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Howdy, neighbor!

Millions of years ago, the American Southwest sat next to East Antarctica

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12:58pm, July 10, 2008
Magazine issue: Vol. 174 #3, August 2, 2008
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What a juxtaposition: About 800 million years ago, East Antarctica, now one of the coldest regions on Earth, abutted what is now California’s Death Valley, one of the hottest.

Both locales were part of an equatorial supercontinent called Rodinia, says John Goodge, a geologist at the University of Minnesota Duluth.

The motion of tectonic plates continually rearranges Earth’s continents, sometimes cramming most or all of them into immense groupings called supercontinents.

One of those assemblages, Rodinia, existed between 750 million and 800 million years ago. Debate has long raged about how today’s landmasses were arranged then, says Goodge. The orientation of magnetic lines locked into rocks that formed at the time — which often can be used to estimate the location and orientation of ancient landmasses — are in many cases contradictory, he notes.

In previous studies, various teams have argued that Australia, southern China, or even Siberia lay along the southwestern edge of Laurentia, a landmass that held most of what is now North America.

Now, geochemical analyses of rock samples taken from the Transantarctic Mountains hint instead that portions of East Antarctica occupied that spot, Goodge and his colleagues report in the July 11 Science. For one thing, the ratios of neodymium isotopes in the ancient sediments in the TransantarcticMountains are the same as those in what was then Laurentia, says Goodge. Also, the hafnium isotope ratios in the 1.44-billion-year-old zircons found in East Antarctica match those of the zircons found in the distinctive granites now found primarily in North America.

Finally, the researchers note, the ratios of various isotopes and elements in a basketball-sized chunk of granite found in East Antarctica — a chunk ripped by a glacier from bedrock now smothered by thick ice, the team speculates — match those of granite found only in what was southwestern Laurentia, which today is the American Southwest.

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