Paleoflood Research Of BZ Profile During Middle-late Holocene

Through detailed field survey of Yihe river and its tributaries, BZ profile of Yihe rivertributary selected as the typical profile of the paleoflood record. The sedimentary sectioncontains two typical flood sediments layers, a layer of limnetic facies sediments and two eoliansediments deposits layers. Collecting samples after field observation. Environmental proxiessuch as grain size, magnetic susceptibility, geochemical elements and so on have been analyzed.Through analysis of the filed sedimentary characteristics and laboratory analysischaracteristics, two period paleoflood have been identified and climate change duringmiddle-late Holocene have been rebuilt. The main conclusions of this paper are as follows:(1) Particle size analysis indicate that sediment of the second and fourth layer are silty sandand sand, the first layer is sandy silt, the third layer is clayey silt and Sandy silt, while the fifthlayer is clayey silt. Medium sand and coarse sand almost exists only in the second and fourthlayer, very coarse sand only found in the second layer. This shows that relative depth and scaleof the first phase of the flood was greater than the second phase of the flood. The median size ofthe second and fourth layer is bigger than other layers and the average grading focused on thecoarser particles. The greater the grain size parameters changed, the bigger the flood energy. Inaddition, the second flood period sediment have the maximum mean grain size and its morefocused on coarse particle, which could told that it deposit during huge flood. Meanwhile,median size is small in the first, third and fifth layers and average grading focused on the fineparticles. The granularity of fifth layer is the finest in the profile. The granularity get coarser ofthe middle third layer maybe caused by increased wind or degraded river. The suspension andsaltation granularity components of the second and the forth deposit varies a lot, and rangeoverlap is small. Size fraction in the first and third layer are all in the range of easily floatingcomponents. Grain size frequency curves for axisymmetric, and sorted better. Therefore the thefirst and third layer was judged as eolian sand. Grain size frequency curves of the fifth layer isunimodal, it illustrates that sedimentary dynamic and material source was single, namely thesediment deposit in stable deepwater environment. In sensitive grain-size population contentsvaried in depth curve of the second and forth layer sediments, Coarse particle content is extremely high, it indicate the strong hydrodynamic process and large-scale flood occurredfrequently. The data indicated boundary beach-flood plain environment. The coarse grainedcomponents of middle third layer is8.64%, it indicate the increase of wind or the increase ofriver drought degree. The sensitive grain-size population contents varied in depth curve of thefifth layer sediments reflects the depth change of the lake, then the climate change.(2) The magnetic susceptibility and the grain size is significantly correlated, especially finesand component and magnetic susceptibility of the correlation coefficient of0.812. So the reasonfor magnetic susceptibility changes, maybe the granularity. The magnetic susceptibility valuesindicate the second and the forth layer were large-scale flood period. The magnetic susceptibilityvalues of the fifth layer is smaller than other layers, it indicate relatively stable environment.Meanwhile, frequency magnetic susceptibility fluctuates significantly in the fifth layer.(3) The content of MgO,Fe2O3,Ni,V,Rb and Pb are the biggest values for the entire section,and the Rb/Sr valve is relatively high. While the value of Na2O, Na2O/K2O and Sr/Mg are thesmallest for the entire section, and the value of K2O and CaO are relatively small. It declare thatthe climate was given priority to warm and wet during4500-4000a BP. The value of Na2O, K2O,CaO, Na2O/K2O and CaO/MgO of the forth layer are the biggest for the entire section, thetendency of the curve indicate the changing climate. The low value of MgO, Fe2O3, Ni, V, Rb, Pband Rb/Sr indicate the cold and dry climate, and the large flood occurred duing that time. TheMgO, Fe2O3, Ni, V, Rb, Pb, Rb/Sr, Na2O, K2O, CaO, Na2O/K2O, CaO/MgO value illustrate thislayer overall trend of climate change is from warm-wet to cold-dry. The climate of the secondlayer varies a lot and mainly cold-dry. MgO, Fe2O3, V, Rb, Rb/Sr, Na2O/K2O, CaO/MgO, Sr/Mgof the first layer indicate the stable climate.(4) Laboratory analysis data show that the two overbank flood sediment layers can beregard as typical flood deposits. The result in accordance with the judgment of the floodsediment from the wild. Reference to sediments date results, the two period of paleofloodoccurred in3.3-2.2ka and4-3.7cal ka BP.(5) The regional climate comparison show that climatic variation of BZ profile agreed withglobal climate change. The two period of paleoflood correspond to3ka BP and4ka BP events.It in consequence of strong summer monsoon, during which precipitation changed dramatically.Through the investigating of Yihe river basin, we select a typical profile. Geomorphology, stratigraphy and chronology has been used to study the sediment. The paleoflood occurred in3.3-2.2ka and4-3.7cal ka BP. Climate proxies have been used to rebuild the environmentclimate of the whole profile. The research achievement provide reference to climate change ofYihe river basin and paleoflood research.