The massive Tibetan Plateau, with an average elevation above 5 kilometers (about 3.1 miles) high, rose to its current height over millions of years following the collision of the Indian subcontinent with Asia. But exactly how and when that uprising began remains debated. New data suggest that the southern half of the plateau reached its current height before 35 million years ago.
David Rowley of the University of Chicago in Illinois and Brian Currie of Miami University in Ohio took measurements of oxygen isotopes contained in lake sediments and nodules made of carbonate soils high on the plateau as a proxy for elevation. The method relies on the assumption that at higher elevations, water loses more of its heavier isotope of oxygen through condensation and rainfall. Therefore, any samples with a bias toward the lighter oxygen isotopes would represent the highest elevations reached.
As published in the Feb. 9 Nature, the isotope values they found showed that Tibetís ancient lakes and surrounding plains already sat higher than 4 kilometers in elevation by 35 million years ago. That timing does not jibe with models of more recent uplift: The standard theory holds that Tibet pushed its way up to its current height only 8 to 10 million years ago, in reaction to the relatively sudden dropping away of the mountainsí root deep in the mantle of the planet. The theory then holds that the fast influx of magma could have quickly added 2 kilometers of height to a plateau only 2 or 3 kilometers high at the time.
The new dates and elevations, combined with current models of mantle and crust behavior in Tibet, have led Rowley and Currie to suggest that the collision between India and Asia pushed up a small portion of southern Tibet, with the uplift spreading northward soon after, building the southern half of the plateau by at least 35 million years ago.
The new isotopic answers are not definitive, however, wrote Andreas Mulch and C. Page Chamberlain of Stanford University in California, in an accompanying comment in Nature, even though the data are a step in the right direction. They wrote that such new methods of determining elevation need to be considered as complements to other proxies.
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