The Holocene Paleoflood And Sheet Erosion-deposit Events In The Fenwei Basin

The Holocenewas the quick development and prosperity period of the mankind and mankind's civilization, and was also the great change period of the global environment. The Fenwei basin, one of the prehistoric culture centers in China, lies in the southeastern Loess plateau. Studied the Holocene loess-palaeosol sequence, the information about natural environment evolution and mankind's activity impact could be obtained.Based on the detailed field investigation in Fenwei basin, the three typical high-resolution Holocene DXF-N, NSC and HXZ loess-palaeosol profile were studied by the proxy climatic data of grain-size, magnetic susceptibility, TOC, CaCO₃ and geochemical elements. The results revealed the rule about the palaeoflood and sheet ersoion-deposit events. The main results are as following: peak value1. Analyzed from the particle-size characteristics in DXF-N loess-palaeosol profile in the front of palaeoalluvial-pluvial fans in Yuncheng basin., and integrated with the archeology, the ¹⁴C age data in loess plateau and the cultural remains found in DXF-N profile, the results showed that the 4 sheet erosion-deposit events were recorded in DXF-N profile in 11500 a B.P. 11000 a B.P., 3850 a B.P.³300 a B.P., 2200 a B.P.²000 a B.P. and 1300 a B.P. 800 a B.P.respectively. The first sheet erosion-deposit event in 11500 a B.P. 11000 a B.P. arose from the climate change, because the temperature risen, a great deal of rainfall caused easily erosion on the bare, loose dust loess deposition. Since Xia and Shang dynasties from 3850 a B.P, mankind activities disturbed the the process of erosion, movement and deposition was advanced, and caused the sheet erosion-deposit events. Under the condition of the dry and cool weather in 2200 a B.P.²000 a B.P. and 1300 a B.P.—800 a B.P, the mankind activities moer strongly disturbed the palaeopluvial-alluvial plans, the sheet erosion-deposit events were accelerated.2. The NSC loess-palaeosol profile lies in the gentle slope ground between Shidihe river palaeoalluvial-pluvial fans and sourthern Weihe T₂ terrace in eastern Guanzhong plain. Analyzed from the particle-size characteristics, and integrated withthe archeology, the ¹⁴C age data in loess plateau and the cultural remains found in NSC profile, the results showed that the sheet erosion-deposit events were recorded in NSC profile in 11500—8500 a B.P.early and after 4000 a B.P., the sheet erosion-deposit event in 11500—8500 a B.P.early just took place in the early when the ice age end toward climate warm, the climatic change caused this sheet erosion- deposit event. Since Xia and Shang dynasties after 4000 a B.P., the human agricultural cultivation activities affected the surface of palaeopluvial-alluvial fan and promoted the slope runoff erosion and deposit process, which caused the sheet erosion- deposit event. Beginning from the 3100 a B.P.. the climate became dryer, but the human activities affected strongly the local eco-environment, and the sheet erosion-depositions events were accelerated. After Sui and Tang dynasties, the intensity of sheet erosion was intensified caused by the human activities.3. Analyzed from the particle-size characteristics, and integrated with the archeology, the ¹⁴C age data in loess plateau and the cultural remains found in HXZ profile, the results showed that the 3 palaeoflood events took place in in 5000 a B.P., 4200 a B.P.⁴200 a B.P. and 3100 a B.P.³000 a B.P.. The palaeoflood in 5000 a B.P. caused mainly by the climatic change from the climate acute fluctuation to the warm and wet. And the palaeoflood in 4200-4000 a B.P. was caused by human activities under the condition of warmer and wetter climate. Because of the instable climatic system after 3100 a B.P., the concentrative rainstorm and human stronger activities caused a paleaoflood event in 3100a B.P.³000 a B.P.. Based on the erosion velocity of Qishuihe river, the river bed gradient and coarseness coefficient, the flood flux of three palaeoflood events in 5000 a B.P., 4200⁴000 a B.P. and 3100 — 3000 a B.P. was 1190 m³/s, 2240 m³/s and 4030 m³/s figureed out respectively according to the current riverbed section counter-rose, and was 1280 m³/s, 2360 m³/s and 4260 m³/s figureed out respectively according to the current Qishuihe riverbed section shrunk by 30%, 25% and 20% because of the erosion velocity and human activities, which could provide reference for the Qishuihe river flood research.4. The acceleration erosion caused by mankind's activity developed gradually, which experienced a very long historical development process. In the early Holocene. the soil erosion was caused by natural factors because of the weak intensity of mankind's activity, the soil erosion belonged to natural erosion. The impact of mankind's activity on soil erosion has began before 4000 a B.P., but the intensity was...