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12280"The Great Dying" --Was Earth's Colossal Extinction Event Caused by Microbes?

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  • derhexerus
    Apr 30, 2014
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      URL to an interesting post in The Daily Galaxy blog

      Did you ever stop to think, and forget to start again?

      The physical environment can produce sudden shocks to the life of our planet through impacting space rocks, erupting volcanoes and other events. But sometimes life itself turns the tables and strikes a swift blow back to the environment. MIT researchers have identified a different culprit — one coming from biology rather than geology. They argue that the carbon disruption and, consequently, the end-Permian extinction were set off by a particular microorganism that evolved a new way to digest organic material into methane.

      The end-Permian (or PT) extinction event occurred 252 million years ago. It is often called the Great Dying because around 90 percent of marine species disappeared in one fell swoop. Similar numbers died on land as well, producing a stark contrast between Permian rock layers beneath (or before) the extinction and the Triassic layers above. Extinctions are common throughout time, but for this one, the fossil record truly skipped a beat.

      "The end-Permian is the greatest extinction event that we know of," said Daniel Rothman, a geophysicist at the Massachusetts Institute of Technology. "The changes in the fossil record were obvious even to 19th Century geologists."

      Understanding the cause of this biological devastation requires understanding the geochemical clues that go along with it. Chief among these clues is a sudden swing in the balance of carbon isotopes stored in rocks from that same time period. If geologists can find what disrupted the carbon, they'll likely know what killed off so much of the Earth's life forms. Several theories have tried to explain the carbon perturbation as, for example, massive volcanism, or a drop in sea level, but none of these environmental causes have fully matched the data.

      With this genetic innovation, these methane-producers, or methanogens, ran rampant across the ocean, overturning the carbon cycle. The resulting changes in ocean chemistry would have driven many species to extinction.

      "This shows how unstable Earth's systems are," Rothman said. "A very small event in the microbial community can have an enormous impact on the environment." The basis of this new theory comes from a reassessment of the carbon data.

      A plot of data on mass extinctions in Earth's history. The end-Permian extinction event is the large peak on the left at 250 million years ago. Credit: University of Chicago For decades, geologists have been aware that the ratio of carbon isotopes (the light verses heavy forms of the element) changed abruptly in geological samples around the time of the end-Permian event. Specifically, the carbon stored in rocks tilted towards the lighter isotope by about 1 percent over a matter of 100,000 years.



      Rothman and his colleagues re-analyzed these isotope fluctuations, incorporating them into a model of dynamical exchange between different reservoirs of carbon material. The results showed that the level of carbon dioxide in the ocean rose faster than exponentially. The increase was slow at first, but picked up pace as time went on.