| The Yellow River Basin is one of the cradles of Chinese civilization, especially in regions of the middle and lower reaches Yellow River, which had been the country’s political, economic and cultural center for a long time. As one of the eight dynasties, Kaifeng City’s prosperity and decline is closely related to the Yellow River. Current research about the impact of the Yellow River on Kaifeng City is mainly limited to the number of the Yellow River flood hazards and influence of the major flood after developing into the city. But there are still no scientific conclusions on the frequency of the Yellow River dike breach and sediment accumulation, the intensity of the flood and the grading standard. Traditional palynology research is used to reveal changes of regional paleoenvironmental while the studies on pollen assemblages between the flood period and sedimentary hiatus are rare. The Yellow River flooded many times near Kaifeng City, however, have formed a relatively complete sedimentary sequence, which provide us a rare research carrier to explore the stratigraphic evidence of flood events and its pollen records in the vicinity of Kaifeng City.In this research, a 25-m long core was obtained from Henan University(ZKjm) which located in the western suburbs of Kaifeng City, Henan Province, China. Based on the results of detailed pollen analysis, along with color identification, AMS14 C dates, grain size and historical documents, sedimentary cycles in the western suburbs of Kaifeng City were identified, at the same time, flood events and their pollen records were revealed since the middle and late Holocene. The results are as follows:(1) According to the ratio of sand and clay of grain size in the western suburbs of Kaifeng City, ZKjm core can be divided into nine sedimentary cycles from the bottom to top. The first sedimentary cycle is below the surface of 20.5~24 m interval, the second sedimentary cycle is below the surface of 17.5~20.5 m interval, the third sedimentary cycle is below the surface of 14.5~17.5 m interval, the fourth sedimentary cycle is below the surface of 11.8~14.5 m interval, the fifth sedimentary cycle is below the surface of 9.8~11.8 m interval, the sixth sedimentary cycle is below the surface of 7.8~9.8 m interval, the seventh sedimentary cycle is below the surface of 4.2~7.8 m interval, the eighth sedimentary cycle is below the surface of 2.5~4.2 m interval, the ninth sedimentary cycle is below the surface of 0.5~2.5 m interval, respectively. Daliang City during Warring States Period was detected around the depth of 14.5 m while Qin, Han, Sui, Tang and Song dynasties cultural layers were identified below the surface of 13~14 m interval which developed on the sediments of BC 225.(2) ZKjm core of the western suburbs of Kaifeng City roughly recorded eight large flood events. The roughly occurrence time of these flood events were 4000 a BP, 3500~3300 a BP, 3100 a BP, BC 225, AD 1387, AD 1461, AD 1642 and AD 1841, respectively. based on the existing research we can concluded that abrupt climate change(climate cooling), soil erosion along the Yellow River, poor governance and human burst were the main reasons for the flooding of the Yellow River.(3) The line slope of saltation load, the ratio of sand and clay of grain size, Ф value of intersected point between bedload and saltation load and the sediment thickness were selected as the evaluation indexes in this paper in order to divide the flood grades quantitatively. Based on the total flood index(TFI), a flood grading system was established. When TFI is <0.16, it means that the flood intensity is weak(Grade Ⅰ); when TFI is 0.16≤TFI<0.36, it means that the flood intensity is moderate(Grade Ⅱ); when TFI is 0.36≤TFI<0.56, it means that the flood intensity is strong(Grade Ⅲ); when TFI is ≥0.56, it means that the flood intensity is severe(Grade Ⅳ). Furthermore, flood events occurred in AD 1387, AD 1461 and AD 1841 belong to Grade Ⅱ; flood events occurred in 3500~3300 a BP, 3100 a BP and AD 1642 belong to Grade Ⅲ; flood events occurred in 4000 a BP and BC 225 belong to Grade Ⅳ and Grade Ⅰ, respectively. In addition to the hydrodynamic conditions, the damage extent of the flood are also concerned with the duration of the flood, human defense extent, weather conditions when the flood occurred and building structure of the affected areas.(4) The boundaries of sedimentary cycle are mainly consistent with the pollen subzones except for individual strata, suggesting pollen can be used as a reference indicator of dividing sedimentary cycles of strata. Notably, the number of the pollen subzones is more than the sedimentary cycles, indicating that the accuracy of the cycle division on some intervals pollen index is slightly higher than granularity. On the other hand, as some floods of the Yellow River lasted longer they may have multiple short-term floods intermittent due to seasonal changes and the changeable of the floods flow path and the geographic scope. However, local pollen of Kaifeng City was mainly deposited during the floods intermittent, which must be difference form that pollen deposited during the floods from the upper reaches of the Yellow River. Therefore, a pollen subzone may not indicate a specific flood event.(5) The difference of pollen assemblage mainly for that the percentage of arboreal pollen of flood deposition period almost higher while the percentage of Planted Poaceae almost lower than neighboring hiatus period during the flood deposition period and the hiatus period of ZKjm core in the western suburbs of Kaifeng City. The reason is mainly that alluvial pollen is sorted during transport by running water, the pollen sources and human activities. Notably, the percentage of arboreal pollen of the hiatus period significantly higher than neighboring flood deposition period in the third and the seventh sedimentary cycle, which mainly account for climatic conditions, human activities and hiatus time. The percentage of Planted Poaceae of hiatus period lower than neighboring flood deposition period in the fourth and the seventh sedimentary cycle, which mainly account for no farmland existence around the core and the "ChongZhen Drought" event respectively. |
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