15 December 2016
Ancient monsoons helped prehistoric humans colonize Tibetan Plateau
Posted by dgaristo
By Emma Hiolski
Humans first colonized the Tibetan Plateau roughly 10,000 years ago when the climate warmed enough to bring heavy rains, new research finds.
A new study of sediments and archaeological artifacts in the Tibetan Plateau reveals a detailed record of monsoons that likely promoted human habitation in the region at the beginning of the Holocene, the present geological epoch.
The new study both helps scientists pinpoint when humans first settled the region but also provides a new take on how prehistoric humans dealt with changing climate in a harsh environment, according to the authors.
“We don’t really see evidence of people coming up onto the plateau until the Holocene,” said Adam Hudson, a geologist with the USGS Global Environmental Change Science Center in Lakewood, Colorado, and lead author of the new study. “So it’s coincident with these warm climate conditions—a significant climate change—that made conditions more favorable for people to be able to move up onto the Tibetan Plateau.”
The Tibetan Plateau is an immense elevated plateau spanning 2.5 million square kilometers (970,000 square miles) in Central and East Asia, surrounded by massive mountains. At an average elevation of 4,500 meters (about 15,000 feet), the Tibetan Plateau has extensive glacier coverage and feeds several rivers critical to the region, including the Yellow, Indus, Ganges and Brahmaputra rivers. The plateau is known as “the water tower of Asia.”
Prior to the Holocene, which began roughly 10,000 years ago, the Tibetan Plateau was entirely covered by glaciers. After the glaciers retreated, the area became more accessible and habitable. However, scientists didn’t have a clear picture of what the climate was like or when exactly humans began populating the region.
In the new study, Hudson worked with a team of scientists from the U.S. and China to find out what the climate of the plateau was like before and during the Holocene.
The researchers sampled the layered sediments of several outcroppings in the Yarlung Tsangpo River Valley in the southwestern portion of the Tibetan Plateau. They identified organic-rich layers—blackened sediments indicating heavy plant growth and decay—and used radiocarbon dating to determine how old the layers were.
Stone and ceramic artifacts embedded in these layers indicated evidence of human activity within those Holocene timeframes. Hudson presented the results of his paleoclimate and archaeologic analyses Tuesday at the 2016 American Geophysical Union Fall Meeting in San Francisco.
The team found the blackened organic layers dated to the early and middle Holocene, from about 10,000 to 4,000 years ago. These layers correspond to a heavy monsoon climate, meaning the plateau featured more extensive wetlands, a higher water table and warmer temperatures then it has today, Hudson said. Beneath those deposits, an older gravel layer indicated a dry, desert environment prior to the Holocene. Higher, more recent deposits depict a transition from wetlands to small ponds and eventually to dry dune deposits about 1,000 years ago that extend up to the modern soil surface.
The appearance of ancient human artifacts—shards of ceramic pottery, stones with shards knapped off—within the wetland layers provided a clearer picture of when humans first migrated to the Yarlung Tsangpo valley. According to the new study, the southwestern area of the plateau was populated more recently (6,500 to 1,300 years ago) than the northeastern portion (9,000 to 8,500 years ago), though it isn’t clear whether humans migrated across the plateau from the northwest or from another region.
Taken together, the paleoclimate and archaeological data can illuminate how human populations responded to the major climate shifts of that epoch, according to Hudson.
“It is our hypothesis that it isn’t a coincidence that it took until the end of the last ice age for people to move onto the plateau full time,” he said. “This is a clear example—one of many, we are finding—of a situation in which poor climate conditions couldn’t be overcome even by the sophisticated behavioral adaptations of prehistoric humans.”
Hudson wants to expand his analyses to other sites across the Tibetan Plateau and deeper below the surface. He plans to work with local museums in Lhasa, Tibet’s capital, to do excavations and a more systematic study of ancient artifacts to further understanding of prehistoric humans’ migration and how different populations interacted with one another.
—Emma Hiolski is a graduate student at the UC Santa Cruz Science Communication Program. You can read more of her work at her website or on Twitter at @EHiolski.