Changes in grain-size and sedimentation rate of the Neogene Red Clay deposits along the Chinese Loess Plateau and implications for the palaeowind system |
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Authors: | Lingjuan?Wen Email author" target="_blank">Huayu?LuEmail author Xiaoke?Qiang |
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Institution: | State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi'an 710075, China |
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Abstract: | The wind system responsible for transporting dust onto the Chinese Loess Plateau during the late Miocene and Pliocene is still
unknown and recent investigations highlight many controversies. This report aims to investigate spatiotemporal changes in
grain-size and sedimentation rate of the Neogene Red Clay deposits in north China and to explore palaeoclimatic changes during
the late Miocene and Pliocene, in particular the palaeowind system that transported dust. Samples were collected from eight
Red Clay sections on the Loess Plateau. Measurement and analysis show that there is a clear southward decrease in the mean
grain-size index and in the coarse particle fraction (>20 ì m). At Jiaxian site on the northern Loess Plateau, the average
mean grain-size is around 20 ì m, while at Lantian site in the south, the mean is around 9 ì m. The coarse particle fraction
>20 ì m makes up 24.4% and 5.6%, at the two sites respectively. This distinct diversity of grain-size in the aeolian Red Clay
deposit between the north and south indicate that the palaeodust was transported mainly by northerly low-level winds. The
grain-size variations in the Red Clay deposits can also be divided into three stages (the lower, the middle and the upper
interval): grain-size of the lower stage is significantly coarser than that of the middle stage, but finer than that of the
upper stage. As a consequence, the intensity of pa-laeowind and desiccation of the dust source region during the late Miocene
and Pliocene can be divided into three stages: 6.2-5.4 Ma, 5.4-3.5 Ma and 3.5-2.6 Ma. Strength of the palaeowind during the
middle stage (5.4-3.5 Ma) is weaker than that of the previous and subsequent stages and the intensity of palaeowind during
the latest stage (3.5-2.6 Ma) is stronger than that of the early time (6.2-5.4 Ma). Variations in the sedimentation rate of
the Red Clay deposit can also be divided into three stages: the earlier, the middle and the later interval. The sedimentation
rate of the earlier stage is higher than during the middle stage, but lower than during the later one. This is a clear indicator
of changes in the palaeowind intensity and aridity in the dust source region and may be linked with global ice volume changes
during the late Miocene and the Pliocene. |
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Keywords: | Red Clay deposit grain size atmospheric circulation the late Miocene and the Pliocene Chinese Loess Plateau |
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