| The research is aimed at studying climate change and large precipitation events of Xiuhe River which is located in Yongxiu county, Jiujiang city, southeast China. The XR-01 profile, which is located on the right bank of the delta, was selected to show these changes. The bottom of the profile is accumulated by alluvial-lacustrine deposits while in the upper part with floodplain deposits. The profile is 5.7 m thick, and we divide the profile into 380 pieces(1.5cm intervals). This research is based on visual records of the profile during fieldwork, coupled with laboratory works including 137 Cs + 210 Pb + AMS-14 C dating, granularity, magnetic susceptibility, element(less than 20 microns) analysis(XRF), carbon isotope index. The main results are:1) Sedimentary facies and granularity analysis of sedimentary process, variations and parameters of grain size and components, revealed a flooding characteristic of this river. Probability cumulative curve of saltation particles are characterized by 2 phases, which reveals its sensitivity to flooding systems.2) On the basis of historical records and meteorological observation, the abnormal events with heavy rain occurred in 1789, 1792-1793, 1822-1823, 1837-1839, 1848-1849, 1858, 1869-1870, 1900-1901, 1931, 1935, 1954-1955, 1973, 1983, 1995 and 1998.3) According to the variations of Rb/Sr ratio and isotope of organic carbon, we can divide the profile into four phases:a. Phase 1(570 ~ 540 cm, 1780 to 1800 AD): Rb / Sr ratio(4.08 in average, with a maximum of 4.76 and a minimum of 3.25) and isotope of organic carbon(-22.52‰in average). This warm and humid stage is beneficial to chemical weathering. The typical flood years are in 1789 and 1792-1793, which is clearly shown in the corresponding layers.b. Phase 2(570 ~300cm, about 1800 to 1900 AD): Rb / Sr ratio(3.86 in average, with a maximum of 4.86 and a minimum of 2.71) and isotope of organic carbon(-22.35‰in average). This phase has the lowest level of Rb / Sr ratio, and it is not conducive to chemical weathering. There are many floods during this phase, such as 1822-1823, 1837-1839, 1848-1849, 1858, 1869-1870.c. Phase 3(300 ~ 200 cm, 1900- 1931 AD): Rb / Sr ratio(4.37 in average, with a maximum of 5.47 and a minimum of 3.74) and isotope of organic carbon(-21.93‰ in average), It is a warm and humid stage with chemical weathering. The typical flood years are in 1901 and 1931. This is a transitional period from the Little Ice Age into the Modern Warm Period. This phase has the most gradual fluctuations of Rb / Sr ratios.d. Phase 4(200 ~ 0cm, 1931-2014 AD): Rb / Sr ratio(4.62 in average, with a maximum of 6.47 and a minimum of 3.64) and isotope of organic carbon(-22.49 in average), warm and humid with chemical weathering. Based on historical records and meteorological observation, serious flood happens in 1935, 1954-1955, 1973 and 1998.The above results show that this profile is in the transition phase of climate change with relatively less flood events. High incidence of regional flood happens in typical cold and warm periods.This study suggest that: 1) Modern Warm Period is developed from the last cold period of Little Ice Age, which has gone through three stages: 1900-1931, 1931-1970 and since 1970 s, respectively. These stages are featured by gradual evolution 2) The accumulation rate of lacustrine and fluvial floodplain deposition are characterized by high speeds. Analysis of granularity and geochemical elements, especially in the south such studies major rivers,have great potential in revealing climate change and regional flood events, which should arose great attention.The results reveal showed similar changes between the Yangtze River region and the northern hemisphere. The regional flood is probably related with phase changes of the climate, which provides a theoretical basis for decision-making in preventing disasters and reducing damages of floods in this area. |
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