Chapter 6 Changing Life in a Changing World

UPDATES FOR CHAPTER 6.

1. Major update, August 2013.
Finally! A convincing crater from the Permian-Triassic boundary. The Araguainha crater in Brazil dates to the PT boundary within measuring error. But it is only 40 km across, too small for the impact to have generated a global crisis -- except for some very special circumstances. The crater was blasted into formations that contained very large oil and gas concentrations, so the impact ejecta would have included huge amounts of hydrocarbons. In addition, there are sedimentary features in Permian rocks for hundreds of miles around the crater, showing that the hydrocarbon-bearing sediments slumped, slid, churned, and were caught up in tsunami, all from the enormous seismic shocks generated by the impact AND by the collapse of the surrounding sediments into the crater. These effects also occurred in the Caribbean after the KT impact at Chicxulub, and here is a previous episode in Permian sediments at the PT boundary. Estimates of the yield of hydrocarbons around the Araguainha crater suggest that this relatively small impact had effects that were greatly multiplied by the particulars of the impact site. If the hypothesis holds up under the scrutiny that will now be focussed on the Araguainha crater, then we have a powerful (and convincing) trigger for the PT crisis that was already primed to occur because of the eruptions of the Siberian Traps. As at the KT boundary, it may have taken a random combination of a huge eruption and an asteroid strike to generate a global extinction. This is a careful study and it is likely to be a landmark paper.

And, of course, there's a teaching moment here. The similarity of the PT and the KT mass extinctions means a lot in terms of a general theory. There will be more encouragement for looking for a convincing end-Triassic impact (there are already papers about candidates). The hydrocarbon factor becomes important too. Is Earth more likely to suffer mass extinctions through time as organic productivity has increased? Probably yes, but carbon has to be buried where it is geologically vulnerable to disturbances such as impacts. It's a bit scary that a fairly small impact like Araguainha could be part of the largest ever mass extinction, but it happened at the best (worst) time and place.

If you write a textbook, something will eventually happen which makes you wish you could rewrite a chapter. This is one of those times, and it's only months since it became too late to change anything!

Tohver, E. et al. 2013. Shaking a methane fizz: Seismicity from the Araguainha impact event and the Permian­Triassic global carbon isotope record. Palaeogeography, Palaeoclimatology, Palaeoecology 387, 66­75. Here is a news story from The Economist by Matt Kaplan: The Economist

Chapter 6: images for lectures

NOTES AND LINKS FOR CHAPTER 6.

The Global Diversity Gradient

Global Tectonics and Global Diversity

Plate tectonics and past geography

Footnotes

Mass Extinctions

  • Bad hypothesis for the Ordovician extinction. ENN site, April 2005. This is a bunch of astronomers with an idea but no data, looking for something to explain. Why use some untestable idea from outer space when there are perfectly reasonable mechanisms here on Earth?

    The Permo-Triassic (P-T) Extinction

    Finally! A convincing crater from the Permian-Triassic boundary. The Araguainha crater in Brazil dates to the PT boundary within measuring error. But it is only 40 km across, too small for the impact to have generated a global crisis -- except for some very special circumstances. The crater was blasted into formations that contained very large oil and gas concentrations, so the impact ejecta would have included huge amounts of hydrocarbons. In addition, there are sedimentary features in Permian rocks for hundreds of miles around the crater, showing that the hydrocarbon-bearing sediments slumped, slid, churned, and were caught up in tsunami, all from the enormous seismic shocks generated by the impact AND by the collapse of the surrounding sediments into the crater. These effects also occurred in the Caribbean after the KT impact at Chicxulub, and here is a previous episode in Permian sediments at the PT boundary. Estimates of the yield of hydrocarbons around the Araguainha crater suggest that this relatively small impact had effects that were greatly multiplied by the particulars of the impact site. If the hypothesis holds up under the scrutiny that will now be focussed on the Araguainha crater, then we have a powerful (and convincing) trigger for the PT crisis that was already primed to occur because of the eruptions of the Siberian Traps. As at the KT boundary, it may have taken a random combination of a huge eruption and an asteroid strike to generate a global extinction. This is a careful study and it is likely to be a landmark paper. Tohver, E. et al. 2013. Shaking a methane fizz: Seismicity from the Araguainha impact event and the Permian­Triassic global carbon isotope record. Palaeogeography, Palaeoclimatology, Palaeoecology 387, 66­75. Here is a news story from The Economist by Matt Kaplan: The Economist

    A methane belch at the Permo-Triassic boundary. I've read this paper, (it was in Geology) and IMHO it was the bad paper of that decade (there are many other candidates). First, this is not a new idea: for example, my Davis colleagues Dan Dorritie and Gary Vermeij published a much cleaner methane hypothesis in Science several years ago. So, to start with, this is SHODDY scholarship, to put it mildly. Second, there is no justification for the claim that methane would be released practically instantaneously and would then explode. Third, if methane was involved in Noah's Flood, there should be a carbon isotope spike ‹ has the author looked for it (NO!). And fourth, whoever said (for the press release) that a single mammal swimming in the ocean could set off a methane disaster may well be unaware that there were NO mammals (count them) in the Permian, let alone swimming ones, and is likely influenced by the science FICTION idea that a butterfly flapping its wings can set off a hurricane. Who reviewed this??? Press release from Northwestern University, August 28, 2003.

    Recovery After Extinctions

    Extraterrestrial Impacts and Meteorite Craters

    References for Chapter 6

    Page last updated September 13, 2013

    Links last checked September 12, 2012

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