Robert Karl Stonjek
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 Posted: Mon Jul 07, 2008 5:50 am Post subject: Geologists push back date basins formed, supporting frozen E |
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Geologists push back date basins formed, supporting frozen Earth theory
The findings appear to remove one of the major obstacles to the Snowball
Earth theory that a frozen Earth was once entirely covered in snow and ice -
and might even lend some weight to a controversial claim that complex life
originated hundreds of million years earlier than most scientists currently
believe.
"In modern geology, to revise the age of basins like this by 500 million
years is pretty unique," says Joe Meert, a UF associate professor of
geology.
Agreed Abhijit Basu, a professor of geological studies at Indiana
University: "The required revision is enormous - 500 million years or about
11 percent of total Earth history."
Meert is one of eight authors of a paper on the research that recently
appeared in the online edition of the journal Precambrian Research.
The Purana basins - which include the subject of the study, the Vindhyan
basin - are located south of New Delhi in the northern and central regions
of India. They are slight, mostly flat depressions in the Earth>s crust that
span thousands of square miles. For decades, Meert said, most geologists
have believed the basins formed 500 million to 700 million years ago when
the Earth>s crust stretched, thinned and then subsided.
Meert said that date may have originated in early radiometric dating of
sediment from the basin. Radiometric dating involves estimating age based on
the decay or radioactive elements. Additionally, he said, apparent fossils
retrieved from the basin seemed to have originated between 500 million and
700 million years ago.
The researchers were working on an unrelated project and had no intention of
re-examining the basins' age. But then a UF graduate student, Laura Gregory,
dated a kimberlite retrieved from the Vindhyan basin to about 1,073 million
years ago. A kimberlite is a volcanic rock that contains diamonds.
Gregory also used paleomagnetism, a technique that estimates where rocks
were formed by using the orientation of their magnetic minerals. Curious
about whether the kimberlite results would apply more generally to the
region, fellow UF graduate student Shawn Malone compared the kimberlite>s
orientation to other rocks from the Vindhyan basin. To his surprise, he
found the orientations were virtually identical.
As a result, the geologists expanded the investigation, using a modified
chain saw to drill wine-cork-sized cores out of dozens of rocks collected
from 56 sites. Their contents all also had the same or very similar magnetic
orientation, Meert said.
Much of the basins are composed of sediments that cannot be dated using any
method. But Meert said the sediment also contains zircon, which can be dated
using laser mass spectrometry - vaporizing tiny bits of the rocks with a
laser, then analyzing their uranium and "daughter" lead contents to tease
out their formation date based on rates of decay.
All the zircon the researchers tested originated 1,020 million years ago,
Meert said.
The Snowball Earth theory posits that the Earth was covered in snow and ice
from about 635 million to 700 million years ago. While much geological
evidence has been found to support that theory worldwide, the Vindhyan and
other Purana basins lacked numerous telltale signs, such as striated or
scratched boulders formed when ice drags small pebbles over bedrock and
boulder beds derived from glaciers known as tillites, Meert said. As a
result, he said, the basins represented a prominent obstacle to the theory.
The new study removes that obstacle because it pushes back the origins of
the basins to well before Snowball Earth would have occurred.
A 2007 study, conducted independently of the UF study and published in the
Journal of Geology, dated rocks from another Purana basin to 1,020 million
years ago, another 500-million-year revision. One of its authors was M.E.
"Pat" Bickford, a professor emeritus at Syracuse University>s department of
earth sciences. Bickford said the revisions of the age of the Purana basins
calls into question the hypothesis that they formed when the supercontinent
Rodinia broke up. Rodinia is thought to have separated into the modern
continents about 700 million years ago, but the revisions make the basins
too old for that split, Bickford said.
The UF research could also support a Swedish paleontologist>s controversial
dating of multicellular creatures called Ediacarans from an older part of
the basin to 1.6 billion years. But, said Meert, "Of all the implications of
this research, the notion that Ediacaran-like organisms may be much older
than 580 million years is probably the most speculative."
Source: University of Florida
http://www.physorg.com/news134309554.html
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Robert Karl Stonjek |
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