Depostion Couplet,annual Freeze-Thaw Layer And Catchment Erosion Within The Landslide-Dammed Reservoirs On The Chinese Loess Plateau

As one of the most rapidly and severely eroded areas on the Earth,the Chinese Loess Plateau?CLP?is characterized by numerous gullies and rolling topography.These gullies easily form reservoirs as a result of natural landslides or anthropogenic check dams,which have subsequently filled rapidly with eroded loess from their catchments.The sediments trapped in the reservoirs are characterized by annual layers with deposition couplets and thus provide a valuable archive of catchment erosion rates,the historical climate and eco-environmental conditions,and changes in human activity on the CLP.Therefore,the sedimentary successions in the reservoirs are one of the idea objects to reconstruct contemporary natural and anthropogenic information on the CLP.In this study,high-resolution XRF core scanning was used for the sedimentary successions of Heshui?HS?landslide-dammed reservoir on the south of CLP and Jingbian?palaeo?landslide-dammed reservoirs?JB-1 and JB-2?on the north of CLP.Along with chromaticity,grain size,mineralogy,water content,bulk density and 137 Cs content distributions of the discrete samples in the representative core sections,the physical and chemical characters and geochemical indexes of deposition couplets were addressed.Furthermore,annual freeze-thaw layers were identified to date the deposition couplets in the reservoirs and then the erosion history,the storm intensity and frequency were reconstructed.Finally,the response of soil erosion to the histroical storms and human activities and the environment differences between the southern and northern CLP were discussed.Individual deposition couplets are a typical structure in landslide-dammed reservoir sedimentary succession on the CLP and are composed of underlying coarse layer and upper fine layer as a result of sorting and stratification processes during an individual hydrological event.The numbers and thichnesses of the couplets mainly depend on the storms frequencies and intensities,respectively.One of the obstacles for investigating landslide-dammed reservoir sedimentary successions is how to build the chronology of the couplets.The freeze-thaw layer is commonly formed within the last fine-grained layer of the sediments of an individual year during the cold winter and following warm spring.The physical and chemical characters of the fine-grained layer are changed significantly by the freezing and thawing processes.The numbers and specific sediment yields of the deposition couplets between each pair of annual freeze-thaw layers represents the frequencies and intensities of large hydrological events within a given year.If there is no couplet between any two annual freeze-thaw layers,it means no large hydrological events occurred during that year;i.e.,it was a drought year or the ground was dry during freezing.Therefore,identifying annual freeze-thaw layers among deposition couplets is an effective way of dating landslide-dammed reservoir sedimentary succession because the freeze-thaw layer is considered to be the boundary of the last hydrological event in a year.Based on the high-resolution chromaticity and XRF element distributions of the sediments in the HS and JB-1 landslide-dammed reservoirs,along with grain size,mineralogy,water content and bulk density distributions of the discrete samples in the representative core sections,the basic features of the deposition couplets were refined.The underlying coarse-grained layer is dominated by coarse silt and fine sand,high lightness,low bulk density and water content,loose and fragile;large and/or heavy minerals?e.g.quartz,zircon,Ti-rich minerals?concentrate on this layer,resulting in high values of Si,Zr,and Ti.The upper fine-grained layer is characterized by clay and fine silt,low lightness,high bulk density and water content,compact structure and horizontal bedding,resulting from a low settling velocity of fine fractions being deposited from the water column during the latter stages of the hydrologic event;light minerals?magnetite,K-feldspar,mica,illite,and calcite?trend to be enriched in this layer,leading to high Fe,K,Ca,Sr,and Rb?isomorphism presenting in Ca-and K-containing minerals,respectively?.High-resolution XRF core scanning analysis are valid to exactly plot out visible and invisible deposition couplets within the landslide-dammed reservoirs on the CLP,whereas grain size and mineralogy of the discrete samples can be used to identify visible ones among the deposition couplets.Rb/Zr ratio by high-resolution XRF core scanning is recommended as an excellent geochemical proxy to plot out deposition couplet through eliminating the scanning errors of single element to a large extent.According to Rb/Zr ratios distributions,165 and 126 deposition couplets were identified in the HS and JB-1 sedimentary succession,respectively,indicating that there were 165 and 126 storms or floods since the formation of the respective landslide-dammed reservoirs.According to the 137 Cs contents distributions of the fine-grained layers on the HS and JB-1 sedimentary successions,the 151 st couplet?labeled from the bottom to the top?in the HS succession was deposited following a strom in 1966 A.D.and the 97 th couplet?labeled from the bottom to the top?in the JB-1 succession was formed following a strom in 1963 A.D.The correlations of Fe counts with the Ca counts by XRF core scanning show three clusters for all of the fine and coarse layers in the representative core sections.All of the coarse silt layers have low element peak areas values and poor correlations,further indicating their same sources.In contrast,the data from all of the fine-grained layers deviate significantly from those of the matrix groups due to the high cps values of Ca,Fe,and Mn in the fine-grained layers.The most important observation is that there are two different clusters for the fine-grained layers,with different slopes and intercepts.There are high concentrations of Ca and Fe and high slopes of their correlations for some fine-grained layers,which can be attributed to be further concentrated of mobile elements in the last fine-grained layers by freezing and thawing processes.Therefore,they are deemed as annual freeze-thaw layers and the others within the year are the intra-annual fine-grained layers.Based on the correlations of Ca/Zr and Fe/Zr by XRF core scanning,56 and 77 annual freeze-thaw layers were identified in the HS and JB-1 sedimentary successions,respectively.Being comparison of the Ca,Fe,Mn cps intensities and the slopes of their biplots,along with grain sizes and mineralogy of the HS and JB-1 core sections,there are different soil properties and experience different freezing and thawing processes within the two catchments in the northern and southern parts of the CLP.The HS sediments have higher Fe and Mn cps values but lower Ca cps values,which are potentially the result of finer material and larger Ca loss owing to stronger eluviation in the southern CLP under relatively humid and warm conditions.In contrast,the Fe versus Ca and Mn plots of the annual freeze-thaw layers of the JB-1 catchment have steeper slopes than those of the HS section due to the strong freezing and thawing processes in the northern CLP under relatively dry and cold conditions.According to the identification of the annual freeze-thaw layers,modern rainfall data and the occurring years of historical flood records within the HS and JB catchments,and ¹³⁷ Cs contents distributions,the HS sedimentary succession are dating from 1811 to 1996,the JB-1 succession recoreds the storm and erosion history during the periods between 1855 and 2014.Based upon the formation times and datings of HS and JB-1 sedimentary succession,sediment yield and specific sediment yield?SSY?of individual couplet and annual SSY?ASSY?were calculated by measured catchment and surface areas,the thickness of the couplets,the gully slopes and bulk density.From 1811 to 1996 within the HS landslide-dammed reservoir,the total sediment yield reached to 4.70 × 10⁶ t,with 2.85 × 10⁴ t of the mean sediment yield of individual couplets and 2.53 × 10⁴ t of the mean annual sediment yield;CSSY and ASSY were averaged as 1.44 × 10⁴ t/km2 and 1.28 × 10⁴ t/km2/yr,respectively.Among 165 erosion storms during 1811 and 1996 in the HS catchment,there were 29 heavy rainfalls,49 storms,69 heavy storms,and 18 extraordinary storms,with the heaviest storm in 1838 A.D.From 1855 to 2014 within the JB-1 landslide-dammed reservoir,the total sediment yield reached to 5.13 × 10⁶ t,with 4.07 × 104 t of the mean sediment yield of individual couplets and 3.35 × 10⁴ t of the mean annual sediment yield;CSSY and ASSY were averaged as 1.42 × 10⁴ t/km2 and 1.12 × 10⁴ t/km2/yr,respectively.Among 126 erosion storms during 160 years since 1855 in the JB catchment,there were 29 heavy rainfalls,80 storms,and 17 heavy storms,with the heaviest storm in 1898 A.D.ASSY in the landslide-dammed reservoirs on the CLP is a combined result of annual storm frequencies?ASFs?and CSSYs.According to the distributions of ASSYs,ASFs and CSSYs,the HS catchment showed large various ASSYS during the periods from 1811 to 1996.There were eight periods with high annual storm frequencies and intensities with high ASSYs during between 1811 and 1996,i.e.1811–1822,1838–1855,1868–1871,1883–1914,1921–1923,1935–1940,1956–1961,and 1985–1996 A.D.During the periods from 1810 s to 1950 s,ASFs were relatively higher and stronger,along with the growth of populations and large-scale reclamation of the lands as the result of the rehabilitation police at the late Qing Dynasty,the war of Hui-Han in Tongzhi reign,resulting in higher erosion within the HS catchment.Whereafter,ASFs were relatively lower and local residents mainly planted the corns since 1950 s,resulting in weak and stable soil erosion within the catchment.Similar to those of the HS catchment,the ASSYs within the JB-1 catchment decreased gradually from 1855 to 2014 and reached constant after the 1930 s.To be concluded,annual erosion changes in the different catchments on the CLP were the coupled results of both contemporary storms and human activities.