| The Yihe River Basin is one of China’s major agricultural production bases since ancienttimes and it had given birth to the brilliant Longshan Culture. It was also home to the DongyiCulture during the Holocene Epoch. However, during the Holocene Epoch, droughts and floodsfrequently occurred in the Yihe River Basin, thus exerted a profound impact on the social andcultural development there. The decline of ancient culture in the basin is closely related to theoccurrence of paleofloods. Therefore, the analysis of the Yihe River Basin before written recordsof paleoflood situation from the perspective of geological history has an important significanceto understand the watershed paleoflood rules and paleoclimate features. Meanwhile, it provides atheoretical basis for the Yihe River Basin disaster mitigation of flood control and water resourcesdispatch.Based on the fieldwork, the Chuanliujie (clj) Profile is selected as the typical profile of thepaleoflood record of the Yihe River watershed.14C and OSL were used to determine the age ofthe profile-clj according to the differences of sediment material component content. The datingresults show that profile-clj records the paleoflood information of the Yihe River Basin about6728-21640 a B.P. period. These indicators of sediment grain size, magnetic susceptibility, losson ignition（LOI）, Fe2O3and Rb, Sr content measurement were measured, then used to explorethe occurrence of the paleoflood of the Yihe River watershed and their paleoclimate backgroundby analyzing them from different angles.Sediment grain size analysis includes the content of different particle-levels, grain sizeparameters, the probability cumulative curves, the change rate of median particle size. The resultshows that the indicators of the content of different particle-levels, grain size parameters, and theprobability cumulative curves have a good indication of the significance that can be used to inferthe occurrences of flood and analyse the hydrodynamic conditions. To distinguish the differentstages of flood, the change rate of median particle size is superior to the median particle size.Since flood sediment grain size is affected directly by the flood size, the sediment grain size is a direct reflection of the flood size, which can be used to reflect the scale and intensity of floods.LOI is a good indicator of the significance of the division of the different periods of flood.When LOI value is high, it indicates the pedogenic role of this period and the well-developedvegetation, which are often signs of a longer flood intermittent period, while when the LOI valueis low, it indicates big floods or frequent flooding period. When it is too difficult for the onefoldgrain size to distinguish the different sedimentary cycles, LOI indicator makes up for lack of thechange rate of median particle size which did not reflect the other two of floods. The twoindactors of the change rate of median particle size and LOI confirm each other, reflecting thatthe Yihe River Basin produced at least 13 floods from the late Pleistocene Epoch to the middleHolocene Epoch.Based on the comprehensive analysis of Sediment magnetic susceptibility, median grain sizeand the changes of Fe2O3content, the essay then discusses the climatic and environmentalchanges recorded in the profile-clj. High magnetic susceptibility values is corresponded to coldarid environments, and low magnetic susceptibility values is corresponded to warm and humidenvironment. The result is more or less consistent with the wet and dry climate change reflectedby the content of Rb and Sr and Rb/Sr ratios in the the profile-clj. The comparative analysis ofthe paleoclimate characteristics and paleoflood scale reflected by sediment grain size suggeststhe paleoflood scale and corresponding paleoclimate characteristics of the 13 paleofloodsrecorded in the profile-clj: (1) Layer 9-11 (10439-21640 a B.P.) The climate conditions then werecold and dry, which is corresponded to the1st-3th floods period, the large floods period. But inthe end of the 1st flood period, there was a small-scale flood period with long intermittentbetween floods. (2) Layer 8 (9564-10439 B.P.) The climate then was warm and dry climate,which is corresponded to the 4th floods period, the occasional small-scale floods period. (3)Layer 7 (8647-9564 a B.P.) The climatic conditions then was cold and dry, which iscorresponded to the 5th flood period, the large-scale floods period. (4) layer 6 (8277-8647 a B.P.)The climatic conditions then was warm and humid, which is corresponded to the 6th floodsperiod, the occasional flooding of small-scale. The weather-shack structure of the 6th layer alsoindicates that a long drought period followed the 6th floods period, which had no flood sedimentdeposition, thus it is lack of geological record of this drought period. (5) Layer 4-5 (7802-8277 aB.P.) It was a warm and humid climate then, and the climate trends changed toward the arid direction. The 5th layer is corresponded to the 7th floods peroid, the largest major floods afterthe Holocene Epoch recorded in the profile-clj. The 4th Layer is corresponded to the 8th and 9thfloods peroid, which occasional flooding of smaller-scale occurred and the flood scale may besmaller than the 5th floods period. (6)Layer 2-3 (7472-7802 B.P.) It was a cold and dry climatethen, and the two layers from bottom to top correspond to the 10th and 11th floods period. The10th layer was frequent flood period and the size of the flood is bigger than that of the 5th floodsperiod. The 11th layer is corresponded to the period which the size of the flood frequentlyalternated. (7) Layer 1 (6728-7472 a B.P.). The 1th layer bottom corresponds to the late 11thfloods period, which was warm and humid, while the middle and upper part of the 1th layerindicates a warm dry climate, which are respectively corresponding to the 12th and 13th floodsperiod. The 13th floods period is smaller floods period, with a flood scale that is bigger than the9th. However, the flood scale of the 12th floods period is smaller than the 9th. In addition, the12th flood period lasted for a long time before and after the flood the interim period.Climatic characteristics of the whole profile imply that the floods recorded in the profile-cljfrom the late Pleistocene Epoch to the early Holocene Epoch are large scale floods. The largescale of the floods in this period was a response to global climate change since deglaciation. Thelarge-scale and small-scale floods occured alternately at different times in the Yihe River Basin,and greater intensity of floods followed after small-scale floods or long-term droughts. |
