黄河中游吉县段全新世古洪水滞流沉积物光释光测年研究

OSL DATING OF THE HOLOCENE PALEOFLOOD EVENTS IN THE JIXIAN REACH ALONG THE MIDDLE YELLOW RIVER

通过对黄河中游河口镇-龙门段峡谷进行详细的野外考察,在多个地点发现了含有古洪水滞流沉积物的全新世地层剖面.对山西省临汾市吉县段冯家集剖面的古洪水滞流沉积物系统采样,进行了沉积学及年代学分析研究.野外观测和粒度成分分析表明冯家集剖面夹有5层典型的古洪水滞流沉积物,其性质为细沙质粉沙,具有水平层理,层与层之间被坡积石渣土分隔.该层系的顶界高出黄河平水位达18m.它们是特大洪水在高水位滞流情况之下的悬移质沉积物,客观地记录了全新世时期黄河中游一期5次特大古洪水事件.通过分离出125～225 μm石英颗粒,应用单片再生剂量法（SAR）测得其光释光年龄值,结合地层对比及各种气候替代性指标的分析,确定这些古洪水事件发生在3200～ 3000a B.P.之间,处于全新世中期大暖期向全新世晚期相对干旱期转折的阶段.这对应我国历史上的商末-西周初期,季风气候状态不稳定,黄河流域既有严重地干旱,又有特大洪水事件发生.本文的研究成果为深入揭示河流水文系统对于全球变化的响应规律提供了新的可靠证据.

Detailed paleo-hydrological investigations were carried out along the Hekou-Longmen reach in the middle reaches of the Yellow River valley.Typical paleoflood slackwater deposits （SWDs） were identified at many sites along the river channel by using the sedimentological criteria.OSL and sediment samples were taken in the Fengjiaji profile （FJJ,36°16′07″N,110°28′39″E） in the Jixian County of Shanxi Province.Both of the field observation and analytical results show that a bedset of 5 paleoflood SWDs is inserted into the profile.They consist of sandy silt with parallel beddings and they are separated by slope clastic deposit between the beds.The top of the bedset is about 18meters above the water table of the Yellow River.They were deposited by suspended sediment load of the flood slack water during the overbank flooding during the Holocene.This is the natural record of the extraordinary paleoflood events.OSL dating of the coarse quarts grains （125～225μ m） was carried out by using the single aliquot regenerative-dose （SAR） protocol.The chronology framework of the paleoflood events on the Yellow River was established with a combination of the OSL dates and stratigraphic correlations.It indicates that these extraordinary plaeoflood events occurred between 3200a B.P.and 3000a B.P.It corresponds to the late period of Shang Dynasty to the early Western Zhou Dynasty.During this period,monsoonal climate shifted from mid-Holocene Climate Optimum to Late Holocene dry conditions.Very serious droughts and great floods occurred in the Yellow River basin in related to the unstable climatic conditions.These flood events are also correlated to the global climatic events during the Holocene.Our study provides new evidence to show the response of hydrological system to the global climatic change.