The Yellow River has an exact birthday! Groundwater "tells" us the Yellow River's "birth story" →

Huang He finally has an accurate birthday.

Chinese scientists including Pang Zhonghe, a researcher at the Institute of Geology and Geophysics of the Chinese Academy of Sciences, conducted research on the origin of the Yellow River and the formation period of the modern Yellow River system. Through precise dating of groundwater, they concluded that the formation period of the modern Yellow River system was 1.25 million years ago, which changed the previous situation of various opinions.

Image source: pixabay Recently, the research results were published online in Earth and Planetary Science Letters.

Combining two dating methods

It is reported that the ancient Yellow River flows eastward from Baoji Gorge to Sanmenxia and then flows into the sea, while the modern Yellow River flows northward from Liupan Mountain, passes through Helan Mountain, crosses Yinshan Hetao, then turns southward from west of Luliang, and finally flows eastward through Sanmenxia into the sea. This process is an important example of how crustal tectonic changes affect the formation and evolution of surface water systems, and is of great significance for understanding paleoclimate and paleohydrological changes.

However, there are different opinions in the academic community about the time when the middle reaches of the ancient Yellow River changed its course. There are different views on the formation time of the modern Yellow River water system, including 150,000 years ago, no later than 1.2 million years ago, and about 5 million years ago.

In the past, scientists used dating methods to determine the age of geological sediments, but this method is easily affected by factors such as deep sources and changes in initial values ​​in groundwater dating. The interpretation accuracy of its dating results is low, and its application in groundwater is very limited. To this end, the Institute of Geology and Geophysics of the Chinese Academy of Sciences, Beijing Normal University, and the University of Science and Technology of China formed a joint research team, selected the deep groundwater in the Weihe Basin for chronological research, and carried out detailed dating work with integrated methods.

As a member of the noble gas family, it has a half-life of 230,000 years and a dating range of up to 1.3 million years. It has no interference sources underground, is chemically stable, and does not react with other substances. It is considered an ideal tracer for ancient groundwater in the range of 100,000 to 1 million years. Due to the extremely low abundance of isotopes in nature, the application of this method has been limited for a long time until the emergence of laser-cooled atom trap technology (ATTA).

Image source: pixabay

According to reports, ATTA is a quantum measurement technology that can achieve counting at the single-atom level with extremely high sensitivity. At normal temperature and pressure, it can count atoms in concentrations as low as 300 and 200 m in a few microliters of krypton gas.

Pang Zhonghe introduced that based on the combination and comparative study of the two methods, not only can more accurate groundwater age data be obtained, but also important factors affecting the dating results can be found according to the differences in groundwater age data, thereby gaining a deeper understanding of the deep geochemical processes and dynamic information experienced by groundwater.

For example, the dating data obtained by the researchers showed that the age of the deep groundwater in the same place was inconsistent with that of the old groundwater, which was due to the influence of "dead chlorine". Pang Zhonghe said that this difference just reflects the water-rock interaction process and its dynamic influence experienced by the groundwater.

Uncovering the Yellow River’s “Birthday” in the Weihe Basin

Why was the Weihe Basin chosen as the location to reveal the origin of the Yellow River? The researchers explained that the Weihe Basin is located between the Loess Plateau and the Qinling Orogenic Belt in China, and preserves more than 7,500 meters of thick fine-grained river-lake sediments. It is a world-class unique terrestrial record for studying the climate and hydrological changes in Cenozoic Asia, and is also a good place to study the changes in the Yellow River channel. At the same time, the development and utilization of geothermal resources in the Weihe Basin has created a large number of deep geothermal water wells, which indirectly created favorable conditions for stratified sampling tests.

According to reports, a total of 8 groups of groundwater samples were collected and measured in this study. By analyzing the age and chemical composition data of groundwater, the researchers found that from the Qinling Mountains to the north, the regional groundwater showed a trend of gradually getting older; and according to geochemical thermometers, the temperature of groundwater can reach 100℃~130℃, at which time the isotope exchange between groundwater and carbonate is enhanced, and the enrichment of O-18 increases. While this part of groundwater continued to migrate northward under the force of gravity, it dissolved the chloride in the ancient Sanmen Lake lake phase sediments, so the Cl element content increased significantly, and eventually mixed with the old groundwater in the center of the basin.

At the same time, due to the uplift of the Ordos Plateau, the erosion and continuous cutting of the Yellow River caused the deep groundwater flow in the region to change from east-west to north-south. This period was about 1.1 million to 1.3 million years ago, which corresponds to the Middle Pleistocene Climate Transition Period and the Quaternary Transition Period.
Ultimately, the study obtained new results for the 1.25 million year old dating time node and new insights into the regional groundwater flow field.

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Source: China Science Daily (ID: china_sci)

Editor: Bai Li