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A lone granite boulder found against all odds high atop
a glacier in Antarctica may provide additional key evidence
to support a theory that parts of the southernmost continent
once were connected to North America hundreds of millions
of years ago.
Writing in the July 11 edition of the journal Science,
an international team of U.S. and Australian investigators
describe their findings, which were made in the Transantarctic
Mountains, and their significance to the problem of piecing
together what an ancient supercontinent, called Rodinia,
looked like. The U.S. investigators were funded by the
National Science Foundation (NSF).
Previous lines of scientific evidence led researchers
to theorise that about 600-800 million years ago a portion
of Rodinia broke away from what is now the southwestern
United States and eventually drifted southward to become
eastern Antarctica and Australia.
The team's find, they argue, provides physical evidence
that confirms the so-called southwestern United States
and East Antarctica (SWEAT) hypothesis.
"What this paper does is say that we have three
main new lines of evidence that basically confirm the
SWEAT idea," said John Goodge, an NSF-funded researcher
with the Department of Geological Sciences at the University
of Minnesota-Duluth.
Added Scott Borg, director of the division of Antarctic
sciences in NSF's Office of Polar Programs, "this
is first-rate work and a fascinating example of scientists
at work putting together the pieces of a much larger puzzle.
Not only do the authors pull together a diverse array
of data to address a long-standing question about the
evolution of the Earth's crust during a critical time
for biological evolution, but the research shows how the
ideas surrounding the SWEAT hypothesis have developed
over time."
As a field researcher during the late 1980's and early
1990's, Borg authored papers on the SWEAT hypothesis.
The boulder find came by serendipity while the researchers
were picking though rubble carried through the Transantarctic
Mountains by ice streams-rivers of ice-that flow at literally
a glacial pace from East Antarctica.
Goodge and his team were searching for rocks that might
provide keys to the composition of the underlying continent
crust of Antarctica, which in most places is buried under
almost two miles of ice.
"We were picking up boulders in the moraines that
looked interesting," Goodge said. "It was basically
just a hodge-podge of material."
One rock in particular, small enough to heft in one hand,
found atop the Nimrod Glacier, was later determined to
be a very specific form of granite with, as Goodge describes
it, "a particular type of coarse-grained texture."
Subsequent chemical and isotopic tests conducted in laboratories
in the United States revealed that the boulder had a chemistry
"very similar to a unique belt of igneous rocks in
North America" that stretches from what is now California
eastward through New Mexico to Kansas, Illinois and eventually
through New Brunswick and Newfoundland in Canada.
That belt of rocks is known to have been a part of what
is called Laurentia, which was a component of the supercontinent
of Rodinia.
"There is a long, linear belt of these igneous rocks
that stretches across Laurentia. But 'bang' it stops,
right there at the (western) margin where we knew that
something rifted away" from what is now the West
Coast of the United States," Goodge said.
"It just ends right where that ancient rift margin
is," Goodge said. "And these rocks are basically
not found in any other part of the world."
That it should turn up on a glacier high up in the mountains
of Antarctica is strong evidence in support of the SWEAT
model that parts of North America continue into part of
the frozen continent at the bottom of the Earth.
"There's no other explanation for how it got where
we found it," Goodge said. "It was bull-dozed
over from that interior region of Antarctica."
The find itself is compelling to geologists, Goodge noted,
because little other physical evidences exists to allow
them directly to put together the jigsaw puzzle of the
long-disappeared Rodinia.
But because the supercontinent existed at a key time
in the development of multi-cellular life on Earth, it
also helps provide a geological context in which this
massive biotic change took place.
"During the Cambrian explosion about 520 million
years ago we started seeing this huge expansion in the
diversity of life forms," Goodge said. "This
was also a time when the Earth was undergoing tremendous
geologic changes."
He added that "something helped trigger that big
radiation in life."
The shifting configuration of the continents, accompanied
by collisions between landmasses, erosion and the influx
of chemicals into the seas may well have provided the
nutrients to that growing diversity of lifeforms.
"There are ideas developing about these connections
between the geo-tectonic world on the one hand and biology
on the other.
The job of geoscientists in this context, he said "is
to reconstruct what the world was like at the time."
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National
Science Foundation -
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