The Tibetan Plateau Is Much Younger Than We Thought

The Lunpola Basin today barely deserves its name, 4,000 meters above sea level and not far below areas around. However, 25 million years ago it was half as high with mountains around and supporting tropical plants. Su Tao

In a world of geological marvels, the Tibetan Plateau stands out. Both extraordinarily high, and astonishingly large, it shapes the climate of much of Asia, including regions home to half of humanity. Its rise changed the world, but we've had trouble working out when that was. Using very different methods, but publishing coincidentally close to each other, two teams have concluded the plateau reached its current size far more recently than we thought.

Professor Su Tao of the Chinese Academy of Sciences discovered 25.5-million-year-old palm fossils in the Lunpola Basin. Palm trees are very restricted in the conditions under which they can grow, and Tibet today doesn't come close. 

After extensive modeling, Su concluded in Science Advances that the palms must have been growing on a valley floor no more than 2 kilometers (1.2 miles) above sea level. The valley would have been fringed by mountains 4.5 kilometers (2.8 miles) above the oceans – towering in most parts of the world, but insignificant altitudes for modern Tibet.

Palms like these could never come close to surviving in Tibet today, and prove the place was very different 25 million years ago. Su Tao

Past attempts to estimate Tibet's age have produced contradictory results depending on the technique. Su's is consistent with some past studies based on plants and animals inhabiting the region. Indeed some have argued the current height is only around 8 million years old

On the other hand, stable isotope paleoaltimetry, which uses the fact the proportion of oxygen-18 and deuterium atoms in rain varies with altitude to reconstruct ancient mountain heights from water trapped in minerals, gave much older dates. One study suggests part of the plateau approached modern heights 100 million years ago, even before India and Asia made contact as part of the plate collision responsible for both the Himalayas and the enormous plateau.

The most widely accepted theory, however, had the plateau at its current height by the Eocene era, 40 million years ago. This has now been challenged by Université Paris-Saclay PhD student Svetlana Botsyun in the journal Science.

Botsyun argues the relationship between isotope ratios and altitude is affected by climatic conditions. Previous estimates have not taken into account that, during the Eocene, Tibet was about 10 degrees further south and bordered by the wide Paratethys sea, rather than surrounded by land as it is today. Allowing for these conditions, while the proportion of oxygen-18 decreases with height today, Botsyun thinks it would have done the opposite 40 million years ago, and the plateau would have been less than 3 kilometers (1.9 miles) high.

Today the Tibetan Plateau is more than 4,000 meters (13,000 feet) above sea level. That's almost twice as high as the tallest mountain on the Australian continent, and 90 percent of the height of the tallest peak in the contiguous United States.

At a time when there was a huge sea where central Asia is now, the predominant winds over Tibet were very different. Botsyun et al/Science Advances

 

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