Study indicates groundwater sapping led to desertification of parts of Inner Mongolia

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Geographical location of the Hunshandake Sandy Lands. Credit: PNAS, doi: 10.1073/pnas.1418090112

Study indicates groundwater sapping led to desertification of parts of Inner Mongolia
(Phys.org)—A combined team of American and Chinese researchers has
found evidence that suggests that parts of Inner Mongolia that were
thought to have been desert for a million years, might have actually
dried up just over four thousand years ago. In their paper published in Proceedings of the National Academy of Sciences,
the team describes their research efforts, their findings and why what
they have uncovered might lead to rethinking much of early Chinese
history.

Most historians believe that the middle portion of the Yellow river was the cradle of Chinese civilization, despite evidence of a prior civilization
living farther to the north in Inner Mongolia. Until now, details of
that earlier civilization, known as the Hongshan Kingdom, (which was
established approximately 6,500 years ago) was sparse, as most assumed
it did not amount to much because it existed in a desert. But evidence
uncovered by this new team of researchers suggests the area was not
desert during that time period after all—it became that way thousands of
years later, leading the people living there little choice but to
emigrate south, quite possibly contributing to the rise of the Xia
Dynasty, the first in China.

To come to these conclusions, the team conducted many field studies,
including excavations of new artifacts in the region. They found
examples of pottery and other stone artifacts that suggested a culture
far more advanced than had been thought—one that was also bigger and
dependant on hunting and fishing, which is not typically the case for
desert dwellers. The team also studied the environmental landscape, and
found that the sand dunes in the area were likely shaped by terrain that
was formed by rivers and lakes. A closer look at quartz dug from the
area showed it had been buried beneath the sand for just 12,000 years, a
sign that up till that time, the area had been a lakeshore. Next, the
team dug down and found sediments from a long dry lake which existed
between 5,000 and 9,000 years ago—pollen in it revealed the presence of
trees in the area.

All of the evidence combined suggested the area was quite fertile
until approximately 4,200 years ago, when desertification set in. The
team suggests the area became drier as water that once flowed into the
area was diverted for some unknown reason to a river flowing east,
forcing the people living there to flee south.

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More information: Groundwater sapping as the cause of
irreversible desertification of Hunshandake Sandy Lands, Inner Mongolia,
northern China, Xiaoping Yang, PNAS, DOI: 10.1073/pnas.1418090112

Abstract

In the middle-to-late Holocene, Earth’s monsoonal regions
experienced catastrophic precipitation decreases that produced green to
desert state shifts. Resulting hydrologic regime change negatively
impacted water availability and Neolithic cultures. Whereas mid-Holocene
drying is commonly attributed to slow insolation reduction and
subsequent nonlinear vegetation–atmosphere feedbacks that produce
threshold conditions, evidence of trigger events initiating state
switching has remained elusive. Here we document a threshold event ca.
4,200 years ago in the Hunshandake Sandy Lands of Inner Mongolia,
northern China, associated with groundwater capture by the Xilamulun
River. This process initiated a sudden and irreversible region-wide
hydrologic event that exacerbated the desertification of the
Hunshandake, resulting in post-Humid Period mass migration of northern
China’s Neolithic cultures. The Hunshandake remains arid and is
unlikely, even with massive rehabilitation efforts, to revert back to
green conditions.

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