Nav: Home

Stanford researchers capture Central Asia's 'de-greening' over millions of years

October 13, 2016

A new study chronicles how central Asia dried out over the last 23 million years into one of the most arid regions on the planet. The findings illustrate the dramatic climatic shifts wrought by the ponderous rise of new mountain ranges over geologic time.

Researchers have long cited the uplift of the Tibetan Plateau and the Himalayan Mountains around 50 million years ago for blocking rain clouds' entry into central Asia from the south, killing off much of the region's plant life.

The new study, published online in the journal Geology, paints a more nuanced picture of Central Asia's desertification. It suggests that the relatively recent rise of lesser-known mountain ranges, such as the Tian Shan and the Altai, further sealed off moisture from the west and north. As a result, great stretches of what we now consider western China, southwestern Mongolia and eastern Tajikistan became barren earth or laced by sand dunes.

"While Central Asia was probably never lush and verdant, it was certainly greener 23 million years ago and probably even greener in the more distant past," said Jeremy Kesner Caves, the lead author of the study and a doctoral student at Stanford's School of Earth, Energy & Environmental Sciences.

"One way to think about this change is that when viewed from space today, Central Asia appears very brown because of its expansive deserts," Caves said. "If viewed from space 23 million years ago, though, Central Asia would have looked somewhat darker, simply due to there being considerably more leaves and vegetation."

Reading carbon

Caves and his co-authors arrived at their conclusions after measuring the carbon isotope values in buried, ancient soil samples. A particular isotope, or version, of carbon found in the samples speaks to the dryness of conditions at the time of the soil's deposition. Wetter, rainier conditions allow for greater numbers of organisms, including plants and soil-dwelling bacteria, to thrive and pull carbon out of their surroundings to fuel their growth and metabolism, leaving telltale carbon isotopes in their environment.

Previously, scientists had relied on these sorts of soil sample measurements primarily to study plant types and atmospheric carbon dioxide levels. Caves and colleagues instead looked at samples over extensive geographic and temporal spans in order to draw a fuller portrait of the climatic changes influencing soil composition.

"Our paper is the first-ever attempt to present maps of carbon isotopes over a geologic time frame of more than a million years," Caves said.

He and several co-authors traveled to Mongolia, eastern Kazakhstan and northern China to collect the bulk of 171 new soil samples, while Russian co-authors collected samples near Lake Baikal. The new specimens were considered alongside more than 2,200 previously collected samples. Because most of those existing samples originated from the Tibetan Plateau, the research team plugged a gap in the geographical coverage by going to little-studied northern central Asia.

The samples themselves "are honestly pretty boring," Caves admitted. "Basically, they look and feel like dirt." But the rocky outcrops exposing the old, hardened soil chunks can dazzle. "The outcrops are striped deep purple, red and green, and they often erode in crazy patterns," Caves said. "Imagine Badlands National Park in South Dakota or the Painted Desert in Arizona."

Asia's de-greening

Overall, the samples were well-distributed from 23 million to 2.6 million years ago during a geological period known as the Neogene. The Earth's climate cooled off substantially as the Neogene wore on, setting the stage for an Ice Age when glaciers crept from polar regions into lower latitudes.

Upon analysis, the samples' carbon isotope values revealed an exceptionally arid region deep in Asia's interior going back 23 million years, initially ringed by areas of higher rainfall. Starting about five million years ago, however, that dry region expanded to the north and west, as new mountain ranges reached heights sufficient to block westerly winds from delivering moisture.

The findings will help researchers disentangle how much of Central Asia's de-greening occurred in response to localized geological changes versus global shifts happening during the Neogene.

Investigating North America

With this compelling demonstration of using ancient soil samples as proxies for regional climate in Asia, Caves now plans to extend his investigations elsewhere on the globe. "I hope to be able to apply this method to other continents, such as North America, where there are large datasets of carbon isotopes," Caves said.

Doing so could illuminate impacts on western North America's climate due to the uplift of the Sierra Nevada in eastern California, as well as the Rocky Mountains further east, which reached near their present elevations around 40 million and 50 million years ago, respectively.

"Only by making these continental-scale maps, like Jeremy has done for Central Asia, can you further understand how the uplift of mountain ranges controlled rainfall patterns against this backdrop of global cooling in the Neogene," said Page Chamberlain, co-author of the study and a professor of Earth system science at Stanford. "North America is really ripe for this kind of research."
Other Stanford co-authors on the study include graduate student Daniel Ibarra. Page Chamberlain is also a member of Stanford Bio-X and an affiliate of the Stanford Woods Institute for the Environment. Other co-authors are from Dartmouth College, the Institute of Zoology in Kazakhstan, the China University of Geosciences, Northwestern University, L.N. Gumilyov Eurasian National University in Kazakhstan, and the Institute of the Earth's Crust in Russia.

Funding was provided by the National Science Foundation, the Stanford University McGee Grants, the Geological Society of America, and the National Basic Research Program of China.

Stanford's School of Earth, Energy & Environmental Sciences

Related Tibetan Plateau Articles:

New perspective: Vegetation phenology variability based on tibetan plateau tree-ring data
Recently, a research group headed by Prof. YANG Bao from the Key Laboratory of Desert and Desertification, Northwest Institute of Eco-Environment and Resources of the Chinese Academy of Sciences, together with coauthors from Russia, Germany, Canada and Sweden, has reconciled these conflicting results based on a 55-year series of vegetation phenology for the TP derived from well-validated process-based Vaganov-Shashkin model (V-S) simulations of tree-ring growth data.
Tibetan Alpine grasslands threatened by climate warming
The researchers from Northwest Institute of Plateau Biology of the Chinese Academy of Sciences found that the stability of the grasslands was affected not by the richness of plant species, but by the effects on dominant species and the asynchrony of the species.
Rising temperatures threaten stability of Tibetan alpine grasslands
A warming climate could affect the stability of alpine grasslands in Asia's Tibetan Plateau, threatening the ability of farmers and herders to maintain the animals that are key to their existence, and potentially upsetting the ecology of an area in which important regional river systems originate, says a new study by researchers in China and the United States.
Microphysical differences in precipitation between Tibet and southern China
Studies of raindrop size distribution (DSD) over different regions helps to advance our understanding of DSD characteristics and provide observational facts regarding the development and evaluation of microphysical parameterization schemes in numerical models over different regions in the future.
Tibetan people have multiple adaptations for life at high altitudes
The Tibetan people have inherited variants of five different genes that help them live at high altitudes, with one gene originating in the extinct human subspecies, the Denisovans.
Mechanism of the influence of the Tibetan-Iranian Plateaus on the circulation and climate in summer
The Iranian-Tibetan Plateaus have both dynamic and thermal influences on Asian climate and global circulation.
The initial collision between Indian and Asian continental
A recent research reveals that India-Eurasia continental collided first in central Tibet at about 65 Ma (SCES, No.3, 2017).
Bursting the bubble: Solution to the Kirchhoff-Plateau problem
Researchers solve a mathematical problem illustrated by soap films spanning flexible loops.
Why did rainfall over Asian inland plateau region undergo abrupt decrease around 1999?
The Asian inland plateau (AIP) is located in the East Asian monsoon marginal areas and mainly includes Mongolia and part of northern China.
Humans occupied tibetan plateau thousands of years earlier than previously thought
A new analysis of an archeological site in the high mountains of Tibet suggests that permanent residents may have set up camp thousands of years sooner than previously thought.

Related Tibetan Plateau Reading:

Best Science Podcasts 2019

We have hand picked the best science podcasts for 2019. Sit back and enjoy new science podcasts updated daily from your favorite science news services and scientists.
Now Playing: TED Radio Hour

Do animals grieve? Do they have language or consciousness? For a long time, scientists resisted the urge to look for human qualities in animals. This hour, TED speakers explore how that is changing. Guests include biological anthropologist Barbara King, dolphin researcher Denise Herzing, primatologist Frans de Waal, and ecologist Carl Safina.
Now Playing: Science for the People

#SB2 2019 Science Birthday Minisode: Mary Golda Ross
Our second annual Science Birthday is here, and this year we celebrate the wonderful Mary Golda Ross, born 9 August 1908. She died in 2008 at age 99, but left a lasting mark on the science of rocketry and space exploration as an early woman in engineering, and one of the first Native Americans in engineering. Join Rachelle and Bethany for this very special birthday minisode celebrating Mary and her achievements. Thanks to our Patreons who make this show possible! Read more about Mary G. Ross: Interview with Mary Ross on Lash Publications International, by Laurel Sheppard Meet Mary Golda...