Magnetostratigraphical dating of loess deposits in china
Geology 15 7: Abstract Although the widespread loess deposits of north China have been well described, there has been limited information on the absolute age of the various layers. You do not currently have access to this article. You could not be signed in. Librarian Administrator Sign In. Email alerts New issue alert. View Full GeoRef Record. Tacheng loess section in Tacheng City in the west of the Junggar Basin.
Magnetostratigraphical dating of loess deposits in china
Due to the different dominant climatic systems, regional topography and aeolian sources, the Xingjiang loess is apparently different from those of the CLP. First, the Xinjiang loess units are generally pale yellow or grayish yellow, with little stratification, and are homogenous and massive with occasionally outcropping pellicle and fleck calcium carbonate.
The paleosols are pale brown or grayish brown and may contain numerous terrestrial snail fossils and ferruginous mottles. However, most of the paleosol and loess units are difficult to distinguish because of the poor pedogenesis [ 18 , 25 , 53 ]. Second, paleoclimatic proxies, such as geochemical elements [ 17 , 54 ], rock magnetism [ 27 ], mineral composition and quart surface texture [ 46 ], reveal that the Xinjiang loess experienced a much drier climate, weaker chemical weathering and poorer pedogenesis than the loess deposits in the CLP. Many magnetostratigraphy and absolute dating techniques OSL, 14 C and electron spin resonance ESR have been conducted on the loess-paleosol sequences in Xinjiang during the past few years [ 15 , 16 , 22 , 27 , 30 , 33 , 56 ].
Although the ages of the Xinjiang loess remain controversial as discussed above , we tentatively try to establish a regional pedostratigraphy by compiling published work to help reveal the regional paleoclimate and paleoenvironment of Xinjiang. There are only a few papers that focus on the ages of the loess in the Kunlun Mountains. In the early 21st century, Fang et al.
Recently, Zan [ 53 ] assigned an age of 0. Much works suggested that the oldest loess deposited since the beginning of the Pliocene [ 58 — 63 ]. The 14 C and luminescence dating results suggested that the Kunlun Mountains loess formed primarily after the last glaciation [ 58 — 63 ], which is equivalent to the age of the Malan loess in the CLP. In the Ili Basin, the oldest loess was dated to nearly ka by paleomagnetic dating Fig 7 [ 33 ]. However, most of the Ili loess strata formed after the last interglacial, which is equal to the age of the Malan loess in the CLP Fig 7. Fig 7 shows that an Sm weak paleosol layer equal to MIS 3 at a higher elevation e.
Because precipitation increases with increasing elevation, this suggests that more precipitation leads to stronger pedogenesis [ 20 ] and a thicker paleosol. For a more detailed description of the distributions and ages of the Ili loess, please refer to reference [ 17 ]. On the northern slopes of the Tianshan Mountains, several sections have been studied [ 39 , 40 , 56 ]. However, these studies focus on rock magnetism and geochemistry analysis to study the regional paleoenvironment rather than dating the loess-paleosol sequences.
However, Lu et al. Because Lujiaowan section is located at the foot of the Tianshan Mountain, the denudation maybe erode the bottom of the Lujiaowan section, so the age of Lujiaowan section is younger than that of the Dongwan loess-paleosol sequence. Moreover, a mid-Pleistocene mammalian fossil was found in the loess section, providing further support for a middle Pleistocene age for the oldest loess on the north of the Tianshan Mountains [ 64 ].
Due to various distances to source regions, the grain size of the Xinjiang surface soils differ from each other. The grain-size contours become increasingly dense i. This phenomenon further suggests that the Gurbantunggut Desert and the Taklimakan Desert are the main sources of loess for the northern Tianshan and Kunlun Mountains, respectively [ 19 ]. This pattern is in accord with the different regional landscape zones, including desert, shrub, and grassland, along with increasing altitude [ 54 ].
Scarce precipitation and strong evaporation lead to the extreme aridity of the Tarim Basin, and stronger wind can transport coarser dust to deposits in the downwind zones on the northern slopes of the Kunlun Mountains. Meteorological observation data have also demonstrated that sandstorms occur more frequently in South Xinjiang than in North Xinjiang due to their different climates [ 65 ]. This pattern indicates that decreasing wind speed leads to the increased settling of fine grains and an apparent intermontane basin defined by the local topography Fig 1.
This phenomenon is correlated with the varying distance from the source region, reduced wind speed caused by increasing elevation, and the blocking effect of the Tianshan Mountains. Studies by Vandenberghe et al. This pattern further revealed that during dust transportation, wind sorting is an important factor in the abrasion and size reduction of the dust particles [ 68 ].
The contour lines in Fig 8A illustrate that there are two directions of dense contours around Shawan: These two directions are in line with the directions of regional dunes and chains [ 16 ], which indicates that the loess around Shawan was transported from the Gurbantunggut Desert by NW and NNE-SSW winds. These two currents Fig 1 meet in the Shawan area, resulting in the thickest loess deposit 81 m on the sixth terrace of the Ningjia River to the north of the Tianshan Mountains Fig 9A.
The distribution and thickness of three sections on the north side of the Tianshan Mountains Fig 7 further support that the loess distributed on the terrace in the middle is the thickest, whereas near the Junggar Basin and the Tianshan Mountains, the loess becomes thinner. However, due to the smaller area of desert to the north of Qitai Fig 1 , only less than 20 m thick loess has been deposited in this area.
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The regional topography around Tacheng Fig 1 is similar to the Ili Basin, which looks like a trumpet, with the mouth facing towards the west. Thus, this area may mainly receive dust and water vapor derived from Central Asia and transported by westerlies. Unlike the 96 m in the eastern Ili Basin, the loess between Tacheng and Yumin is rarely more than 30 m thick, which could be the result of a smaller dust source to the west of the Tacheng area.
No evident characteristics were observed around Bole because of fewer sampling sites 2 samples. Based on the topographic characteristics of the closed valley around Bole Fig 1 , the loess thins from east to west. Because the loess distribution around Bole is not located in a downwind direction of the Gurbantunggut Desert, we infer that the loess originates from fluvial sediment via local mountain-valley wind circulation, as evidenced by local meteorological data provided by the China Meteorological Data Sharing Service System.
This phenomenon is similar to that of the loess sediment in the Jinsha River Valley [ 69 ]. Moreover, Deng et al. Because of the blocking effect of the Tianshan Mountains, abundant dust was deposited on the highest terrace of the Ili River around Qingshuihe, making it the thickest loess m deposit in the Ili Basin. Fig 8C shows that the coarser particles appear to the west of Xinyuan rather than more proximally, and this figure also demonstrates that the loess around Xinyuan in the Ili Basin has a thickness of 96 m on the seventh terrace of the Kunes River Fig 9B.
Song et al [ 17 ] noted that, in general, the thickest loess is located around Xinyuan and thins to both sides.
Due to their location in the lower reach of the Ili River, the few well-developed river terraces around Yining on the west of Xinyuan provide an adverse condition for dust accumulation, with the exception of the Qingshuihe loess hill Fig 8A , which is closer to the source region. To the east of Xinyuan, due to the elevation increase, the energy of the westerlies decreases gradually, which means that only finer particles Fig 8C are carried to this high-elevation area. However, the Xinyuan zone is an ideal site for loess accumulation, due to the adequate wind energy and well-developed terrace of the Kunes River.
Timing and Spatial Distribution of Loess in Xinjiang, NW China
Because of the high-elevation m nature of the Zhaosu Basin, only several meters of loess cover the Tekes river terrace. This pattern indicates that this area is mainly dominated by two winds: These two winds meet in the Hotian-Yutian area and deposited the thickest loess sediment m on the flat terrace of the Keriya River. These winds even lift fine dust up to above m [ 15 ]. Based on 17 selected meteorological stations for the period from to , Zhu et al. However, between Kashi and Hotian and between the Yutian and Qiemo, the thickness of the loess gradually decreases and finally disappears because these locations are not downwind of the Taklimakan Desert.
However, the Kunlun loess started to be deposited in the beginning of the Pliocene, which suggests earlier aridification occurred in the Tarim Basin rather than in the Ili Basin and the Junggar Basin. In summary, there are three foremost factors that determined the thickness, distribution and formation age of the Xinjiang loess [ 55 ], including: However, we must note that our results lack certain sampling sites, especially in the Tarim Desert.
We hope that we will investigate the grain-size spatial distribution of the Xinjiang topsoil in more detail in the future. The Xinjiang loess is mainly distributed on terraces of rivers, low uplands, piedmonts and the margins of deserts. The thickness varies from several to several hundred meters. The majority of the Xinjiang loess developed since the middle Pleistocene; however, the oldest Xinjiang loess was deposited during the early Pliocene. The deposition of the Xinjiang loess is attributed to persistent aridification and the appearance of the grassland and desert steppe environments.
It is also suggested that persistent drying, adequate wind energy and suitable accumulation conditions are the three main controlling factors that determined the distribution, thickness and formation age of the Xinjiang loess. We are grateful to editor Prof. Chengsheng LI and the two anonymous reviewers for their valuable comments, criticisms and suggestions on original and revised manuscript. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript.
National Center for Biotechnology Information , U.
- Timing and Spatial Distribution of Loess in Xinjiang, NW China.
- Magnetostratigraphical Dating Of Loess Deposits In China?
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