's Orenburg region are 10 million years younger than previously thought and put an end to an old controversy.
251 million years ago, nine in every ten species of marine animals died out during the Permian-Triassic event, the largest mass extinction in the Earth's history. The catastrophe was possibly caused by global warming, a meteorite impact, massive volcanic eruptions in the area known today as Siberian traps, or a combination of the above.
'The Permian-Triassic mass extinction is well documented in marine rocks,' says lead author Dr Graeme Taylor from the University of Plymouth. 'But if it was a truly global event, the extinctions must be proved on land too.'
The continental rocks of Orenburg, west of the Ural Mountains, are key to this problem due their rich fauna of early land vertebrate fossils, the tetrapods. However, there are not many of them and some sections in the area have no fossils at all. This causes a problem because without fossils, it's difficult to put an accurate date to the rocks.
Thanks to this uncertainty, it was 'suggested that ten million years worth of rock were missing in Russia and that the rocks present there were ten million years older than they actually are,' says Taylor. 'This would mean that the fossil disappearance in Russia would then pre-date that of everywhere else, seriously undermining the idea of a single mass extinction event.'
Fortunately there are other methods to determine the age of rocks. One of them uses the Earth's magnetic polarity record. Strange as it may seem, the magnetic north that compasses point to is not always at the North Pole. Sometimes in the past it was actually at the South Pole. The reasons behind magnetic pole reversal are not exactly clear, but they are a common event in the Earth's history.
Every rock keeps a snapshot of the Earth's magnetic field at the time of its formation, stored in its iron-rich minerals. This is the same for all rocks throughout the world at any given time, because the magnetic field is constant. When the poles switch over from normal to reverse, all newly-formed rocks record this event.
After a few million years the result is a sequence of normal-reverse polarities recorded in a pile of rocks. Scientists keep this information in so-called magnetic columns that resemble very long barcodes with normal polarity painted black and reverse polarity coloured white. Because the length of normal and reverse periods is highly variable, every period in the history of the Earth can be identified by its unique sequence of black/white bars in the magnetic column.
The team collected samples from three main geological sections in the area and measured the magnetic field recorded by the rocks. The data was used to compile the polarity sequence for the area, which was then compared to the global magnetic column. Since the sequence and length of normal and reverse poles is unique, it is possible to assign a precise date to the rocks.
The data shows that 'there is in fact no Permian-Triassic gap. The record is complete and the mass extinction event is further strengthened as being a major turning point in the history of life on Earth and as the most catastrophic event to have, so far, affected our planet,' says Taylor.
Graeme K. Taylor, Christopher Tucker, Richard J. Twitchett, Timothy Kearsey, Michael J. Benton, Andrew J. Newell, Mikhail V. Surkov, Valentin P. Tverdokhlebov. 2009. Magnetostratigraphy of Permian/Triassic boundary sequences in the Cis-Urals, Russia: No evidence for a major temporal hiatus. Earth and Planetary Science Letters 281, 36-47. doi:10.1016/j.epsl.2009.02.002
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