黄河源区玛曲段末次冰消期古洪水事件及其光释光测年研究

Study of the sedimentology and OSL dating of the Last Deglaciation paleoflood events along Maqu section in the source regions of the Yellow River

通过对黄河源区玛曲段河谷开展野外考察,在太吾若(TWR)发现典型的古洪水滞流沉积剖面。根据野外沉积学宏观特征判别,并结合粒度分布、磁化率、地球化学元素和石英颗粒微形态特征等室内实验分析测定结果,准确鉴别出TWR剖面所夹一组多层洪水滞流沉积物(SWD),属典型的河流洪水在高水位滞流环境下沉积的悬移质泥沙。结果表明,TWR剖面古洪水SWD和现代洪水SWD沉积学分类为沙质粉沙,现代土壤为黏土质粉沙,现代风成沙为中沙。古洪水SWD粒度自然分布频率曲线为正偏,呈单峰,主峰高且峰值集中,分选性良好。古洪水SWD和现代洪水SWD磁化率较低,介于现代土壤和现代风成沙之间,化学元素明显区别于现代土壤和现代风成沙,表明它们尚未受到风化成壤作用影响,为古洪水悬移质快速沉积形成的滞流沉积物。石英颗粒微形态特征的分析表明,古洪水SWD和现代洪水SWD石英颗粒表面分布明显的三角痕、V形坑、撞击坑和凹面等水成沉积物的典型特征,属于河流沙类型。采用单片再生剂量法(SAR)光释光技术测年,证明在13.6~13.0 ka BP,黄河源地区经历了一期多次的大洪水事件,发生时段对应末次冰消期向全新世转折的时期,与欧洲和格陵兰冰芯记录末次冰消期中的Bolling/Aellrod暖期相对应,此时青藏高原地区冰川大规模消融,这期古洪水事件正是流域内冰融水大量下泄汇入黄河而形成的大洪水。

Pedo-sedimentological investigations were carried out in the source regions of the Yellow River.A set of paleoflood slack water deposits（SWDs） was identified along Maqu section of the Yellow River based on sedimentary criteria and analytical results,which contain climate and hydrological information.Sediment samples were systematically taken from loess-soil profiles at Taiwuruo study site and magnetic susceptibility,grain-size distribution,geochemical elements and micromorphology were analyzed in laboratory for identifying the paleoflood SWDs.The results show that these SWDs are defined as sandy silt,which are similar to the modern flood SWD; the modern soil is defined as clayey silt; the modern aeolian sand is defined as medium sand.The shape of particle size distribution curve of the paleoflood SWDs is very similar to that of the modern flood SWD,very different from modern aeolian sand and modern soil.The magnetic susceptibility of the paleoflood SWDs and the modern flood SWD are slightly higher than that of the modern aeolian sand,but significantly lower than that of the modern soil.Chemical elements of the paleoflood SWDs are distinctly different from modern soil and modern aeolian sand.The analysis of the morphological characteristics of the quartz particles shows that the surface of the paleoflood SWD and the modern flood SWD are distributed with V-shaped pits,triangular marks,concave surfaces typically caused by physical collision and friction in running water.The morphological characteristics of the quartz particles reflect different microscopic differences in sedimentary environment.Analytical results,including magnetic susceptibility,particle-size distribution,concentrations of chemical elements and micromorphology,indicate that these well-sorted paleoflood SWDs were deposited from the suspended sediment load in floodwaters.This hydro-climatic event was dated to 13.6-13.0 ka BP by using the optically stimulated luminescence（OSL）dating method.The period corresponds to the transition period of the Last Deglaciation to the Holocene,corresponds to the Bolling/Aellrod warming event in the Last Deglaciation in Europe and the Greenland ice core.At this time,the phenomenon of glacier ablation was prevalent in the Tibetan Plateau,which may be concluded that the paleoflood events were closely related to glacier ablation in the same basin.These results are of great significance to mitigate flood disasters and to exploit water resources,as well as to understand hydrological response to global climate change.