Xia and other scientists found that Lop Nur was not a "wandering lake" formed by large-scale wind erosion on the ground, and its water body change was controlled by the change of incoming water lake system.
According to Xia, from the altitude point of view, Lop Nur and Kalashun in the south of it are both small depressions on the plain, and Lop Nur is lower.
"The lowest point of Lop Nur is 778 meters, and the lowest point of Kara Heshun Lake adjacent to Lop Nur is 788 meters, with a difference of 10 meter. The water flows downwards, and it is unlikely that Lop Nur will flow back to Kalashun. " The expedition led by Xia also saw that the dry bottom of the lake is a hard salt shell, which is difficult to break with a hammer. "Wind erosion is not easy to lower the bottom of the lake again."
According to Sven Hedin's forecast, sediments with a height exceeding 100 meters will be formed around 1500 years. However, the results of borehole sampling dating at the bottom of the lake show that the depth of sediments at the bottom of the lake is 1.5 meters, which was deposited 3600 years ago. Moreover, the sediments contain pollen of Typha Typha and Cyperaceae, and the pollen of these aquatic plants is distributed in different degrees. Xia believes that this shows that in the past 10 thousand years, Lop Nur has often had water deposits.
Generally, the water flows into Kala Heshun first, and finally reaches the destination Lop Nur. Kara and Heshun Lake are also freshwater lakes, not terminal lakes.
Therefore, "Lop Nur is a north-south wandering lake, which is not realistic. In the historical period, the water body of Lop Nur has never been poured into Kara and Heshun Lake. " Summer said to.
On the spot, Lop Nur and its surrounding areas are wide and shallow depressions with little height difference. In the autumn of 2003, Xia led a scientific research team to measure a 50-kilometer-long horizontal line at the bottom of the lake, with a maximum elevation difference of only 3.02 meters.
Due to frequent changes and diversions in the lower reaches of Tarim River and Peacock River, the position, size and shape of Lop Nur, the terminal lake, will change greatly.
Why did Lop Nur's "big ears" form?
"The shape of the dry lake basin in Lop Nur and the images taken on satellites are very similar to the outline of human ears, so they are called' big ears'. How this big ear was formed, scientists have different opinions and debates.
Xia participated in the desert investigation team of Chinese Academy of Sciences.1When he visited the United States in May, 1980, he was a guest at Albaz, a remote sensing expert in Washington, and saw this picture of "big ears" hanging in Al's house. Ayrton pointed to "helix", "earlobe" and "ear hole" and asked what they represented in summer. Xia Sung Hoon couldn't answer for a moment, but he told Al, "I'll tell you later."
By nesting the "big ears" on the topographic map with topographic elevation, Xia found that the scope of the "big ears" is exactly the contour line of Lop Nur at an altitude of 780 meters, with a measuring area of 5,350 square kilometers.
"We finally gave the big ear's answer:' spiral' is the trace of lake water shrinkage and evaporation; Ear hole' is a peninsula extending into the lake, which divides Lop Nur into two lakes; The' earlobe' is a delta formed by Kara and Shunhu pouring into Lop Nur. "After comprehensive analysis of leveling, spectral measurement, profile sampling and analysis, the scientific researchers of Xiahe Project Team have further gained a new understanding of the" big ears "in Lop Nur: the formation of the" big ears "in Lop Nur is controlled by the original lake shore topography, especially by the peninsula extending into the lake; The "spiral line" in the image of "big ear" is the annual and seasonal law line in the formation of shrinking salt shells in lakes.
Xia and other scientists determined that the "big ears" were formed by the rapid retreat of lakes, and the specific time was between 4 and 5 years in the early 1960s.
What makes Loulan's ancient country no longer prosperous?
190 1 year, Sven Hedin discovered the ancient city of Loulan in the north of Lop Nur with the help of local tour guides. This time, "the desert reappears Pompeii" caused a sensation in the world. Chinese and foreign scholars believe that the ancient Loulan country once flourished on the Silk Road, and the ancient city of Loulan is the most important historical relic discovered at present. The former hub of politics, economy and transportation in the western regions has now become a wasteland. There are many reasons why Loulan has changed so much.
The dry climate leads to a large number of population migration; Alpine glaciers are shrinking, and the decrease of river water volume causes the decline of the ancient city; The change of rivers makes residents migrate and Loulan is abandoned ... It has also been suggested that human activities are the main factor causing environmental changes in Lop Nur. "We believe that the rise and fall of Loulan is a comprehensive reflection of changes in socio-economic and natural conditions." Xia believes that a single factor cannot fully reveal the reasons for the rise and fall of Loulan.
"The road breaks the city's empty space" and "the water breaks the city's empty space" are two major factors for the rise and fall of Loulan. Xia and other experts believe that the change of traffic route is the most direct and sensitive factor for the rise and fall of Loulan. During the Western Han Dynasty, Loulan became an important transportation hub in the western regions, shouldering the heavy responsibility of "transporting water and grain to meet the Chinese envoys". In order to protect this passage, officials reclaimed wasteland, which brought the prosperity of Loulan and became the gateway of the ancient Silk Road. However, after the road at the southern foot of Tianshan Mountain gradually replaced the road passing through Loulan, it unconsciously lost its position as a relay station for Chinese and Western traffic, because the road was broken and the city was empty.
Another reason is that the ancient city of Loulan is located in the lower reaches of Peacock River, and the water system has changed greatly, which has a great impact on people's production and life. In order to live, people have to find another place to live, that is, "water breaks the city."
"For a century, Lop Nur has changed from a vast lake to a dry' big ear'. The vicissitudes it has experienced have left us with a history of the development of arid areas. " Summer, who has been engaged in desert research for 50 years, began to think about the future of Lop Nur.
Lop Nur finally dried up at 1965.
1, 3700-2500 years ago, an important drought event occurred in the evolution of Lop Nur. At that time, both East Lake and West Lake dried up, and East Lake began to dry up from this time until AD 1958.
2.2500 to 1200 years ago, the climate improved, the West Lake rose, and the lake surface was once expanded, covering an area of about 2,000 square kilometers, making it a salty lake.
3. 1200-700 years ago, Lop Nur had the best climate and environment-the medieval warm period, and it was a freshwater lake-saltwater lake with an area of 2,500 square kilometers.
4. Starting from 700 years ago, the lake gradually dried up, and once expanded around 1930. West Lake area 1.900 square kilometers. 1.958 In the summer, Tianshan Mountain was flooded, and the east and west lakes were flooded at the same time, forming a lake of 5,350 square kilometers, but it dried up in 1.965.