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"my family lives on a high loess slope, and a strong wind blows over the slope."

There is an ancient land in northern China. It is dressed in yellow and is called the Loess Plateau. For thousands of years, the Loess Plateau kept an old secret until a scientist named Liu Dongsheng and his team began to crack it.

"the yellow land is a place where we can recuperate for generations. It is a huge library of geological documents, which contains all kinds of information about the changes of the earth's environment. Like a key, it can solve countless mysteries."

Liu Dongsheng, the Father of Loess

The history of the Neogene paleoclimate environment is hidden in a book of "secrets" written by nature, and scientists around the world are explaining and reading these "secrets". The three most widely circulated books include deep-sea sediments, polar ice cores and Chinese loess.

The Loess Plateau is one of the longest (about 2000 million years) and most complete land paleoclimate records discovered so far. The Loess Plateau is made up of loess.

So what is loess? Figuratively speaking, loess is like the accumulation of "sandstorms" that are common and occur around us.

To put it simply, loess is a product of eolian deposition, that is, after the wind-blown dust falls, it is deposited under gas (relative to underwater). It is mainly composed of silty particles with a particle size of 0.01 to 0.05 mm, including quartz (about 60%), feldspar, mica and a small amount of heavy minerals, rich in carbonic acid (7% and 30%). Loess is porous, soft and collapsible.

Where does loess come from? At the beginning of the 20th century, geologists in China and even the world had a heated discussion on the causes of the Loess Plateau, and put forward a variety of theories, such as eolian theory, hydrogenetic theory, residual theory and polygenetic theory. After years of field scientific investigation, Liu Dongsheng and his partners found that the loess on the Loess Plateau is getting thinner and thinner from northwest to southeast. The mineral composition of these loess is different from that of the local bedrock, but it is the same as that of the Gobi Desert and sandy wasteland in Central Asia and Mongolia.

As a result, they affirmed and developed the "new style theory":

During the Quaternary period, the existence and uplift of the Qinghai-Tibet Plateau prevented the invasion of the southwest monsoon blowing from the Indian Ocean, and the climate within the continent became drier and drier, which was conducive to the generation and transportation of wind and dust. In the ancient geological period, the northwest winter monsoon blew the yellow dust from Central Asia and Mongolia Plateau to the east continuously, and the larger coarse sand was left in Xinjiang and Inner Mongolia, where large deserts and Gobi were formed. The rest of the dust continued southward with the wind and was blocked by the Qinghai-Tibet Plateau and Qinling Mountains, and was controlled eastward by the Taihang Mountains, and finally settled down in Gansu, Shaanxi and Shanxi. At the same time, as the wind weakens, coarse dust particles are thrown down, followed by medium particles, and finally fine particles. This area has little rainfall and belongs to semi-arid area, which is beneficial to the retention of sediments dominated by silt and gradually overlaps to form the Loess Plateau.

Loess shows the principle and mechanism of climate change the heavy loess is mainly formed by the wind force in the form of sandstorms in the past 2.6 million years, and sandstorms mostly occur in the dry period. In the historical process of the formation of loess, the arid climate period and the warm and humid climate period alternate with each other, thus showing the replacement change of loess and paleosol in the loess stratum. The eolian deposition (loess and paleosol) on the Loess Plateau can directly indicate the drying process of the material source area, the dynamic mechanism of wind transport, deposition rate, grain size change and so on. These can also correspond to the corresponding parts of polar ice cores and deep-sea deposits. It not only tells us the temporal and spatial characteristics and laws of loess deposition, but also helps us to interpret the other two "Secrets".

The sandstorm event in China is affected by the Siberian high and related to the wind blowing from the northwest, while the precipitation on the Loess Plateau is mainly affected by the summer monsoon from the southeast, related to the temperature of the sea and the distance between the ocean and the mainland. The main climate controlling factors of the Loess Plateau, the winter monsoon and the summer monsoon, truthfully transmit the climate conditions of the global glacial and interglacial periods to the interior of the Asian continent to form the Loess Plateau, while the loess and paleosol layers of the Loess Plateau faithfully record regional and global climate information in their own codes.

The color of loess indicates that the color of loess is different from that of paleosol. Iron is a stable element in the loess stratum, which is neither moved out nor lost, only the valence state changes. The difference of soil redness is mainly related to the change of iron oxide content. During the paleosol development period, the climate was relatively warm and humid, iron in the soil was oxidized to form magnetite and part of hematite, resulting in high redness, in addition, the strengthening of weathering and soil formation led to the easy decomposition of unstable coarse-grained iron oxides and the increase of fine-grained iron oxides, which deepened the color of the soil; on the contrary, during the loess accumulation period, the climate was relatively cold and dry, the content of iron oxides was low and the redness was low. Therefore, the color change in the loess-paleosol sequence can reflect the intensity of weathering and soil formation controlled by the intensity of the summer monsoon circulation, which can be used to reconstruct the intensity of the ancient summer monsoon.

The paleoclimate indicated by the grain size of loess in the deposition of loess, the grain size represents the intensity of wind, which can be used as an index of the strength of winter monsoon. Generally speaking, in the period of dry and cold climate, the wind is stronger, the particles in the sediment are thicker and the particle size is larger, while in the warmer period, the deposited particles are fine and small. Therefore, the study of the grain size composition and characteristics of loess profile is helpful to understand the formation environment of loess during the sedimentary period.

The magnetic susceptibility of paleoclimate loess indicated by the magnetic susceptibility of loess is closely related to the intensity of weathering or soil weathering. The degree of weathering determines the amount of magnetite and the fineness of its particles, which determines the magnetic susceptibility. The magnetic susceptibility of the paleosol layer is higher than that of the loess layer, and the magnetic susceptibility in the loess layer records the Quaternary paleoclimate change, which can be used as an alternative index of the East Asian summer monsoon.

Case study:

Chinese loess records climate fluctuation on the millennium scale

The millennium-scale climate fluctuation recorded by Chinese loess (Sun, 2012) compares the average grain size of Gulang (red) and Jingyuan (black) loess profiles with NGRIP ice core δ 18O (blue) and Ca2+ (orange) records and Huludong (purple) and Wulu cave (green) stalagmite δ 18O. The sky blue bar represents the Heinrich-like events identified in the three records. The black number (7-17, 34-60 kyr) indicates the well-matched Dmuro event identified in the three records. The grain size records of Gulang loess section show 13 different events (marked "a" to "m") in the range of 20-34 kyr.

* Dmuro cycle: in the study of Greenland ice core, it is found that a series of rapid and substantial cold and warm events occurred in the climate of Greenland during the last glacial period, namely Dmuro cycle. Each warm period is followed by a cold period, and the temperature can change in just a few decades.

* Heinrich event: a rapid cooling event marked by a large-scale glacier drift event in the North Atlantic and representing the climate effect of a large-scale iceberg influx. Under the climatic background of the last glacial period, six strong glacier drift events occurred in the North Atlantic, which represented six major Heinrich events.

Loess sediments are closely related to the global climate on the orbital scale, and can record rapid climate events on the millennium-centennial scale. Both the Dmuro cycle and the Heinrich event are recorded in the loess of China.

The academic history of loess research the climate change during the Quaternary period has always been the core issue of Quaternary research. The theory of Quaternary glaciation originated in the last 20 years of the 18th century and the early 19th century. For a long time in academic circles, it was bound by the classical theory of four glaciations put forward by Penck and Broneck who studied the advance and retreat of Alpine glaciers. This model has been used to understand the law of Quaternary climate change.

Until the 1950s, Emiliani's study of foraminiferal shell oxygen isotopes in Caribbean sediments and Liu Dongsheng's study of Chinese loess-paleosol overlap challenged the classical theory of four glaciations from a short time scale and a long Quaternary time scale, respectively.

After that, the deep-sea sedimentary records obtained by the international ocean drilling program and the polycyclic characteristics of Quaternary climate change recorded by polar ice cores have been recognized by the international academic circles, and the Chinese loess-paleosol series has also been widely accepted by the international academic circles. it has become the most complete continental sedimentary record reflecting the polycyclic characteristics of Quaternary climate. Together with deep-sea sediments and ice cores, Chinese loess has become the three pillars of paleo-global change research. The paleoclimate change curve obtained from various alternative indicators obtained by the study of loess-paleosol sequence in China has become the standard of international comparison.

Loess, a thick book, records the changes of the environment, and she looks forward to our more in-depth interpretation.

References:

[1] Liu Dongsheng. Loess and environment [J]. Journal of Xi'an Jiaotong University (Social Science Edition), 2002 (04): 7-12.

[2] Han Jiamao. Liu Dongsheng and the study of Chinese loess [J]. Fossils, 2004 (02): 10-13: 1.

[3] Jiao translated diffuse. Loess in China and its paleoclimatic significance [J]. Wind of Science and Technology, 2019 (11): 226.

Liu Xiuming, Liu Dongsheng, Friedrich Heller, Xu Tongchun. Study on magnetic susceptibility of loess and Quaternary paleoclimate in China [J]. Geosciences, 1992 (S1): 279285.

[5] Sun, Y., Clemens, S., Morrill, C. Et al. Influence of Atlantic meridional overturning circulation on the East Asian winter monsoon. Nature Geosci 5,46-49 (2012). Https://doi.org/10.1038/ngeo1326

[6] uncover the secrets of loess, the story of Liu Dongsheng https://www.meipian.cn/ 31bscecj

[7] Loess Plateau (topographic region of China) _ Baidu Encyclopedia https://baike.baidu.com/ item/% E9% BB%84% E5% 9C% 9F% E9% AB%98% E5%8E%9F / 198310?fr=aladdin

[8] Loess magnetic susceptibility _ Baidu encyclopedia https://baike.baidu.com/ item/% E9% BB%84% E5% 9C% 9F% E7 A3% 81% E5% 8C% 96 E7%8E%87/362123?fr=aladdin

This article comes from Wechat official account: stone Science Popularization Studio (ID:Dr__Stone), author: Li Yongli, some professional text content modification: Zhou Xin teacher, Editor: Xing Si, Beauty Editor: Li Ying, later: Jiangling

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