Tuesday, October 5, 2010

The Extinction happened on Tuesday at 9:15am

Sam Bowring is a professor at MIT who specializes in Uranium-lead zircon geochronology and has gotten really good at it. The precision that he is getting on his dates are amazing. Basically zircon is a mineral (ZrSiO4) that accepts element such as U which has a large ionic radius into its structure, but rejects Pb (lead). Zircon has an almost ubiquitous presence in the crust of Earth. It occurs in igneous rocks (as primary crystallization products), in metamorphic rocks and in sedimentary rocks (as detrital grains). U-Pb dating provides an age range of about 1 million years to over 4.5 billion years, and with routine precisions in the 0.1-1 percent range. The method relies on two separate decay chains, the uranium series from 238U to 206Pb, with a half-life of 4.47 billion years and the actinium series from 235U to 207Pb, with a half-life of 704 million years. Sam will be talking about two major extinctions that occurred on Earth.

Permo-Triassic Extinctions
A major extinction event occurred approximately 250 million years ago and marks the boundary between the Permian Period and the Triassic Period. At the end of the Permian more than 85% of all species in the oceans, approximately 70% of land vertebrates, and significant numbers of plants and insects vanished. The Permian extinction caused the most fundamental reorganization of ecosystems and animal diversity in the past 500 million years. The marine communities of today are largely a result of the recovery following this extinction. In addition, dinosaurs and mammals arose in the aftermath of the extinction.

A better understanding of how long the end-Permian extinction and its recovery took will allow for new insights and better understanding into the role of mass extinctions in evolution. Mass extinctions are marked in the fossil record by the abrupt disappearance of taxa, sometimes associated with a discrete "boundary bed"; in the case of the end Cretaceous extinction this bed is a layer rich in impact ejecta with distinctive chemical signatures. A critical question is how abrupt such extinctions really were. A satisfactory answer must involve statistical analysis of the stratigraphic and fossil record. Differences in sediment accumulation rate and the preservation potential of organisms can lead to an artificially abrupt and/or drawn out extinction signal, especially if the extinction is of short duration, say, less than 1 million years. Because sedimentation rates vary, stratigraphic thickness does not convert to time directly. Therefore, understanding an extinction requires constraining its tempo by combining high-precision geochronology with paleontological studies. Sam has found that the carbon isotope event associated with the major shift in carbon reservoirs as organisms disappeared from the planet was 165 thousand years long which indicates that there was a catastrophic addition of 12-carbon into the earths system. Furthermore he has found that most of the extinctions occurred in less than 1 milion years.

P-T Video featuring Sam Bowring


  1. Sounds fun. This is a topic I feel fairly confident I should be able to keep up with the lecture on. Carbon dating is one of the GeoSci principles I think is easy to understand and comes up in a number of other subjects (who wants to practice some logarithms?) Being able to determine exactly when a mass extinction event occurred would give a good idea of the cause of the event by comparing the time frame of the extinction to other "known" events: meteor impact, global weather change, volcanic eruption, etc. This gives us a better idea about the survivability of our own species and helps us determine what threats we should invest money in efforts to prevent.

  2. Quite an interesting paper. I still would like to know what the author thinks is the smoking gun. Precisely dating the extinction and determining its duration is handy, but he didn't state what he thought was the most likely cause. Additionally, how do you resolve disputes between biostratigraphy and radiometric dating, since most of the geological periods were originally determined using biostratigraphy?

  3. The topic is pretty mainstream enough for me, a person with no prior experience in geosciences, to understand.

    One thing that interested me was the fact that there were survivors of the Permian extinction. The video posted displayed fossils--could those fossils shed any light on methods we could use to become less susceptible to the toxic emissions?

  4. The paper was very technical for me and it was pretty hard to understand. Hopefully the lecture will be user friendly and comprehensive. Other than that, this subject is very interesting and I hoping he will go over why certain species survived and other didn't although this is not the point of the paper. Mass extinction's are important to study because Earth could potentially experience an event that causes similar effects i.e. overturn of a stratified ocean among others mentioned.

  5. Very interesting topic and paper - while I couldn't follow the detailed methodology of the dating presented in the paper, the discussion on the implications of the results was straight forward. The mechanisms in the scenarios presented can have real value in the world we live in now. I remember watching a documentary about the threat methane hydrates could possibly pose to humans in the not-so-distant future if our oceans warm up. I also hope that during the presentation, we learn a bit more about the amazing 10% of species that miraculously survived.

  6. In the lecture I got lost just a little bit after Denver, but I think he did a wonderful job making his topic clear and relevant to a non Geosci major. The graph he started with (time plot of total number of species on earth to show mass extinction timeframes). His photo of the dig outside of Denver which made the sediment layers very clear was also very easy to understand, and very interesting when he overlaid it with a list of the times when the rock layers we formed. After that his lecture got considerably more technical, and while I could follow the concept he was talking about, most of the details were well above my head.

  7. I agree with Nick. I got lost throughout the talk, but was able to follow the Denver photo fairly well. I feel like I missed the point about why all of this is important. It seemed like his dating leads to more precise dating but is unable to really provide and concrete evidence to support one theory over another. (It is very possible that I missed something after I got lost). I was also unsure about how reliable the lead dating really is. While it did have a smaller margin of error than other methods, it seems like there are still a lot of factors to consider.

    Overall, I enjoyed the small portion of the talk that I did comprehend. :)