汉江上游郧西郧县段古洪水事件光释光测年及其对气候变化的响应

通过对汉江上游的野外考察,发现汉江上游郧西—郧县段第一级阶地前沿全新世土壤剖面中夹有多层古洪水滞流沉积物,选择该河段的归仙河口(GXH)剖面进行了详细野外观察,结合粒度成分和磁化率指标分析,证明它们是典型的古洪水滞流沉积物。采用石英的单片再生剂量法(SAR)获得了该剖面中9个样品的光释光年龄值。基于测定的光释光年龄、考古断代和地层对比结果,确定这四期洪水事件分别发生在距今12500-12000 a、7500-7200 a、3100-2800 a,1000-900 a。通过与汉江上游地区、国内和世界各地的多种指标气候变化记录的对比分析,进一步探讨了汉江上游郧西郧县段全新世以来发生的多期古洪水事件发生的气候背景,这一认识有助于深入理解区域洪涝灾害对全球气候变化的响应规律。

Palaeoflood OSL chronology and its response to climate change in the Yunxi-Yunxian reach in the upper Hanjiang River valley

The Hanjiang River is the biggest tributary of the Yangtze River and it will supply water resource to Beijing through the national"South-to-North Water Diversion Project". Holocene slackwater deposits along the river channels were used to study the magnitude and frequency of the paleoflood that occurred prior to gaged records and historical datasets all over the world. These pedo-stratigraphic sequences are studied using a multi-discliplinarily approach to reconstruct Holocene hydro-climatic variations. Our field investigations was carried out in the upper reaches of Hanjiang River valley. Four bedsets of palaeoflood slackwater deposit (SWD) were found interbedded in the eolian loess-soil profile at the GXH site, which were identified by the sedimentary criteria and analytical results. Analytical results, including grain-size distribution and magnetic susceptibility, indicate that these SWD consists of sandy silt, sourced from the suspended sediment load of the palaeo-floodwater. Each bedset of the SWD represents one group of palaeoflood event. OSL dating for the quarts grains at different sizes separated from the samples by using the single aliquot regenerative-dose (SAR) protocol. All OSL measurements were made on a Ris? TL/OSL DA-20 dating system using a blue light (470 nm) for optical stimulation. The dose rates were converted and calibrated by using the measured concentrations of U, Th, K elements. The OSL dates were figured out by using the Age.exe software written by Grün (2003). According to the optically stimulated luminescence (OSL) dating method in combination with archaeological dating of retrieved anthropogenic remains, and with pedostratigraphic correlations with the previously studied Holocence pedo-stratigraphy in the upper reaches of Hanjiang River valley, these palaeoflood events were dated to 12500-12000 a (SWD 1),7500-7200 a (SWD 2), 3100-2800 a (SWD 3), 1800-1700 a (SWD 4), respectively. These flooding events are therefore considered to be a regional expressions of known climate events in the northern hemisphere and demonstrate that Holocene climate was far from stable. In the reconstructed temperature time series based on high-resolution climatic proxies from tree-rings, stalagmites, ice-cores and lake sediments across the world, these extraordinary palaeoflood events are well correlated to the decline time of global climatic or climatic transition. Highly unstable climate may result in great variation of precipitation. which can cause floods and droughts. These results are of great significance in mitigating flood disasters, and in hydraulic engineering, as well as in understanding hydrological response to global climatic change.