* Astronomy

Title: Giant comets and mass extinctions of life
Author: W. M. Napier

I find evidence for clustering in age of well-dated impact craters over the last 500 Myr. At least nine impact episodes are identified, with durations whose upper limits are set by the dating accuracy of the craters. Their amplitudes and frequency are inconsistent with an origin in asteroid breakups or Oort cloud disturbances, but are consistent with the arrival and disintegration in near-Earth orbits of rare, giant comets, mainly in transit from the Centaur population into the Jupiter family and Encke regions. About 1 in 10 Centaurs in Chiron-like orbits enter Earth-crossing epochs, usually repeatedly, each such epoch being generally of a few thousand years duration. On time-scales of geological interest, debris from their breakup may increase the mass of the near-Earth interplanetary environment by two or three orders of magnitude, yielding repeated episodes of bombardment and stratospheric dusting. I find a strong correlation between these bombardment episodes and major biostratigraphic and geological boundaries, and propose that episodes of extinction are most effectively driven by prolonged encounters with meteoroid streams during bombardment episodes. Possible mechanisms are discussed.

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Title: Mass Extinction And The Structure Of The Milky Way
Author: M. D. Filipovic , J. Horner , E. J. Crawford , N. F. H. Tothill

We use the most up to date Milky Way model and solar orbit data in order to test the hypothesis that the Sun's galactic spiral arm crossings cause mass extinction events on Earth. To do this, we created a new model of the Milky Way's spiral arms by combining a large quantity of data from several surveys. We then combined this model with a recently derived solution for the solar orbit to determine the timing of the Sun's historical passages through the Galaxy's spiral arms. Our new model was designed with a symmetrical appearance, with the major alteration being the addition of a spur at the far side of the Galaxy. A correlation was found between the times at which the Sun crosses the spiral arms and six known mass extinction events. Furthermore, we identify five additional historical mass extinction events that might be explained by the motion of the Sun around our Galaxy. These five additional significant drops in marine genera that we find include significant reductions in diversity at 415, 322, 300, 145 and 33 Myr ago. Our simulations indicate that the Sun has spent ~60% of its time passing through our Galaxy's various spiral arms. Also, we briefly discuss and combine previous work on the Galactic Habitable Zone with the new Milky Way model.

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The Race to Catalogue Living Species Before They Go Extinct

The U.S. has spent several billion dollars looking for life on other planets. Shouldnt we spend at least that much finding and identifying life on Earth?
That is the argument behind a taxonomy analysis by a trio of scientists in Science, published on January 25.
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Toxic seas 'spurred' extinctions of dinosaurs and many other species

Poisonous seawater probably may have driven two of the earth's best-known mass extinctions, say Penn State University researchers.
It is understood that an asteroid the size of Mt. Everest ended the reign of the dinosaurs when it struck Mexico's Yucatan Peninsula 65 million years ago. The impact resulted firestorms, darkness and deadly gases that made trouble for life on land.
According to geochemist Lee Kump of Penn State University, something more was going on in the oceans, where 93 percent of nannoplankton-the base of the marine food web-went extinct.
Dust and smoke kicked up by the asteroid would have throttled photosynthesis for several months, but it took some 270,000 years for plankton populations to bounce back.
Even in the Northern Hemisphere, which suffered a direct hit, recovery should have been much faster. In 2010 Kump and some of his Penn State colleagues explained this lag by proposing that toxic metals from the asteroid contaminated the oceans.
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