Climate Change in Loess
By Xi Chen
The history of the Neogene paleoclimate and environment are hidden in a “secret collection” written by nature in code. Scientists all over the world are explaining and reading these “secrets”, among which the three most widely read books include deep-sea sedimentation, polar ice cores, and Chinese loess.
The Loess Plateau is one of the longest (about 20 million years) and most complete land paleoclimate recorders discovered so far. The Loess Plateau is composed of loess, a word used to describe wind-blown materials that form layers of sedimentary rocks.
What is loess
Figuratively speaking, loess is like the common accumulation of “dust storms” around us.
In short, loess is a product of aeolian deposition; that is, after wind blown sands and dust fall on land, they can form geological strata under air deposition (relative to underwater). Loess is mainly composed of silt particles with particle sizes of 0.01-0.05 mm and including the minerals quartz (about 60%), feldspar, mica, and a small amount of heavy minerals. Loess makes for porous, soft, and friable rock.
Where does loess come from
At the beginning of the 20th century, the world’s geoscientists had a heated discussion on the causes of the Loess Plateau, and put forward many hypotheses, such as aeolian deposition, water genesis, formation from alluvial action, or even a multiple-cause hypothesis.
The aeolian theory holds that loess is caused by local rock weathering. The desert areas in Mongolia, Central Asia, and Northwest China have a dry climate and great temperature differences. Due to the effect of thermal expansion and cold contraction, rocks and gravel are “processed and crushed” into fine sand and dust. The strong northwest wind over time will have lofted millions of tons of fine sand and dust into the sky — allowing for the material to then go south with the wind. Thus, the coarse particles first settle down and gather into a desert, and the fine particles are transported to the north foot of the Qinling Mountains. After two or three million years of transportation and accumulation, the Loess Plateau was finally formed.
Many scholars support this theory, but there are also many opponents. Opponents argue that loess in some areas was not deposited directly by the wind, but was carried by rivers or other forces and then blown ashore from the floodplain. Much of the loess around the Yellow River was formed in this way.
The work of Chinese scientists seems to find strong evidence for the “loess aeolian theory”. They found a large number of plant sporopollen fossils mixed in the loess stratum. Through the analysis of these sporopollen fossils, we can judge the climatic environment when the loess was formed. Chinese scientists pointed out that the climate at the time of the formation of the loess was a relatively arid and windy environment, which was conducive to the transportation and accumulation of loess. At the same time, through the analysis of loess particles in different areas, it can be seen that the loess particles are finer in the southeast direction, on the contrary, the closer they are to the desert area, the coarser they are. These facts are undoubtedly a powerful argument for the loess aeolian theory.
The hydrogenesis theory holds that loess is formed by the accumulation of sediment carried by flowing water. Supporters of this view found that there are obvious stratification on some loess stratigraphic sections. This stratification is undoubtedly the best evidence of the formation of flowing water. However, opponents disagree. They found that the phenomenon of loess stratification on the Loess Plateau is not obvious. On the contrary, in the tens of meters thick loess layer, there is almost a very detailed loess layer up and down.
Meanwhile, there remain other, less common ideas. The residual theory holds that the weathering of bedrock forms soil in situ, while the polygenetic theory holds that loess is the result of the joint action of the above factors.
Loess shows the principle and mechanism of climate change
The thick loess is mainly formed by wind in the form of dust storms in the past 2.6 million years, and dust storms mostly occur in dry periods. In the historical process of loess formation, the arid climate period and the warm and humid climate period alternate with each other, thus showing the replacement change of loess and paleosol on the loess stratum. The aeolian dust deposits (loess and paleosol) in the Loess Plateau can directly indicate the aridity process of its material source area and the dynamic mechanism of wind transport (showing factors such as deposition rate, particle size change, etc.). These can also correspond to factors of formation and past climate explored within polar ice cores and deep-sea sediments. It not only tells us the temporal and spatial characteristics and mechanisms of loess deposition, but also helps us interpret the other two “secrets”.
The sandstorm events in China are affected by the Siberian high, which is 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, which is related to the temperature of sea water and the distance between the ocean and the mainland. Winter monsoon and summer monsoon, the main climate controlling factors of the Loess Plateau, transmitted the climate conditions of the global glacial and interglacial periods to the interior of the Asian continent to form the Loess Plateau. The Loess and paleosol layer of the Loess Plateau faithfully records the regional and global climate information in its own code.
The color of loess is different from that of ancient soils. Iron tends to be a stable element in the loess stratum — it won’t be leached out but only changes in redox chemistry. 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 wet, and the iron in the soil was oxidized to form magnetite and some hematite, resulting in more redness in the coloration of the soil. In addition, the strengthening of weathering and pedogenesis 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, the climate of loess accumulation period is relatively cold and dry, the content of iron oxide is low, and there is less redness in the materials. Therefore, the color change in loess paleosol sequence can reflect the strength of weathering and pedogenesis controlled by the circulation intensity of summer monsoon, which can be used for the reconstruction of paleosummer monsoon intensity.
In loess deposits, the grain size represents the strength of wind force and can be used as an index of wind strength in winter. Generally speaking, when the climate is dry and cold, the wind is strong, and the particles in the sediments are coarse and the particle size is large. In the warmer climate period, the deposited particles are fine and the particle size is small. Therefore, the study of grain size composition and characteristics of loess profile is conducive to understanding the formation environment of loess deposition period.
The magnetic susceptibility of loess is closely related to the weathering intensity of soils and rocks that form the loess materials. The weathering degree 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. The magnetic susceptibility in the loess layer records the Quaternary paleoclimate change and can be used as an alternative index of the East Asian summer monsoon.
Loess, a thick book, records the changes of the environment. It looks forward to our deeper interpretation
Xi Chen is a senior student of the University of Science and Technology of China and a Research Associate for the BMSIS Young Scientist Program. She is working with Dr. Jihua Hao on abiotic synthesis of organic molecules in the subsurface ocean of Enceladus.