汉江上游郧西段归仙河口剖面全新世古洪水事件研究

The Holocene Palaeoflood Events at the Guixianhekou Site in the Yunxi Reach of the Upper Hanjiang River

对汉江上游郧西段一级阶地上归仙河口剖面的磁化率、烧失量和粒度等进行系统分析，用OSL方法对地层进行断代。结果表明：黄土剖面中夹有4层古洪水滞流沉积物（SWD），其颜色呈灰白色或浊黄橙色，结构和质地均匀且松散，与相邻上下层呈突变接触关系；磁化率和烧失量均显著低于相邻上下黄土或古土壤层；在粒度特征上，古土壤S属于粘土质粉砂，黄土L1、L0和MS属于砂质粉砂，古洪水SWD1、swD2和SWD4属于砂质粉砂，古洪水SWD3属于粉砂质砂；与上下相邻层相比，古洪水SWD的颗粒整体偏粗，中值粒径和平均粒径偏大，粘粒含量低，砂粒含量大。这4期的古洪水事件分别发生在1000～900aB．P.，3100～3000aB．P．．7500～7000aB．P．和12500～12000aB．P．，对应于晚更新世末期以来气候波动、转折时期，同时也是对全球气候变化的区域响应。

Four layers of palaeofiood slaekwater deposits （SWD） had been inserted into the Holocene loess- palaeosol stratum at the first terrace land in the Yunxi Reach of the Hanjiang River valley. Palaeoflood SWDs were identified by the variation of Magnetic susceptibility, loss-on-ignition and grain size distribution in the Guixianhekou （GXHK） profile. These results showed that the color of palaeoflood SWDs were grayish or yellow-orange, and abrupt vertical change in particle size, color, structure and texture. The Magnetic sus- ceptibility and loss-on-ignition values of palaeoflood SWDs were lower than those of the adjacent loess and palaeosol. Moreover, the grain size of palaeoflood SWDs were coarser than the adjacent loess and palaeosol. Loess deposits （L0, Lt and L1 ） were sandy silt and palaeosol was clayey silt. Palaeoflood SWD3 was silty sand in contrast to palaeoflood SWD1, SWD2 and SWD4 were sandy silt, the result why that palaeoflood SWD layers might be developed from different sedimentary process. Palaeoflood SWDs had higher sand con- tent and lower clay content, and their medium size and mean size were obvious larger than loess deposits. These features perfectly showed the presence and sedimentary characteristics of palaeoflood SWD and recor- ded the occurrence of palaeoflood events. According to OSL dating and stratigraphic correlation with the SJH and QFC profile in the Hanjiang River, four palaeoflood events were dated to 1 000-900 a B. P. , 3 100-3 000 a B. P. , 7 500-7 000 a B. P. and 12 500-12 400 a B. P. , respectively. The four palaeoflood SWDs re- corded four extraordinary floods events, corresponding to part of the Holocene abrupt climatic variability during 1000-900 aB.P., 3 100-3000 aB.P., 7 500-7000 aB.P. and 12 500-12 000aB. P. Palaeoflood events at the GXHK site in the upper Hanjiang River provided a regional express on global climate change.