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TOPIC: Extinction


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RE: Extinction
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The first evidence the giant prehistoric kangaroos that roamed Tasmania were hunted into extinction by humans has been found in a cave in a rainforest-clad region in the north-west of the island state.
A team of Australian and British researchers report in today's Proceedings of the National Academy of Sciences that radiocarbon dating of fossil remains of seven megafauna species pushes the survival of the giant prehistoric animals on the Australian island state forward to 41,000 years ago.
Humans are thought to have arrived in Tasmania 43,000 to 40,000 years ago, when the island became temporarily connected by a land bridge to mainland Australia.


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The extinction of many ancient species may be due to humans rather than climate change, experts say.
Large prehistoric animals in Tasmania may have been wiped out by human hunting and not temperature changes, a team of international scientists argue.
This pattern may have been repeated around the globe on islands such as Great Britain, the scientists say.
The findings were published in the American scientific journal - Proceedings of the National Academy.

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Anoxic event
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Undersea volcanic activity has been blamed for a mass extinction in the seas 93 million years ago.
In the so-called "anoxic event" of the late Cretaceous Period, the ocean depths became starved of oxygen, wiping out swathes of marine organisms.

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Undersea volcanic activity triggered a mass extinction of marine life and buried a thick mat of organic matter on the sea floor about 93 million years ago, which became a major source of oil, according to a new study.

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Mass extinction events
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If you are curious about Earth's periodic mass extinction events, such as the sudden demise of the dinosaurs 65 million years ago, you might consider crashing asteroids and sky-darkening super volcanoes as culprits.
But a new study, published online June 15 in the journal Nature, suggests that it is the ocean, and in particular the epic ebbs and flows of sea level and sediment over the course of geologic time, that is the primary cause of the world's periodic mass extinctions during the past 500 million years.

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Extinction
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The full recovery of ecological systems, following the most devastating extinction event of all time, took at least 30 million years, according to new research from the University of Bristol.
About 250 million years ago, at the end of the Permian, a major extinction event killed over 90 per cent of life on earth, including insects, plants, marine animals, amphibians, and reptiles. Ecosystems were destroyed worldwide, communities were restructured and organisms were left struggling to recover. This was the nearest life ever came to being completely wiped out.

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Some of the mass extinctions in Earth's long history resulted from catastrophic events such as asteroid impacts or massive volcanic eruptions. But two researchers who have studied the fossil remains of coral-like marine creatures during two of the biggest of these die-offs argue that prolonged environmental stress might have wiped out many species more quietly. The findings could help scientists predict the effects of climate change on today's marine life.
The demise of the dinosaurs and other species 65 million years ago is well known, but it was preceded by two even bigger mass extinctions. The Great Dying, as scientists call it, ended the Permian period 250 million years ago by killing off 90% of marine creatures and 70% of the land dwellers. The Triassic extinction about 200 million years ago exterminated 20% of Earth's marine life and half the terrestrial species--while allowing dinosaurs eventually to flourish. In both cases, there's no evidence for an asteroid impact or volcanic conflagration, so the causes of the events remain unresolved.

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With surprising and mysterious regularity, life on Earth has flourished and vanished in cycles of mass extinction every 62 3 million years.
The pattern emerges from a computer study of fossil records going back for more than 542 million years, to the time of the great Cambrian Explosion, when almost all the ancestral forms of multicellular life emerged.
The fossil records in Sepkoski's compendium cover the first and last known appearances of 36,380 separate marine genera, including millions of species that once thrived in the world's seas.
The cycles are so clear that the evidence "simply jumps out of the data."
It had been suggested that a faraway dwarf star, named "Nemesis", was orbiting the sun, or an unknown "Planet X" somewhere far out beyond the solar system that's disturbing the comets in the distant region called the Oort Cloud and might be possible causes for the 62-million-year cycles.
Or perhaps there's some kind of "natural timetable" deep inside the Earth that triggers cycles of massive volcanism. There's even a bit of evidence: A huge slab of volcanic basalt known as the Deccan Traps in India has been dated to 65 million years ago just when the dinosaurs died, he noted. And the similar basaltic Siberian Traps were formed by volcanism about 250 million years ago, at the end of the Permian period, when the greatest of all mass extinctions drove more than 70 percent of the entire world's marine life to death.
Anther far-out ideas is that the solar system passes through an exceptionally massive arm of our own spiral Milky Way galaxy every 62 million years, and that that increase in galactic gravity might set off a hugely destructive comet shower that would drive cycles of mass extinction on Earth.

Here are details of the five worst mass extinctions in Earths history and their possible causes:

1) Cretaceous-Tertiary extinction, about 65 million years ago, probably caused or aggravated by impact of a several-mile-wide asteroid that created the Chicxulub crater now hidden on the Yucatan Peninsula and beneath the Gulf of Mexico. Some argue for other causes, including gradual climate change or flood-like volcanic eruptions of basalt lava from Indias Deccan Traps. The extinction killed 16 percent of marine families, 47 percent of marine genera (the classification above species) and 18 percent of land vertebrate families, including the dinosaurs.

The Jurassic-Cretaceous boundaries, 144.6 0.8 million years before present may, or may not, involve a major impact event. An article on the "Discovery of distal ejecta from the 1850 Ma Sudbury impact event," which is "the second largest and third or fourth oldest extraterrestrial Earth impact site," says that "The debris (ejecta), landed 650 km west northwest of Sudbury near Thunder Bay, Ontario, Canada, and 875 km west of Sudbury near Hibbing, Minnesota, United States."

2) End Triassic extinction, roughly 199 million to 214 million years ago, most likely caused by massive floods of lava erupting from the central Atlantic magmatic province -- an event that triggered the opening of the Atlantic Ocean. The volcanism may have led to deadly global warming. Rocks from the eruptions now are found in the eastern United States, eastern Brazil, North Africa and Spain. The death toll: 22 percent of marine families, 52 percent of marine genera. Vertebrate deaths are unclear.

3) Permian-Triassic extinction, about 251 million years ago. Many scientists suspect a comet or asteroid impact. Others believe the cause was flood volcanism from the Siberian Traps and related loss of oxygen in the seas. Still others believe the impact triggered the volcanism and also may have done so during the Cretaceous-Tertiary extinction. The Permian-Triassic catastrophe was Earths worst mass extinction, killing 95 percent of all species, 53 percent of marine families, 84 percent of marine genera and an estimated 70 percent of land species such as plants, insects and vertebrate animals. researchers discovered tiny capsules of cosmic gas trapped inside rocks from the Permian-Triassic, deposited by a space rock colliding with the planet. Isotopes of helium and argon gases commonly found in space were found within buckyballs or fullerenes (sphere of carbon atoms).
Researchers funded by NASA and the National Science Foundation have located the site of an impact crater.

spacer.gif impact

Asteroid impact

The Bedout crater (pronounced Bedoo) is believed to be associated with the largest extinction event in Earth's history about 251 million years ago; caused by an asteroid roughly 6 to 12 kilometres wide...
Permian-Triassic impact on the giant landmass, called Pangaea, was the most severe mass extinction in the history of life on Earth. They used a new extraterrestrial tracer, fullerene; a third form of carbon besides diamond and graphite. .

4) Late Devonian extinction, about 364 million years ago, cause unknown. It killed 22 percent of marine families and 57 percent of marine genera. Little is known about land organisms at the time.

5) Ordovician-Silurian
extinction, about 439 million years ago, caused by a drop in sea levels as glaciers formed, then by rising sea levels as glaciers melted. 25 percent of marine families and 60 percent of marine genera became extinct.


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Fossil record supports evidence of impending mass extinction
Global temperatures predicted for the coming centuries may trigger a new mass extinction event, where over 50 per cent of animal and plant species would be wiped out, warn scientists at the Universities of Leeds and York.

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Past mass extinctions coincide with hotter temperatures, researchers say in new report
Whenever the world's tropical seas warm several degrees, Earth has experienced mass extinctions over millions of years, according to a first-of-its-kind statistical study of fossil records.
Scientists fear it may be about to happen again, but in a matter of several decades, not tens of millions of years.
Four of the five major extinctions over 520 million years of Earth history have been linked to warmer tropical seas, something that indicates a warmer world overall, according to the study published Wednesday.

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