| Palaeoflood study relies on palaeoflood slackwater deposits to receive palaeohydrological information. These slackwater deposits are actually suspended sediment load in flood flow deposited in areas of flow separation and preserve after the flood recession. They are indicators of Quaternary environment. Fluvial geomorphology and palaeohydrological investigations have been launched out towards several gorges along the Yellow River supported by the grants from the National Nature Science Foundation of China. Palaeoflood slackwater deposits of the Holocene extraordinary floods were found out in the Jingpingcun (JPC) site and Longmenxia (LMX) site, respectively. After identification in the field and physicochemical analysis in the lab, conclusions are as follows:(1) The palaeoflood SWDs are dull yellow orange, silt, friable. The stratigraphy is horizontal, and pinch out to the upper layer. It shows clearly the palaeoflood SWDs are well-sorted due to the long transportation distance.(2) Compared with JPC, and LMX profiles, the magnetic susceptibility and loss-on-ignition in JPC profile are the lowest. The reason is that the JPC profile located at the gorge section while the water characterised as limpid and less sand. The deposits covering the ground formed under dry and cold climate conditions which mean weak weathering and pedogenesis. Besides, Plot of Palaeoflood SWD12 in the JPC profile is located at the ? section in the CM diagram, while other plots are located at the VI section which represents a suspension depositional environment. All of these discoveries illustrate that diversity of sedimentary environment.(3) The data of clay/silt in LHK and PDG profiles is the smallest among these three studied profiles and the particle size in this profile is the coarsest among the three profiles. The reason is that the silt is originated from the Mu Us Desert which contributed great quantity coarse grains into the Yellow River. Besides, the curves of palaeoflood units SWDs in the LHK and PDG profiles exhibit characteristics of single peaks with narrow points due to its long transportation.(4) The magnetic susceptibility and loss-on-ignition data in the LMX profile appear as the highest among the three profiles which show the same characteristics with loess and palaeosols. The reason is that the LMX profile located at the southeast boundary of the Loess Plateau where characterised as sub-humid climate, which leads the deposits influenced severely by the weathering.(5) The JPC-SWD1 was dated at 3180±150 a, and the JPC-SWD12 was dated at 3070±130 a. These flood events identified in the JPC site occurred at 3300?3000 a, a period during which the monsoonal climate shifted from the middle Holocene climatic optimum to late Holocene dry conditions. In the Longmenxia reaches of the Yihe River valley, these flood events were dated at 3100-3000 a,1800-1700 a,770-610 a, and 420-340 a using the optically stimulated luminescence method in combination with the pedostratigraphic correlations. These extraordinary flood events are well correlated chronologically with the known Holocene climatic events that occurred in the Northern Hemisphere. Thus, the monsoonal climate was highly variable with both floods and droughts occurring frequently during these episodes.These results are important for understanding the response of river systems in eastern Asia to global changes. |
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