China News Service, Tibet, Ali, August 11 (Reporter Sun Zifa) Like a sapphire blue pearl inlaid in Pangong Lake (also known as Pangong Tso) in western Tibet, China, not only the beautiful scenery attracts tourists, but also as the western part of the Qinghai-Tibet Plateau. The largest lake, the tectonic causes of Pangong Lake, changes in water temperature and water level, the acidity and saltiness of the lake water, and the impact on the surrounding ancient and modern humans have also attracted scientists' attention and continued research for a long time.
China's second Qinghai-Tibet Plateau Comprehensive Scientific Expedition Research (Qinghai-Tibet Scientific Expedition) "History of Human Activities and Its Influence" team, which will carry out the scientific expedition mission in the summer of 2022, recently went to Pangong Lake to carry out the "Comprehensive investigation of the relationship between people and land on the Qinghai-Tibet Plateau". .
The leader of this scientific expedition team and researcher Hou Juzhi of the Institute of Qinghai-Tibet Plateau Research, Chinese Academy of Sciences has long been engaged in the research on the ecological environment of lakes on the Qinghai-Tibet Plateau. The lake "has a soft spot".
Typical biseasonal convective mixed lake
What kind of lake is Pangong Lake?
Hou Juzhi said that Pangong Lake is located at the junction of Ritu County and Kashmir region in Ali Prefecture, Tibet Autonomous Region of China.
Pangong Lake is an east-west lake with a total area of 604 square kilometers and a maximum water depth of 41.7 meters.
The average annual precipitation in the Pangong Lake area is 87 mm, mainly in summer, the evaporation is 2465 mm, and the annual average temperature is 2 °C.
The lake water in the eastern basin of Pangong Lake is mainly supplied by glacial meltwater. The three main rivers entering the lake are Domaqu (Wujiang) in the north of the Qinghai-Tibet Plateau, Angmaiqu in the east, and Maga Zangbo in the south.
He pointed out that the monitoring data of lake water temperature in the past ten years shows that Pangong Lake is a typical bi-seasonal convective mixed lake. The lake water turns over in spring and autumn as a whole, the lake water is stratified in summer, and the surface lake water freezes in winter. From November to April of the following year, the lake surface is completely frozen, and the temperature of the lake water is close to the lowest value of the year (2°C). The time gradually increased from 4°C to 8°C; during the lake water stratification period (July-October), the lake water was distributed in a thermosphere, the water temperature decreased with the increase of depth, and the temperature range of the lake surface water temperature was 12.9°C-16.9°C; During the overturning period (November), the lake water undergoes vertical convection and mixing. At the same time, the temperature of the lake water at different layers is the same. During this period, the overall temperature of the water body gradually decreases, from 8°C to 2°C.
Since 16,000 years, the annual average temperature in the lake area has changed by more than 3°C
Hou Juzhi said that in June 2011, the scientific research team he led used an advanced sampling platform to drill the core samples of the lake in the southeast of Pangong Lake.
Dr. Wang Mingda (now an associate professor at Liaoning Normal University) of the scientific research team used biomarker compound indicators to reconstruct the annual average temperature and precipitation isotopic changes in the region over the past 16,000 years.
The reconstruction results show that, since 16,000 years, the annual average temperature in the Bangong Lake area has changed greatly, with a change range of more than 3 °C.
During the deglacial period-early Holocene (about 16,000 to 8,000 years ago), the temperature fluctuated significantly, ranging from 0.8 to 3.6 °C.
The most significant feature of the reconstructed annual average temperature of Pangong Lake is that the temperature of the Middle Holocene (about 8000-4000 years ago) was about 1.5°C lower than that of the Early Holocene and the Late Holocene. Contradicted.
Since the late Holocene (about 4000 years ago), the temperature has gradually recovered, and the average annual temperature has risen to 3.5 °C.
The record of leaf wax fatty acid hydrogen isotope (leaf wax hydrogen isotope) in Pangong Lake shows that the isotope variation range has exceeded 100‰ since 16,000 years, indicating that the hydrological conditions in this area have undergone drastic changes.
During the last deglacial period (about 16,000-12,000 years ago), the wet and dry fluctuations in the Pangong Lake area were obvious; in the Early-Middle Holocene (about 12,000-4,000 years ago), there was no significant occurrence of leaf wax hydrogen isotopes. Changes, the amplitude is less than 20‰, the isotope is slightly positive around 8000 years ago, which may indicate the occurrence of drought events; since the late Holocene, the hydrogen isotope of leaf wax has been gradually positive, indicating that the climate has become arid.
Researcher Hou Juzhi pointed out that compared with other paleoclimate reconstruction records, it was found that there may be differences in the temperature and precipitation isotopic driving factors of Pangong Lake over the past 16,000 years.
The reconstructed annual average temperature of the lake area is mainly controlled by the change of the average annual solar radiation. Greenhouse gases and glacial activities may also affect the temperature of the lake area at different stages. The reconstructed precipitation isotope changes are similar to the stalagmite oxygen isotope trends in the monsoon area, which basically reflect the changes in the monsoon intensity. The latter is mainly controlled by the summer solar radiation in the northern hemisphere.
The lake level has increased by about 1 meter in the last 5 years
Hou Juzhi said that the latest research shows that from 2016 to 2022, the water level of Pangong Lake will fluctuate and rise, and the lake surface elevation will increase by about 1 meter.
In response to the brackishness and distribution of the lake, he pointed out that because the middle part of Bangong Lake is narrow and shallow, and the water depth is less than 5 meters, the communication between the east and the west of the lake is not smooth, and the lower water of the lake is independent of each other. The eastern 2/3 of the public lake is a freshwater lake, and the western 1/3 is a saltwater lake.
According to the field observation by their scientific research team on July 27, 2012, the thermocline of Pangong Lake appeared at a water depth of 12-26 meters, the lake salinity was 0.47 grams per liter and the vertical profile was basically unchanged; up to 14 meters.
Hou Juzhi said that the earlier research on Pangong Lake began in the 1980s. The Sino-French joint expedition in Pangong Lake based on the oxygen isotope research in the center of the lake believed that Pangong Co was mainly affected by the monsoon in the Holocene, but in the distance The sudden increase in oxygen isotopes in 2000 may be the result of the lake changing from an open lake to a closed lake.
In 2011, their scientific research team sampled and studied in the center of Pangong Lake, which also confirmed the previous research results.
At the same time, Pangong Lake is also the first lake with low temperature in the Middle Holocene and relatively high temperature in the early and late Holocene using glycerol tetraether lipids (GDGTs). This phenomenon has also been reconstructed by several other lakes in the Qinghai-Tibet Plateau. However, the specific causes of this phenomenon still need further research.
Hou Juzhi said that, including issues such as Holocene temperature changes and direct dating of ancient lake shorelines, there are still many mysteries in Pangong Lake that need to be solved urgently. , the relevant scientific research team of the Qinghai-Tibet Plateau Institute of the Chinese Academy of Sciences, etc., will further increase the comprehensive scientific investigation, sampling and comparative analysis of ancient and modern research work on Bangong Lake in the follow-up.
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