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NileQueen
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Cosmic Rays and Extinction events 62 Million year cycleIn Science:
55 Mya was the Paleocene Eocene Thermal Maximum (PETM) which has been associated with massive methane dissociation from the seafloor...
http://sciencenow.sciencemag.org/cgi/content/full/2007/801/1
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Picture of solar system
Deadly wobble.
The solar system's rollercoaster-like path around the galaxy periodically makes it vulnerable to lethal radiation from intergalactic space.
Credit: Adrian Melott/University of Kansas
Of Cosmic Rays and Dangerous Days
By Phil Berardelli
ScienceNOW Daily News
1 August 2007
Researchers may have uncovered the reason why Earth's biodiversity mysteriously plummets periodically. They have found that a rollercoaster-like wobble in the sun's orbit around the center of the Milky Way galaxy regularly moves Earth closer to a source of dangerous intergalactic cosmic rays.
Over the last 500 million years or so, the number of species on Earth has tended to dip regularly about every 62 million years. The last time this happened, about 55 million years ago--or about 10 million years after the great K-T extinction event that wiped out the dinosaurs--biodiversity sank by about 10%; around 115 million years ago, it dropped by a similar amount. So far, evolutionary biologists have only been able to establish that the phenomenon seems cyclical, but they haven't isolated a cause.
Now, researchers from the University of Kansas in Lawrence think they have found a possible answer. Physicist and co-author Adrian Melott says that he began suspecting a galactic cause after noticing a 2005 paper that calculated that the drop in species diversity occurs regularly on a time scale of tens of millions of years, which—for a cyclical event--is too long for something happening within the solar system. So he and Kansas colleague Mikhail Medvedev began examining the possibilities. At about the same time as the drops in biodiversity, the researchers determined, the sun reaches the highest point in its orbit relative to the galactic plane, where most Milky Way stars reside. At that point, the scientists report in the 1 August Astrophysical Journal, the solar system is closest to an incoming source of potentially lethal cosmic rays created by interactions between the Milky Way's magnetic field and radiation generated by a cluster of nearby galaxies.
These galaxies are located in the direction of the constellation Virgo, and the radiation consists of particles called muons, which are so powerful they can penetrate about 2.5 kilometers of sea water or 900 meters of rock--enough to reach just about every living thing on Earth and damage its DNA. Because the zenith of the Sun's oscillations match almost exactly with the times of the dips in the fossil record, the researchers found, "we've noticed an incredible coincidence," Melott says.
Physicist Richard Muller of Lawrence Berkeley Laboratory in California calls the paper's hypothesis "intriguing" and something that should be "of great interest" to both the astrophysics and evolutionary biology communities. The problem, says Muller, who co-wrote the 2005 paper in Nature that piqued Melott's interest, is that killer cosmic rays may not have been the direct cause of the drops in biodiversity. There could be other candidates, such as significant climate change. "We've got to try to understand the mechanism better," he says.
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Essan
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There's a good book which looks at the causes of extinctions and does look into possiblity of a cycle of some sort being involved in extinction events:
Catastrophes and lesser calamities
Recommended 
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NileQueen
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Thanks, Andy.
There's also a good recent report in Nature on extinctions at
geological time boundaries.
http://www.nature.com/nature/journal/v448/n7150/full/448122a.html
excerpt
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News Feature
Nature 448, 122-125 (12 July 2007) | doi:10.1038/448122a; Published online 11 July 2007
Mass extinctions: Reading the book of death
Nick Lane1
Nick Lane is the author of Power, sex, suicide: Mitochondria and the meaning of life.
Top of pageAbstractStudies of mass extinctions tend to emphasize the sheer scope of the carnage. But subtle differences between the species that died and those that survived can be crucial, finds Nick Lane.
The extinction at the end of the Permian period, some 251 million years ago, is the most fascinating mass-murder mystery in Earth's history. Simply put, a party or parties unknown killed off up to 96% of all species then alive. Palaeontologists and Earth-system scientists have suggested any number of possible culprits, from a comet or asteroid collision to a collapse of the ozone layer, from a dramatic suite of volcanic eruptions to ocean waters bubbling with sulphurous fumes.
Given that the evidence for an impact, although championed by some, seems sketchy and inconclusive to the majority, the most dramatic of these suspects is volcanism — the sequence of eruptions that gave rise to the basalts of the Siberian Traps. The scale of the eruptions was vast, with something like 3 million cubic kilometres of basalt flowing on to the surface, and the main lava flows took place at almost exactly at the same time as the end-Permian extinction itself, give or take a few hundred thousand years. The vast clouds of gas and ash they spewed out look like the smoke from a very large gun indeed1.
But if the erupting traps look increasingly like the trigger for the great die-off, how did they actually do their damage? The fiery birth of a small continent's worth of new landscape can change everything from the brightness of the sky to the chemistry of the oceans. How can scientists work out which of these effects were inconveniences, and which were fatal?
One answer is to study not the volcanoes, nor their victims, but the creatures that survived. A mortality rate of 96% sounds pretty indiscriminate — but recent physiological comparisons between the creatures that died and those that survived reveal intriguing patterns2. And these patterns do not just provide clues as to how the great dying actually unfurled. They also reveal how the extinctions at the end of the Permian period shaped the subsequent history of the world that the survivors inherited.
Until recently, perhaps the most popular 'kill mechanism' evoked for the extinction was global warming3. Volcanic carbon dioxide, so the story goes, raised temperatures enough to destabilize vast reserves of methane hydrates in the sea floor, pumping temperatures up further. But although there's little question that global temperatures rose at the end of the Permian by as much as 6 °C, there are problems with this picture.
REUTERS
Eruptions, such as Hekla's, could cause ozone depletion.
Among the things to go extinct at the end of the Permian are a lot of plants. There are plausible arguments as to how temperature could kill off animals — for example, it would force a higher metabolic rate on them, requiring an increase in their calorie intake that they might simply not have been able to satisfy. But there's no obvious reason why such heat would damage plants. What's more, the physiology of the survivors doesn't fit the story. Animals that seem to have started off with higher metabolic rates suffered less than those with slow metabolisms. Other things being equal, you might expect animals that are already generating more heat in their own bodies to do worse when things heat up further.
Another possible kill mechanism is a collapsed ozone layer. Mutant pollen and spores in sediments from the time of the extinction hint at an increase in ultraviolet levels brought about by a dearth of ozone. David Beerling, a plant physiologist who concentrates on palaeoclimate issues at the University of Sheffield, UK, says his eyes were opened to this risk when he learned that a NASA aircraft flying through the plume above the 2000 Hekla eruption in Iceland showed substantial amounts of hydrochloric acid had been injected directly into the stratosphere. Simulations by Beerling and his colleagues at Sheffield and the University of Cambridge, UK, suggest that the Siberian Traps could have removed 70% of the ozone shield from the Northern Hemisphere, enough to do serious environmental damage4.
Even so, Beerling doubts that an ozone collapse was responsible for the mass extinction: "Animals have too many places to hide on land, and in the oceans they could easily escape the worst of it," he says. And as with global warming, an excess of ultraviolet radiation does not seem to fit the pattern of survival that is actually seen. Living deep in the sea doesn't seem to have helped.
Animals have too many places to hide on land, and in the oceans they could easily escape ozone collapse. ~David Beerling
But better ways of breathing, which are often seen in creatures with a higher metabolic rate, did. In the sea, creatures with gills and an active internal circulation, such as snails, clams, crabs and fish, fared better than stationary filter feeders. A similar pattern holds on land, with groups that were not so good at pumping air through their respiratory systems — such as the giant dragonflies iconic of pre-Permian times — suffering disproportionately. These patterns focus attention not on harsher sunlight, but on tainted air.
Oxygen collapse
One possible factor is a worldwide dearth of oxygen. It used to be thought that today's oxygen level of 21% had remained fairly constant over geological time. But in the late 1980s, Robert Berner of Yale University in New Haven, Connecticut, shocked the establishment by arguing that in the Carboniferous period that preceded the Permian, 300 million years ago, the oxygen level had reached 30% or more, and that it had then fallen dramatically to a mere 13% in the late Permian and the early part of the subsequent Triassic. But after a few refinements, Berner's models for this period have been widely accepted.
Berner blames the oxygen collapse on another spate of volcanism that predates the Siberian Traps by eight million years or so. The warming those eruptions triggered, he argues, would have brought about more arid conditions, drying out some of the coal swamps of the time. It had been the capacity of those swamps to bury organic carbon where it could not be respired back into carbon dioxide, which had accounted for the high oxygen levels. As the swamps dried out, less organic carbon got buried this way, and so more was respired back to carbon dioxide — using up oxygen in the process. Over millions of years the slower rate of carbon burial drew oxygen levels down to an unprecedented low point5.
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It's unlikely that any of these respiratory adaptations evolved in response to the suffocating conditions at end of the Permian; rather, the animals most likely to survive the disaster were those already adapted to deal with suffocating conditions on a daily basis — creatures used to burrowing on land, or scavenging in the stagnant muds of ocean shelves; or just those that came up trumps with an air-sac system that happened to be particularly good.
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