渭河中游全新世黄土剖面光释光测年及记录的古洪水事件

OSL Dating of Holocene Loess-Paleosol Profiles in the Middle Reaches of Weihe River and Paleoflood Events

通过对渭河流域进行广泛的考察,在中游咸阳附近一处阶地发现保存完好的全新世黄土-古土壤剖面里夹有古洪水滞流沉积层,对其进行了年代学和沉积学研究。对于采集的全新世地层样品,进行粒度、磁化率测量分析,证明所夹沉积物是典型的古洪水滞流沉积物,该层记录了古洪水事件发生的气候水文信息。应用红外后蓝光的SAR光释光测年技术,获得该剖面9个OSL年龄值,确定渭河在距今3.2～2.8ka之间为一个洪水多发时期,并建立了渭河古洪水事件的年代序列。这一结果揭示了古洪水的频发与全新世中期向晚期过渡的转折时期,气候由温湿向干旱化发展,大气系统失稳,气候变化剧烈,降水量异常变化,变率增大,是导致渭河流域特大古洪水多发的主要原因。

Through extensive field investigation, paleoflood stagnant zones were found to occur in layers of the loess--paleosol profiles in the middle reaches of Weihe River. Authors carried out chronological and sedimentological study on the samples collected from the stagnant zones. Analyses of grain- size distribution and magnetic susceptibility for the samples collected from the Holocene strata suggest that the sediments hosted in the profiles are the paleoflood stagnant material and have recorded climatic and hydrological information of paleofloods during that period. Nine Optically Stimulated Luminescence （OSL） ages were obtained using the post IR SAR dating method, indicating that flooding occurred frequently during 3. 2 ~ 2. 8ka in middle reach of Weihe River. This study then reconstructed the chronology framework of the palaeoflood events in the middle reaches of Weihe River. The results reveal that palaeoflood occurred very frequently at the turning period from the middle Holocene to the late Holocene, during which the climate in the Weihe reaches turned to be dry. The leading reasons for extraordinary palaeofloods were unstable atmosphere system, dramatic changing climate, and variable precipitation.