Abstract:

The upper Hanjiang River basin,the water source area of Middle Route of South to North water transfer project of China,is controlled by subtropical monsoonal climate.The rainstorms in the summer and longlasting rains in the autumn often cause great floods.The most recent large floods occurred on 19 July,2010 and 19 September,2011.Hydrologic environment information of large floods is often recorded by slackwater deposits (SWD) in the convex river bank,tributary mouth and wide channel etc.Slackwater deposits (SWD) are actually suspended sediment load of the flood at a highstage stagnant environment.Flood SWD,the product of soil erosion and rock weathering,is closely related to the surface materials during flood period and human actives.In the recent years,palaeoflood SWD were found at several sites along the upper Hanjiang River valley.These SWDs often inserted in the loesspalaeosol and slope clastic deposits.Geochemical elements including major elements and heavy metals accumulation in palaeoflood and modern flood SWD in the upper Hanjiang River valley were measured and analyzed.These results showed that palaeoflood SWD had high Na2O and CaO content and low K2O.UCCnormalized abundances of major elements SiO2,Al2O3,Fe2O3,TiO2,MgO and MnO for palaeoflood SWD were similar to that of modern flood SWD.On the one hand,the chemical index of alteration (CIA) and the weathering index of parker (WIP) were applied to evaluate the chemical weathering intensity of palaeoflood SWD and modern flood SWD in the upper Hanjiang River.The average CIA value of palaeoflood SWD was 64,whereas modern flood SWD was 60.Moreover,the average WIP value of palaeoflood SWD was also close to that of modern flood SWD.On the other hand,the triangle diagram ACNK of flood SWD showed that the weathering intensity of palaeoflood SWDs was intermediate weathering,while modern flood SWD was weak weathering.In addition,Modern flood SWD was a tool for evaluating floodrelated environment impacts on heavy metal concentrations within river sediments,which affect river and groundwater quality,soil quality and floodplain systems.The sources of modern flood SWD were mainly from soil erosion on modern ground during flood periods.Heavy metals Zn,Co,Cr,V and Ba content of modern flood SWD were higher than palaeoflood SWD 16%,60%,20%,77%,42%,respectively.UCCnormalized abundances of Zn,Co,Cr,V and Ba of modern flood SWD were obviously higher than that of palaeoflood SWD.The phenomenon implied that modern flood SWD significantly influenced by heavy metals of anthropogenic input on the Hanjiang drainage basin.The differences of chemical weathering and anthropogenic impact on the hydroclimatic system of the upper Hanjiang River basin were indicated by geochemical character of extraordinary floods in the long timescale.These results enlarge the knowledge of the relationship between human activities and natural weathering,at the same time,they are important for understanding soil and water conservation and ecological environment during flood period in the upper Hanjiang River