EARTH & ENVIRONMENT
Tibetan Plateau isn’t as old as thought
World’s ‘roof’ reached current height after 25 million years ago
BY CAROLYN GRAMLING
A plant fossil discovered in rocks from
the Tibetan Plateau and a new analysis
of the area’s geochemistry are rewriting
the uplift history of the region dubbed the
“roof of the world.” And this research suggests that the story of the rise is far more
complicated than just raising the roof.
Previous studies had suggested that
the plateau reached its current height,
about 4. 5 kilometers above sea level, on
average, by at least 40 million years ago.
But chemical evidence left in the region’s
rocks indicates the rise couldn’t have happened before about 40 million years ago,
researchers report in the March 1 Science.
Another team reports that as recently
as 25 million years ago, the region wasn’t
yet a flat, windswept plateau. Instead,
the area was a diverse landscape of steep
mountains surrounding a deep valley
where palm trees grew, the team says
March 6 in Science Advances.
Scientists want to pinpoint the timing of the rise in large part because the
Tibetan Plateau has had such a profound
effect on climate. The plateau altered
atmospheric patterns, causing the onset
of monsoons in South Asia and the drying
out of Asia’s interior, says paleoclimatologist Svetlana Botsyun of the University of
Tübingen in Germany. In fact, the plateau
is so tall that it affects the atmosphere
globally, altering temperature, precipitation, humidity and cloud cover, she says.
About 55 million years ago, the Indian
subcontinent rammed northward into
Asia, and the land between them buck-
led. The Himalaya mountain range was
born, and the Tibetan Plateau north of
the mountains was pushed upward.
One method to estimate how long that
uplift took is paleoaltimetry, which uses
oxygen isotopes to estimate changes
in elevation over time. At higher elevations, the ratio of oxygen- 16 to oxygen- 18
in rainfall is higher than at lower elevations. Previous studies of oxygen isotopes
preserved in carbonate rocks, along with
the rocks’ ages, suggested that the plateau
must have been more than 4 kilometers
high by at least 40 million years ago.
Botsyun and colleagues call that estimate into question. Factors other than
elevation — such as changes in atmospheric carbon dioxide concentrations
or different sources of the water that
becomes rain — also affect oxygen isotope ratios in rainfall, Botsyun notes.
Those factors were at play in the
Eocene, about 56 million to 34 million
years ago. The epoch had much higher
atmospheric CO2 levels than today, a
wide, shallow sea covered parts of China,
and India wasn’t quite where it is now.
Botsyun’s team took these factors into
account in a series of computer simulations to re-create atmospheric circulation
during the Eocene and then recalculated
the oxygen isotope ratios. “We can’t tell
when the uplift [did] happen,” Botsyun
says. But the results suggest that, by
40 million years ago, the plateau was no
higher than 3 kilometers above sea level.
That timing gels with the age of a fossilized palm tree leaf found in the central
Tibetan Plateau. Paleobotanist Tao Su
of the Chinese Academy of Sciences in
Today the Tibetan Plateau in central and
eastern Asia sits at an average elevation of
4. 5 kilometers above sea level.
Mengla and colleagues dated the fossil to
about 25 million years ago. Based on the
ability of the plant’s living relatives — all
tropical and subtropical species — to tol-
erate cold, the team concludes that, at the
time, the region couldn’t have been more
than about 2. 3 kilometers above sea level.
The team also suggests that the region
wasn’t a high, flat plateau yet. Instead,
the palm tree lived in a valley sur-
rounded by high mountain ranges to
the north and south. That conclusion is
based on simulations of past climate that
varied the topography of the region and
incorporated previous estimates of tem-
perature based on leaf fossils at sites near
where the palm tree fossil was found.
What we think of as the Tibetan
Plateau is actually a jigsaw of tectonic
blocks of land, each with its own eleva-
tion history, says Robert Spicer, one of
Su’s collaborators. By the time India
began its collision, parts of what is now
the plateau were already as high as
4. 5 kilometers above sea level, says
Spicer, an earth scientist at the Open
University in Milton Keynes, England.
The valley in which the tree lived
existed from about 60 million to at least
20 million years ago, he says. Other fossil
finds suggest that the valley was home to
a diverse subtropical ecosystem. But as
the collision of India and Asia went on,
the land continued to rise, and sediment
eroded from the mountains filled in the
valley, forming a tall, flat plateau, he says.
The fossil- and isotope-derived eleva-
tions agree on a key point: The central
part of what’s now the Tibetan Plateau
rose to its current height later than
once thought, Spicer says. But to really
understand the history of the plateau,
paleoaltimetry needs to incorporate the
geologic history of the region, he adds.
“You need to get the topography right.
If you have the wrong Tibetan geometry,
you cannot produce realistic results.”
Botsyun agrees. Adding in a deep valley landscape to her simulations “would
be really interesting,” she says. s