Geologists discover ancient buried canyon in South Tibet

This photo shows the Yarlung Tsangpo Valley close to the Tsangpo Gorge, where it is rather narrow and underlain by only about 250 meters of sediments. The mountains in the upper left corner belong to the Namche Barwa massif. Previously, scientists had suspected that the debris deposited by a glacier in the foreground was responsible for the formation of the steep Tsangpo Gorge -- the new discoveries falsify this hypothesis. -  Ping Wang
This photo shows the Yarlung Tsangpo Valley close to the Tsangpo Gorge, where it is rather narrow and underlain by only about 250 meters of sediments. The mountains in the upper left corner belong to the Namche Barwa massif. Previously, scientists had suspected that the debris deposited by a glacier in the foreground was responsible for the formation of the steep Tsangpo Gorge — the new discoveries falsify this hypothesis. – Ping Wang

A team of researchers from Caltech and the China Earthquake Administration has discovered an ancient, deep canyon buried along the Yarlung Tsangpo River in south Tibet, north of the eastern end of the Himalayas. The geologists say that the ancient canyon–thousands of feet deep in places–effectively rules out a popular model used to explain how the massive and picturesque gorges of the Himalayas became so steep, so fast.

“I was extremely surprised when my colleagues, Jing Liu-Zeng and Dirk Scherler, showed me the evidence for this canyon in southern Tibet,” says Jean-Philippe Avouac, the Earle C. Anthony Professor of Geology at Caltech. “When I first saw the data, I said, ‘Wow!’ It was amazing to see that the river once cut quite deeply into the Tibetan Plateau because it does not today. That was a big discovery, in my opinion.”

Geologists like Avouac and his colleagues, who are interested in tectonics–the study of the earth’s surface and the way it changes–can use tools such as GPS and seismology to study crustal deformation that is taking place today. But if they are interested in studying changes that occurred millions of years ago, such tools are not useful because the activity has already happened. In those cases, rivers become a main source of information because they leave behind geomorphic signatures that geologists can interrogate to learn about the way those rivers once interacted with the land–helping them to pin down when the land changed and by how much, for example.

“In tectonics, we are always trying to use rivers to say something about uplift,” Avouac says. “In this case, we used a paleocanyon that was carved by a river. It’s a nice example where by recovering the geometry of the bottom of the canyon, we were able to say how much the range has moved up and when it started moving.”

The team reports its findings in the current issue of Science.

Last year, civil engineers from the China Earthquake Administration collected cores by drilling into the valley floor at five locations along the Yarlung Tsangpo River. Shortly after, former Caltech graduate student Jing Liu-Zeng, who now works for that administration, returned to Caltech as a visiting associate and shared the core data with Avouac and Dirk Scherler, then a postdoc in Avouac’s group. Scherler had previously worked in the far western Himalayas, where the Indus River has cut deeply into the Tibetan Plateau, and immediately recognized that the new data suggested the presence of a paleocanyon.

Liu-Zeng and Scherler analyzed the core data and found that at several locations there were sedimentary conglomerates, rounded gravel and larger rocks cemented together, that are associated with flowing rivers, until a depth of 800 meters or so, at which point the record clearly indicated bedrock. This suggested that the river once carved deeply into the plateau.

To establish when the river switched from incising bedrock to depositing sediments, they measured two isotopes, beryllium-10 and aluminum-26, in the lowest sediment layer. The isotopes are produced when rocks and sediment are exposed to cosmic rays at the surface and decay at different rates once buried, and so allowed the geologists to determine that the paleocanyon started to fill with sediment about 2.5 million years ago.

The researchers’ reconstruction of the former valley floor showed that the slope of the river once increased gradually from the Gangetic Plain to the Tibetan Plateau, with no sudden changes, or knickpoints. Today, the river, like most others in the area, has a steep knickpoint where it meets the Himalayas, at a place known as the Namche Barwa massif. There, the uplift of the mountains is extremely rapid (on the order of 1 centimeter per year, whereas in other areas 5 millimeters per year is more typical) and the river drops by 2 kilometers in elevation as it flows through the famous Tsangpo Gorge, known by some as the Yarlung Tsangpo Grand Canyon because it is so deep and long.

Combining the depth and age of the paleocanyon with the geometry of the valley, the geologists surmised that the river existed in this location prior to about 3 million years ago, but at that time, it was not affected by the Himalayas. However, as the Indian and Eurasian plates continued to collide and the mountain range pushed northward, it began impinging on the river. Suddenly, about 2.5 million years ago, a rapidly uplifting section of the mountain range got in the river’s way, damming it, and the canyon subsequently filled with sediment.

“This is the time when the Namche Barwa massif started to rise, and the gorge developed,” says Scherler, one of two lead authors on the paper and now at the GFZ German Research Center for Geosciences in Potsdam, Germany.

That picture of the river and the Tibetan Plateau, which involves the river incising deeply into the plateau millions of years ago, differs quite a bit from the typically accepted geologic vision. Typically, geologists believe that when rivers start to incise into a plateau, they eat at the edges, slowly making their way into the plateau over time. However, the rivers flowing across the Himalayas all have strong knickpoints and have not incised much at all into the Tibetan Plateau. Therefore, the thought has been that the rapid uplift of the Himalayas has pushed the rivers back, effectively pinning them, so that they have not been able to make their way into the plateau. But that explanation does not work with the newly discovered paleocanyon.

The team’s new hypothesis also rules out a model that has been around for about 15 years, called tectonic aneurysm, which suggests that the rapid uplift seen at the Namche Barwa massif was triggered by intense river incision. In tectonic aneurysm, a river cuts down through the earth’s crust so fast that it causes the crust to heat up, making a nearby mountain range weaker and facilitating uplift.

The model is popular among geologists, and indeed Avouac himself published a modeling paper in 1996 that showed the viability of the mechanism. “But now we have discovered that the river was able to cut into the plateau way before the uplift happened,” Avouac says, “and this shows that the tectonic aneurysm model was actually not at work here. The rapid uplift is not a response to river incision.”

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The other lead author on the paper, “Tectonic control of the Yarlung Tsangpo Gorge, revealed by a 2.5 Myr old buried canyon in Southern Tibet,” is Ping Wang of the State Key Laboratory of Earthquake Dynamics, in Beijing, China. Additional authors include Jürgen Mey, of the University of Potsdam, in Germany; and Yunda Zhang and Dingguo Shi of the Chengdu Engineering Corporation, in China. The work was supported by the National Natural Science Foundation of China, the State Key Laboratory for Earthquake Dynamics, and the Alexander von Humboldt Foundation.

Researchers resolve the Karakoram glacier anomaly, a cold case of climate science

The researchers found that low-resolution models and a lack of reliable observational data obscured the Karakoram's dramatic shifts in elevation over a small area and heavy winter snowfall. They created a higher-resolution model that showed the elevation and snow water equivalent for (inlaid boxes, from left to right) the Karakoram range and northwest Himalayas, the central Himalayas that include Mount Everest, and the southeast Himalayas and the Tibetan Plateau. For elevation (left), the high-resolution model showed the sharp variations between roughly 2,500 and 5,000 meters above sea level (yellow to brown) for the Karakoram, while other areas of the map have comparatively more consistent elevations. The model also showed that the Karakoram receive much more annual snowfall (right) than other Himalayan ranges (right), an average of 100 centimeters (brown). The researchers found that the main precipitation season in the Karakoram occurs during the winter and is influenced by cold winds coming from Central Asian countries, as opposed to the heavy summer monsoons that provide the majority of precipitation to the other Himalayan ranges. -  Image by Sarah Kapnick, Program in Atmospheric and Oceanic Sciences
The researchers found that low-resolution models and a lack of reliable observational data obscured the Karakoram’s dramatic shifts in elevation over a small area and heavy winter snowfall. They created a higher-resolution model that showed the elevation and snow water equivalent for (inlaid boxes, from left to right) the Karakoram range and northwest Himalayas, the central Himalayas that include Mount Everest, and the southeast Himalayas and the Tibetan Plateau. For elevation (left), the high-resolution model showed the sharp variations between roughly 2,500 and 5,000 meters above sea level (yellow to brown) for the Karakoram, while other areas of the map have comparatively more consistent elevations. The model also showed that the Karakoram receive much more annual snowfall (right) than other Himalayan ranges (right), an average of 100 centimeters (brown). The researchers found that the main precipitation season in the Karakoram occurs during the winter and is influenced by cold winds coming from Central Asian countries, as opposed to the heavy summer monsoons that provide the majority of precipitation to the other Himalayan ranges. – Image by Sarah Kapnick, Program in Atmospheric and Oceanic Sciences

Researchers from Princeton University and other institutions may have hit upon an answer to a climate-change puzzle that has eluded scientists for years, and that could help understand the future availability of water for hundreds of millions of people.

In a phenomenon known as the “Karakoram anomaly,” glaciers in the Karakoram mountains, a range within the Himalayas, have remained stable and even increased in mass while many glaciers nearby — and worldwide — have receded during the past 150 years, particularly in recent decades. Himalayan glaciers provide freshwater to a densely populated area that includes China, Pakistan and India, and are the source of the Ganges and Indus rivers, two of the world’s major waterways.

While there have been many attempts to explain the stability of the Karakoram glaciers, the researchers report in the journal Nature Geoscience that the ice is sustained by a unique and localized seasonal pattern that keeps the mountain range relatively cold and dry during the summer. Other Himalayan ranges and the Tibetan Plateau — where glaciers have increasingly receded as Earth’s climate has warmed — receive most of their precipitation from heavy summer monsoons out of hot South and Southeast Asian nations such as India. The main precipitation season in the Karakoram, however, occurs during the winter and is influenced by cold winds coming from Central Asian countries such as Afghanistan to the west, while the main Himalayan range blocks the warmer air from the southeast throughout the year.

The researchers determined that snowfall, which is critical to maintaining glacier mass, will remain stable and even increase in magnitude at elevations above 4,500 meters (14,764 feet) in the Karakoram through at least 2100. On the other hand, snowfall over much of the Himalayas and Tibet is projected to decline even as the Indian and Southeast Asian monsoons increase in intensity under climate change.

First author Sarah Kapnick, a postdoctoral research fellow in Princeton’s Program in Atmospheric and Oceanic Sciences, said that a shortage of reliable observational data and the use of low-resolution computer models had obscured the subtleties of the Karakoram seasonal cycle and prevented scientists from unraveling the causes of the anomaly.

For models, the complication is that the Karakoram features dramatic shifts in elevation over a small area, Kapnick said. The range boasts four mountains that are more than 8,000 meters (26,246 feet) high — including K2, the world’s second highest peak — and numerous summits that exceed 7,000 meters, all of which are packed into a length of about 500 kilometers (300 miles).

Kapnick and her co-authors overcame this obstacle with a high-resolution computer model that broke the Karakoram into 50-kilometer pieces, meaning that those sharp fluctuations in altitude were better represented.

In their study, the researchers compared their model with climate models from the United Nations’ Intergovernmental Panel on Climate Change (IPCC), which averages a resolution of 210-kilometer squares, Kapnick said. At that scale, the Karakoram is reduced to an average height that is too low and results in temperatures that are too warm to sustain sufficient levels of snowfall throughout the year, and too sensitive to future temperature increases.

Thus, by the IPCC’s models, it would appear that the Karakoram’s glaciers are imperiled by climate change due to reduced snowfall, Kapnick said. This region has been a great source of controversy ever since the IPCC’s last major report, in 2007, when the panel misreported that Himalayan glaciers would likely succumb to climate change by 2035. More recent papers using current IPCC models have similarly reported snowfall losses in this region because the models do not accurately portray the topography of the Karakoram, Kapnick said.

“The higher resolution allowed us to explore what happens at these higher elevations in a way that hasn’t been able to be done,” Kapnick said. “Something that climate scientists always have to keep in mind is that models are useful for certain types of questions and not necessarily for other types of questions. While the IPCC models can be particularly useful for other parts of the world, you need a higher resolution for this area.”

Jeff Dozier, a professor of snow hydrology, earth system science and remote sensing at the University of California-Santa Barbara, said that the research addresses existing shortcomings in how mountain climates are modeled and predicted, particularly in especially steep and compact ranges. Dozier, who was not involved in the research, conducts some of his research in the Hindu Kush mountains west of the Karakoram.

Crucial information regarding water availability is often lost in computer models, observational data and other tools that typically do not represent ranges such as Karakoram accurately enough, Dozier said. For instance, a severe 2011 drought in Northern Afghanistan was a surprise partly due to erroneous runoff forecasts based on insufficient models and surface data, he said. The high-resolution model Kapnick and her co-authors developed for Karakoram potentially resolves many of the modeling issues related to mountain ranges with similar terrain, he said.

“The Karakoram Anomaly has been a puzzle, and this paper gives a credible explanation,” Dozier said. “Climate in the mountains is obviously affected strongly by the elevation, but most global climate models don’t resolve the topography well enough. So, the higher-resolution model is appropriate. About a billion people worldwide get their water resources from melting snow and many of these billion get their water from High Mountain Asia.”

The researchers used the high-resolution global-climate model GFDL-CM2.5 at the Geophysical Fluid Dynamics Laboratory (GFDL), which is on Princeton’s Forrestal Campus and administered by the National Oceanic and Atmospheric Administration (NOAA). The researchers simulated the global climate — with a focus on the Karakoram — based on observational data from 1861 to 2005, and on the IPCC’s greenhouse-gas projections for 2006-2100, which will be included in its Fifth Assessment Report scheduled for release in November.

The 50-kilometer resolution revealed conditions in Karakoram on a monthly basis, Kapnick said. It was then that she and her colleagues could observe that the monsoon months in Karakoram are not only not characterized by heavy rainfall, but also include frigid westerly winds that keep conditions in the mountain range cold enough for nearly year-round snowfall.

“There is precipitation during the summer, it just doesn’t dominate the seasonal cycle. This region, even at the same elevation as the rest of the Himalayas, is just colder,” Kapnick said.

“The high-resolution model shows us that things don’t happen perfectly across seasons. You can have statistical variations in one month but not another,” she continued. “This allows us to piece out those significant changes from one month to the next.”

Kapnick, who received her bachelor’s degree in mathematics from Princeton in 2004, worked with Thomas Delworth, a NOAA scientist and Princeton lecturer of geosciences and atmospheric and oceanic sciences; Moestasim Ashfaq, a scientist at the Oak Ridge National Laboratory Climate Change Science Institute; Sergey Malyshev, a climate modeler in Princeton’s Department of Ecology and Evolutionary Biology based at GFDL; and P.C.D. “Chris” Milly, a research hydrologist for the U.S. Geological Survey based at GFDL who received his bachelor’s degree in civil engineering from Princeton in 1978.

While the researchers show that the Karakoram will receive consistent — and perhaps increased — snowfall through 2100, more modeling work is needed to understand how the existing glaciers may change over time as a result of melt, avalanches and other factors, Kapnick said.

“Our work is an important piece to understanding the Karakoram anomaly,” Kapnick said. “But that balance of what’s coming off the glacier versus what’s coming in also matters for understanding how the glacier will change in the future.”

The paper, “Snowfall less sensitive to warming in Karakoram than in Himalayas due to a unique seasonal cycle,” was published online in-advance-of-print Oct. 12 by Nature Geoscience.

Snail shells show high-rise plateau is much lower than it used to be

This is the Zhada Basin on the southwest Tibetan Plateau, with the Himalayas to the south. -  Joel Saylor
This is the Zhada Basin on the southwest Tibetan Plateau, with the Himalayas to the south. – Joel Saylor

The Tibetan Plateau in south-central Asia, because of its size, elevation and impact on climate, is one of the world’s greatest geological oddities.

At about 960,000 square miles it covers slightly more land area than Alaska, Texas and California combined, and its elevation is on the same scale as Mount Rainier in the Cascade Range of Washington state. Because it rises so high into the atmosphere, it helps bring monsoons over India and other nations to the south while the plateau itself remains generally arid.

For decades, geologists have debated when and how the plateau reached such lofty heights, some 14,000 feet above sea level, about half the elevation of the highest Himalayan peaks just south of the plateau.

But new research led by a University of Washington scientist appears to confirm an earlier improbable finding – at least one large area in southwest Tibet, the plateau’s Zhada Basin, actually lost 3,000 to 5,000 feet of elevation sometime in the Pliocene epoch.

“This basin is really high right now but we think it was a kilometer or more higher just 3 million to 4 million years ago,” said Katharine Huntington, a UW associate professor of Earth and space sciences and the lead author of a paper describing the research.

Co-authors are Joel Saylor of the University of Houston and Jay Quade and Adam Hudson, both of the University of Arizona. The paper was published online in August and will appear in a future print edition of the Geological Society of America Bulletin.

The Zhada Basin has rugged terrain, with exposed deposits of ancient lake and river sediments that make fossil shells of gastropods such as snails easily accessible, and determining their age is relatively straightforward. The researchers studied shells dating from millions of years ago and from a variety of aquatic environments. They also collected modern shell and water samples from a variety of environments for comparison.

The work confirms results of a previous study involving Saylor and Quade that examined the ratio of heavy isotope oxygen-18 to light isotope oxygen-16 in ancient snail shells from the Zhada Basin. They found the ratios were very low, which suggested the basin had a higher elevation in the past.

Oxygen-18 levels decrease in precipitation at higher elevations in comparison with oxygen-16, so shells formed in lakes and rivers that collect precipitation at higher elevations should have a lower heavy-to-light oxygen ratio. However, those lower ratios depend on a number of other factors, including temperature, evaporation and precipitation source, which made it difficult to say with certainty whether the low ratios found in the ancient snail shells meant a loss of elevation in the Zhada Basin.

So the scientists also employed a technique called clumped isotope thermometry, which Huntington has used and worked to refine for several years, to determine the temperature of shell growth and get an independent estimate of elevation change in the basin.

Bonding, or “clumping” together, of heavy carbon-13 and oxygen-18 isotopes in the carbonate of snail shells happens more readily at colder temperatures, and is measured using a tool called a mass spectrometer that provides data on the temperature of the lake or river water in which the snails lived.

The scientists found markedly greater “clumping,” as well as lower ratios of oxygen-18 to oxygen-16 in the ancient shells, indicating the shells formed at temperatures as much as 11 degrees Celsius (20 F) colder than average temperatures today, the equivalent of as much as 5,000 feet of elevation loss.

Just why the elevation decline happened is open to speculation. One possibility is that as faults in the region spread, the Zhada Basin lowered, Huntington said. It is unknown yet whether other parts of the southern plateau also lowered at the same time, but if elevation loss was widespread it could be because of broader fault spreading. It also is possible the crust thickened and forced large rock formations even deeper into the Earth, where they heated until they reached a consistency at which they could ooze out from beneath the crust, like toothpaste squeezed from the tube.

She noted that climate records from deep-sea fossils indicate Earth was significantly warmer when the cold Zhada Basin snail shells were formed.

“Our findings are a conservative estimate,” Huntington said. “No one can say this result is due to a colder climate, because if anything it should have been warmer.”

Asian monsoon much older than previously thought

University of Arizona geoscientist Alexis Licht (bottom left) and his colleagues from the French-Burmese Paleontological Team led by Jean-Jacques Jaeger of the University of Poitiers, France (center with hiking staff) used fossils they collected in Myanmar to figure out that the Asian monsoon started at least 40 million years ago. -  French-Burmese Paleontological Team 2012
University of Arizona geoscientist Alexis Licht (bottom left) and his colleagues from the French-Burmese Paleontological Team led by Jean-Jacques Jaeger of the University of Poitiers, France (center with hiking staff) used fossils they collected in Myanmar to figure out that the Asian monsoon started at least 40 million years ago. – French-Burmese Paleontological Team 2012

The Asian monsoon already existed 40 million years ago during a period of high atmospheric carbon dioxide and warmer temperatures, reports an international research team led by a University of Arizona geoscientist.

Scientists thought the climate pattern known as the Asian monsoon began 22-25 million years ago as a result of the uplift of the Tibetan Plateau and the Himalaya Mountains.

“It is surprising,” said lead author Alexis Licht, now a research associate in the UA department of geosciences. “People thought the monsoon started much later.”

The monsoon, the largest climate system in the world, governs the climate in much of mainland Asia, bringing torrential summer rains and dry winters.

Co-author Jay Quade, a UA professor of geosciences, said, “This research compellingly shows that a strong Asian monsoon system was in place at least by 35-40 million years ago.”

The research by Licht and his colleagues shows the earlier start of the monsoon occurred at a time when atmospheric CO2 was three to four times greater than it is now. The monsoon then weakened 34 million years ago when atmospheric CO2 then decreased by 50 percent and an ice age occurred.

Licht said the study is the first to show the rise of the monsoon is as much a result of global climate as it is a result of topography. The team’s paper is scheduled for early online publication in the journal Nature on Sept. 14.

“This finding has major consequences for the ongoing global warming,” he said. “It suggests increasing the atmospheric CO2 will increase the monsoonal precipitation significantly.”

Unraveling the monsoon’s origins required contributions from three different teams of scientists that were independently studying the environment of 40 million years ago.

All three investigations showed the monsoon climate pattern occurred 15 million years earlier than previously thought. Combining different lines of evidence from different places strengthened the group’s confidence in the finding, Licht said. The climate modeling team also linked the development of the monsoon to the increased CO2 of the time.

Licht and his colleagues at Poitiers and Nancy universities in France examined snail and mammal fossils in Myanmar. The group led by G. Dupont-Nivet and colleagues at Utrecht University in the Netherlands studied lake deposits in Xining Basin in central China. J.-B. Ladant and Y. Donnadieu of the Laboratory of Sciences of the Climate and Environment (LSCE) in Gif-sur-Yvette, France, created climate simulations of the Asian climate 40 million years ago.

A complete list of authors of the group’s publication, “Asian monsoons in a late Eocene greenhouse world,” is at the bottom of this release, as is a list of funding sources.

Licht didn’t set out to study the origin of the monsoon.

He chose his study site in Myanmar because the area was rich in mammal fossils, including some of the earliest ancestors of modern monkeys and apes. The research, part of his doctoral work at the University of Poitiers, focused on understanding the environments those early primates inhabited. Scientists thought those primates had a habitat like the current evergreen tropical rain forests of Borneo, which do not have pronounced differences between wet and dry seasons.

To learn about the past environment, Licht analyzed 40-million-year-old freshwater snail shells and teeth of mammals to see what types of oxygen they contained. The ratio of two different forms of oxygen, oxygen-18 and oxygen-16, shows whether the animal lived in a relatively wet climate or an arid one.

“One of the goals of the study was to document the pre-monsoonal conditions, but what we found were monsoonal conditions,” he said.

To his surprise, the oxygen ratios told an unexpected story: The region had a seasonal pattern very much like the current monsoon – dry winters and very rainy summers.

“The early primates of Myanmar lived under intense seasonal stress – aridity and then monsoons,” he said. “That was completely unexpected.”

The team of researchers working in China found another line of evidence pointing to the existence of the monsoon about 40 million years ago. The monsoon climate pattern generates winter winds that blow dust from central Asia and deposits it in thick piles in China. The researchers found deposits of such dust dating back 41 million years ago, indicating the monsoon had occurred that long ago.

The third team’s climate simulations indicated strong Asian monsoons 40 million years ago. The simulations showed the level of atmospheric CO2 was connected to the strength of the monsoon, which was stronger 40 million years ago when CO2 levels were higher and weakened 34 million years ago when CO2 levels dropped.

Licht’s next step is to investigate how geologically short-term increases of atmospheric CO2 known as hyperthermals affected the monsoon’s behavior 40 million years ago.

“The response of the monsoon to those hyperthermals could provide interesting analogs to the ongoing global warming,” he said.

Geologists prove early Tibetan Plateau was larger than previously thought

This is Syracuse University professor Gregory Hoke. -  Syracuse University
This is Syracuse University professor Gregory Hoke. – Syracuse University

Earth scientists in Syracuse University’s College of Arts and Sciences have determined that the Tibetan Plateau-the world’s largest, highest, and flattest plateau-had a larger initial extent than previously documented.

Their discovery is the subject of an article in the journal Earth and Planetary Science Letters (Elsevier, 2014).

Gregory Hoke, assistant professor of Earth sciences, and Gregory Wissink, a Ph.D. student in his lab, have co-authored the article with Jing Liu-Zeng, director of the Division of Neotectonics and Geomorphology at the Institute for Geology, part of the China Earthquake Administration; Michael Hren, assistant professor of chemistry at the University of Connecticut; and Carmala Garzione, professor and chair of Earth and environmental sciences at the University of Rochester.

“We’ve determined the elevation history of the southeast margin of the Tibetan Plateau,” says Hoke, who specializes in the interplay between the Earth’s tectonic and surface processes. “By the Eocene epoch (approximately 40 million years ago), the southern part of the plateau extended some 600 miles more to the east than previously documented. This discovery upends a popular model for plateau formation.”

Known as the “Roof of the World,” the Tibetan Plateau covers more than 970,000 square miles in Asia and India and reaches heights of over 15,000 feet. The plateau also contains a host of natural resources, including large mineral deposits and tens of thousands of glaciers, and is the headwaters of many major drainage basins.

Hoke says he was attracted to the topography of the plateau’s southeast margin because it presented an opportunity to use information from minerals formed at the Earth’s surface to infer what happened below them in the crust.

“The tectonic and topographic evolution of the southeast margin has been the subject of considerable controversy,” he says. “Our study provides the first quantitative estimate of the past elevation of the eastern portions of the plateau.”

Historically, geologists have thought that lower crustal flow- a process by which hot, ductile rock material flows from high- to low-pressure zones-helped elevate parts of the plateau about 20 million years ago. (This uplift model has also been used to explain watershed reorganization among some of the world’s largest rivers, including the Yangtze in China.)

But years of studying rock and water samples from the plateau have led Hoke to rethink the area’s history. For starters, his data indicates that the plateau has been at or near its present elevation since the Eocene epoch. Moreover, surface uplift in the southernmost part of the plateau-in and around southern China and northern Vietnam-has been historically small.

“Surface uplift, caused by lower crustal flow, doesn’t explain the evolution of regional river networks,” says Hoke, referring to the process by which a river drainage system is diverted, or captured, from its own bed into that of a neighboring bed. “Our study suggests that river capture and drainage reorganization must have been the result of a slip on the major faults bounding the southeast plateau margin.”

Hoke’s discovery not only makes the plateau larger than previously thought, but also suggests that some of the topography is millions of years younger.

“Our data provides the first direct documentation of the magnitude and geographic extent of elevation change on the southeast margin of the Tibetan Plateau, tens of millions years ago,” Hoke adds. “Constraining the age, spatial extent, and magnitude of ancient topography has a profound effect on how we understand the construction of mountain ranges and high plateaus, such as those in Tibet and the Altiplano region in Bolivia.”

Rising mountains dried out Central Asia, scientists say

A record of ancient rainfall teased from long-buried sediments in Mongolia is challenging the popular idea that the arid conditions prevalent in Central Asia today were caused by the ancient uplift of the Himalayas and the Tibetan Plateau.

Instead, Stanford scientists say the formation of two lesser mountain ranges, the Hangay and the Altai, may have been the dominant drivers of climate in the region, leading to the expansion of Asia’s largest desert, the Gobi. The findings will be presented on Thursday, Dec. 12, at the annual meeting of the American Geophysical Union (AGU) in San Francisco.

“These results have major implications for understanding the dominant factors behind modern-day Central Asia’s extremely arid climate and the role of mountain ranges in altering regional climate,” said Page Chamberlain, a professor of environmental Earth system science at Stanford.

Scientists previously thought that the formation of the Himalayan mountain range and the Tibetan plateau around 45 million years ago shaped Asia’s driest environments.

“The traditional explanation has been that the uplift of the Himalayas blocked air from the Indian Ocean from reaching central Asia,” said Jeremy Caves, a doctoral student in Chamberlain’s terrestrial paleoclimate research group who was involved in the study.

This process was thought to have created a distinct rain shadow that led to wetter climates in India and Nepal and drier climates in Central Asia. Similarly, the elevation of the Tibetan Plateau was thought to have triggered an atmospheric process called subsidence, in which a mass of air heated by a high elevation slowly sinks into Central Asia.

“The falling air suppresses convective systems such as thunderstorms, and the result is you get really dry environments,” Caves said.

This long-accepted model of how Central Asia’s arid environments were created mostly ignores, however, the existence of the Altai and Hangay, two northern mountain ranges.

Searching for answers


To investigate the effects of the smaller ranges on the regional climate, Caves and his colleagues from Stanford and Rocky Mountain College in Montana traveled to Mongolia in 2011 and 2012 and collected samples of ancient soil, as well as stream and lake sediments from remote sites in the central, southwestern and western parts of the country.

The team carefully chose its sites by scouring the scientific literature for studies of the region conducted by pioneering researchers in past decades.

“A lot of the papers were by Polish and Russian scientists who went there to look for dinosaur fossils,” said Hari Mix, a doctoral student at Stanford who also participated in the research. “Indeed, at many of the sites we visited, there were dinosaur fossils just lying around.”

The earlier researchers recorded the ages and locations of the rocks they excavated as part of their own investigations; Caves and his team used those age estimates to select the most promising sites for their own study.

At each site, the team bagged sediment samples that were later analyzed to determine their carbon isotope content. The relative level of carbon isotopes present in a soil sample is related to the productivity of plants growing in the soil, which is itself dependent on the annual rainfall. Thus, by measuring carbon isotope amounts from different sediment samples of different ages, the team was able to reconstruct past precipitation levels.

An ancient wet period


The new data suggest that rainfall in central and southwestern Mongolia had decreased by 50 to 90 percent in the last several tens of million of years.

“Right now, precipitation in Mongolia is about 5 inches annually,” Caves said. “To explain our data, rainfall had to decrease from 10 inches a year or more to its current value over the last 10 to 30 million years.”

That means that much of Mongolia and Central Asia were still relatively wet even after the formation of the Himalayas and the Tibetan Plateau 45 million years ago. The data show that it wasn’t until about 30 million years ago, when the Hangay Mountains first formed, that rainfall started to decrease. The region began drying out even faster about 5 million to 10 million years ago, when the Altai Mountains began to rise.

The scientists hypothesize that once they formed, the Hangay and Altai ranges created rain shadows of their own that blocked moisture from entering Central Asia.

“As a result, the northern and western sides of these ranges are wet, while the southern and eastern sides are dry,” Caves said.

The team is not discounting the effect of the Himalayas and the Tibetan Plateau entirely, because portions of the Gobi Desert likely already existed before the Hangay or Altai began forming.

“What these smaller mountains did was expand the Gobi north and west into Mongolia,” Caves said.

The uplift of the Hangay and Altai may have had other, more far-reaching implications as well, Caves said. For example, westerly winds in Asia slam up against the Altai today, creating strong cyclonic winds in the process. Under the right conditions, the cyclones pick up large amounts of dust as they snake across the Gobi Desert. That dust can be lofted across the Pacific Ocean and even reach California, where it serves as microscopic seeds for developing raindrops.

The origins of these cyclonic winds, as well as substantial dust storms in China today, may correlate with uplift of the Altai, Caves said. His team plans to return to Mongolia and Kazakhstan next summer to collect more samples and to use climate models to test whether the Altai are responsible for the start of the large dust storms.

“If the Altai are a key part of regulating Central Asia’s climate, we can go and look for evidence of it in the past,” Caves said.

First evidence that dust and sand deposits in China are controlled by rivers

Northern China holds some of the world's most significant wind-blown dust deposits, known as loess. The origin of this loess-forming dust and its relationship to sand has previously been the subject of considerable debate. -  Royal Holloway University
Northern China holds some of the world’s most significant wind-blown dust deposits, known as loess. The origin of this loess-forming dust and its relationship to sand has previously been the subject of considerable debate. – Royal Holloway University

New research published today in the journal Quaternary Science Reviews has found the first evidence that large rivers control desert sands and dust in Northern China.

Northern China holds some of the world’s most significant wind-blown dust deposits, known as loess. The origin of this loess-forming dust and its relationship to sand has previously been the subject of considerable debate.

The team of researchers led by Royal Holloway University, analysed individual grains of fine wind-blown dust deposited in the Chinese Loess Plateau that has formed thick deposits over the past 2.5 million years. As part of this, they also analysed the Mu Us desert in Inner Mongolia and the Yellow River, one of the world’s longest rivers, to identify links between the dust deposits and nearby deserts and rivers.

The results showed that the Yellow River transports large quantities of sediment from northern Tibet to the Mu Us desert and further suggests that the river contributes a significant volume of material to the Loess Plateau.

“The Yellow River drains the northeast Tibetan plateau and so the uplift of this region and the development of Yellow River drainage seems to control the large scale dust deposits and sand formation in this part of China,” said lead researcher Tom Stevens from the Department of Geography at Royal Holloway.

“Identifying how this dust is formed and controlled is important, since it drives climate change and ocean productivity and impacts human health. Its relationship to the river and Tibet implies strong links between tectonics and climate change. This suggests that global climate change caused by atmospheric dust may be influenced by the uplift of Tibet and changes in major river systems that drain this area.”

Geochemical ‘fingerprints’ leave evidence that megafloods eroded steep gorge

This 2005 image shows a concentration of grains of zircon taken from sand deposits, where it occurs with other heavy minerals such as magnetite and ilmenite. -  U.S. Geological Survey
This 2005 image shows a concentration of grains of zircon taken from sand deposits, where it occurs with other heavy minerals such as magnetite and ilmenite. – U.S. Geological Survey

The Yarlung-Tsangpo River in southern Asia drops rapidly through the Himalaya Mountains on its way to the Bay of Bengal, losing about 7,000 feet of elevation through the precipitously steep Tsangpo Gorge.

For the first time, scientists have direct geochemical evidence that the 150-mile long gorge, possibly the world’s deepest, was the conduit by which megafloods from glacial lakes, perhaps half the volume of Lake Erie, drained suddenly and catastrophically through the Himalayas when their ice dams failed at times during the last 2 million years.

“You would expect that if a three-day long flood occurred, there would be some pretty significant impacts downstream,” said Karl Lang, a University of Washington doctoral candidate in Earth and space sciences.

In this case, the water moved rapidly through bedrock gorge, carving away the base of slopes so steep they already were near the failure threshold. Because the riverbed through the Tsangpo Gorge is essentially bedrock and the slope is so steep and narrow, the deep flood waters could build enormous speed and erosive power.

As the base of the slopes eroded, areas higher on the bedrock hillsides tumbled into the channel, freeing microscopic grains of zircon that were carried out of the gorge by the fast-moving water and deposited downstream.

Uranium-bearing zircon grains carry a sort of geochemical signature for the place where they originated, so grains found downstream can be traced back to the rocks from which they eroded. Lang found that normal annual river flow carries about 40 percent of the grains from the Tsangpo Gorge downstream. But grains from the gorge found in prehistoric megaflood deposits make up as much as 80 percent of the total.

He is the lead author of a paper documenting the work published in the September edition of Geology. Co-authors are Katharine Huntington and David Montgomery, both UW faculty members in Earth and space sciences.

The Yarlung-Tsangpo is the highest major river in the world. It begins on the Tibetan Plateau at about 14,500 feet, or more than 2.5 miles, above sea level. It travels more than 1,700 miles, crossing the plateau and plunging through the Himalayas before reaching India’s Assam Valley, where it becomes the Brahmaputra River. From there it continues its course to the Ganges River delta and the Bay of Benga

At the head of the Tsangpo Gorge, the river makes a sharp bend around Namche Barwa, a 25,500-foot peak that is the eastern anchor of the Himalayas. Evidence indicates that giant lakes were impounded behind glacial dams farther inland from Namche Barwa at various times during the last 2.5 million years ago.

Lang matched zircons in the megaflood deposits far downstream with zircons known to come only from Namche Barwa, and those signature zircons turned up in the flood deposits at a much greater proportion than they would in sediments from normal river flows. Finding the zircons in deposits so far downstream is evidence for the prehistoric megafloods and their role in forming the gorge.

Lang noted that a huge landslide in early 2000 created a giant dam on the Yiggong River, a tributary of the main river just upstream from the Gorge. The dam failed catastrophically in June 2000, triggering a flood that caused numerous fatalities and much property damage downstream.

That provided a vivid, though much smaller, illustration of what likely occurred when large ice dams failed millions of years ago, he said. It also shows the potential danger if humans decide to build dams in that area for hydroelectric generation.

“We are interested in it scientifically, but there is certainly a societal element to it,” Lang said. “This takes us a step beyond speculating what those ancient floods did. There is circumstantial evidence that, yes, they did do a lot of damage.”

The process in the Tsangpo Gorge is similar to what happened with Lake Missoula in Western Montana 12,000 to 15,000 years ago. That lake was more than 10,000 feet lower in elevation than lakes associated with the Tsangpo Gorge, though its water discharge was 10 times greater. Evidence suggests that Lake Missoula’s ice dam failed numerous times, unleashing a torrent equal to half the volume of Lake Michigan across eastern Washington, where it carved the Channeled Scablands before continuing down the Columbia River basin.

“This is a geomorphic process that we know shapes the landscape, and we can look to eastern Washington to see that,” Lang said.

Tibetan Plateau may be older than previously thought

The growth of high topography on the Tibetan Plateau in Sichuan, China, began much earlier than previously thought, according to an international team of geologists who looked at mountain ranges along the eastern edge of the plateau.

The Indian tectonic plate began its collision with Asia between 55 and 50 million years ago, but “significant topographic relief existed adjacent to the Sichuan Basin prior to the Indo-Asian collision,” the researchers report online in Nature Geoscience.

“Most researchers have thought that high topography in eastern Tibet developed during the past 10 to 15 million years, as deep crust beneath the central Tibetan Plateau flowed to the plateau margin, thickening the Earth’s crust in this area and causing surface uplift,” said Eric Kirby, associate professor of geoscience, Penn State. “Our study suggests that high topography began to develop as early as 30 million years ago, and perhaps was present even earlier.”

Kirby, working with Erchie Wang, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, and Kevin Furlong, professor of geosciences, Penn State, and colleagues from Waikato University, New Zealand and Arizona State University, looked at samples taken from the hanging wall of the Yingxiu-Beichuan fault, the primary fault responsible for the 2008, Wenchuan earthquake. The researchers used a variety of methods including the decay rate of uranium and thorium to helium in the minerals apatite and zircon and fission track dating, an analysis of tracks or trails left by decaying uranium in minerals again in apatite and zircon.

“These methods allow us to investigate the thermal regime from about 250 degrees Celsius (482 degrees Fahrenheit) to about 60 degrees (140 degrees Fahrenheit),” said Kirby. “The results show that the rocks cooled relatively slowly during the early and mid-Cenozoic — from 30 to 50 million years ago — an indication that topography in the region was undergoing erosion.”

The results also suggest that gradual cooling during this time was followed by two episodes of rapid erosion, one beginning 30 to 25 million years ago and one beginning 15 to 10 million years ago that continues today.

“These results challenge the idea that the topographic relief along the margin of the plateau developed entirely in the Late Miocene, 5 to 10 million years ago,” said Kirby. “The period of rapid erosion between 25 to 30 million years ago could only be sustained if the mountains were not only present, but actively growing, at this time.”

The researchers also note that this implies that fault systems responsible for the 2008 earthquake were also probably active early in the history of the growth of the Tibetan Plateau.

“We are still a long way from completely understanding when and how high topography in Asia developed in response to India-Asia collision,” notes Kirby. “However, these results lend support to the idea that much of what we see today in the mountains of China may have developed earlier than we previously thought.”

Heat and moisture from Himalayas could be a key cause of the South Asian monsoon

Harvard climate scientists suggest that the Tibetan Plateau-thought to be the primary source of heat that drives the South Asian monsoon-may have far less of an effect than the Himalayas and other surrounding mountains. As the monsoon brings needed rainfall and water to billions of people each year, understanding its proper origin, especially in the context of global climate change, is crucial for the future sustainability of the region.

The researchers say the their findings, published in the January 14th issue of Nature, have broad implications for how the Asian climate may have responded to mountain uplift in the past, and for how it might respond to surface changes in the coming decades.

Often called the “roof of the world,” the Tibetan Plateau is a vast area of 2.5 million square kilometers with an average elevation of more than 4,500 meters. Scientists have long theorized that the massive release of heat from the surface of the plateau-with air being heated to higher temperatures over the plateau than air at the same height over lower-level surfaces nearby-has been a major contributor to the strength of the monsoon.

“The South Asian monsoon supplies water to billions of people, many of whom live in developing nations and agricultural societies that are highly vulnerable to variations in this water supply,” explains co-author Zhiming Kuang, Assistant Professor of Climate Science in Harvard’s School of Engineering and Applied Sciences (SEAS) and Department of Earth and Planetary Sciences (EPS).

While the heating by the plateau does enhance rainfall along its southern edge, Kuang and his colleague William Boos, Daly Postdoctoral Fellow in EPS and an environmental fellow at the Harvard University Center for the Environment (HUCE), used an atmospheric circulation model to show that the large-scale South Asian summer monsoon circulation remains unaffected when the plateau is removed.

It turns out that the narrow geography of the Himalayas and other nearby mountain ranges can, in fact, produce an equally strong monsoon by insulating warm, moist air over continental India from the cold dry extratropics, the area between the subtropics and polar regions.

“Because heat from the plateau has been seen as the main contributor to the power of the monsoon, much attention has been given to changes in the plateau’s albedo, or its reflectivity level of the sun’s radiation,” says Kuang.

For example, a decrease in snow cover over the Tibetan Plateau resulting from an increase in global temperatures can affect reflectivity, and hence, the level of heat. The revised theory, emphasizing the important role the mountains play in trapping warm and moist air, suggests that climate scientists should pay as much attention to changes over the Indian subcontinent due to, for example, land use.

How the region’s natural environment is modified through activities such as building, mining, and agriculture, Zhang explains, can influence albedo and moisture, thus altering the temperature/humidity of the boundary layer air.

By considering the influence of both the plateau and the mountains on the strength of the monsoon, the Harvard researchers expect a clearer picture will emerge about the potential changes in the South Asian water supply in the coming decades.

“Ultimately, our revised view has implications for future projections of how the South Asian monsoon might be altered in a warmer world and can be used to infer aspects about the earth’s climate history,” says Boos.