December 11, 2013
Lesser Known Asian Mountain Ranges Led To Expansion Of Gobi Desert
Lee Rannals for redOrbit.com - Your Universe Online
Researchers say two lesser known mountain ranges in Central Asia helped lead to the arid conditions that regions like the Gobi Desert face today.
Scientists speaking at the annual meeting of the American Geophysical Union in San Francisco say the formation of the Hangay and the Altai mountain ranges may have led to the expansion of Asia’s largest desert.
"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," Page Chamberlain, a professor of environmental Earth system science at Stanford University, said in a statement.
Previous studies showed the formation of the Himalayan mountain range and the Tibetan Plateau 45 million years ago led to the creation of some of Asia’s driest environments. However, the latest study challenges this past theory.
"The traditional explanation has been that the uplift of the Himalayas blocked air from the Indian Ocean from reaching central Asia," Jeremy Caves, a doctoral student in Chamberlain's terrestrial paleoclimate research group who was involved in the study, said in a statement.
Scientists thought the rise of the two main mountain ranges create a distinct rain shadow that led to wetter climates in India and Nepal and drier climates in Central Asia. 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. This long-accepted model mostly ignores the existence of the Altai and Hangay mountain ranges.
The team, from Stanford and Rocky Mountain College in Montana, traveled to Mongolia in 2011 and 2012 to collect samples of ancient soil, as well as stream and lake sediments from remote sites in Asia. They chose sites by looking through scientific literature for studies of the region conducted by researchers in the past decades.
"A lot of the papers were by Polish and Russian scientists who went there to look for dinosaur fossils," Hari Mix, a doctoral student at Stanford who also participated in the research, said in a statement. "Indeed, at many of the sites we visited, there were dinosaur fossils just lying around."
In the past, researchers recorded the ages and locations of the rocks they excavated as part of their own investigations. The team used those age estimates to select the most promising sites for their study.
The team measured carbon isotope amounts from different sediment samples of different ages, allowing them to reconstruct past precipitation levels. The new data shows rainfall in central and southwestern Mongolia had decreased by 50 to 90 percent over the last several tens of millions 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."
This new estimation means 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 scientists believe once the Hangay and Altai ranges formed, it created rain shadows 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 said they are not discounting the effect of the Himalayas and the Tibetan Plateau entirely because portions of the Gobi Desert likely already existed before the Hangay and Altai began forming.
"What these smaller mountains did was expand the Gobi north and west into Mongolia," Caves said.