汉江上游尚家河段全新世古洪水事件光释光测年研究

汉江上游是南水北调中线工程重要的水源区。通过对汉江上游河谷的详细调查，在郧县尚家河段全新世黄土-古土壤剖面上部发现一层典型古洪水滞流沉积物，对其进行了年代学和沉积学的研究。对于系统采集的样品，进行粒度分析测定，证明其为典型的古洪水滞流沉积物（SWD），记录了古洪水事件发生的气候水文信息。采用古洪水水文学方法恢复推算出这次特大古洪水事件的洪峰流量为63 720 m3/s，其规模大于实测最大洪水。应用光释光测年（OSL）技术中的单片再生剂量法（SAR），测得样品的年龄在940±140~3 190±100 a之间，确定古洪水SWD是汉江上游发生在1 000~900 a B.P.期间的特大洪水事件的沉积物。这期古洪水事件的发生年代对应于我国北宋/辽时期的气候恶化阶段，由于东亚夏季风环流异常变化，降水变率大，气候处于不稳定期，出现旱涝灾害频繁发生的现象。研究结果对于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证据。

OSL Dating of the Palaeoflood Event in the Shangjiahe Reach in the Upper Hanjiang River Valley

Paleohydrological investigations were carried out in the upper reach of the Hanjiang River, which is the source of the “South-to-North Water Diversion Project”.  During the fieldwork, typical palaeoflood slackwater deposit (SWD) was identified in the Holocene eolian loess-soil profile on the river banks in the Shangjiahe reach in Yunxian county of Hubei province. Sediment and OSL dating samples were taken from the profile. Laboratory analysis of the grain-size distribution and the magnetic susceptibility show that the SWD was deposited from floodwater during an extraordinary palaeoflood event. The peak flood discharge was estimated to be 63 720 m3/s, which is much larger than the recorded floods. OSL dating on quartz grains separated from the samples was carried out by using the single aliquot regenerative-dose (SAR) protocol. The measured ages of the samples are between 940±140 and 3 190±100 a. The extraordinary palaeoflood event recorded by the SWD is therefore dated to between 900 a B.P. and 1000 a B.P., corresponding to the Northern Song/Liao Dynasty. This is a period with abnormal changes of climate and increased monsoonal variability. Both the serious droughts and extraordinary floods occurred during this period. This result is of great significance in understanding the river’s hydrological response to global climatic change.