近50a沂蒙山区大型水库泥沙来源及其对流域环境演变的响应

Investigation on the sediment sources is important for us to know the pattern of sediment yields from the catchment, as well as adopt effective strategy for soil conservation and pollution control targeting at different sediment sources in the catchment. Sediments deposited in reservoir contain records of sediment yields from the upstream catchment, therefore the temporal trend of the sediment yields could be obtained by examining these deposits. The Yimeng Mountain Region is characterized by alternating granite (or gneiss) and limestone, sediment yields from the two contrasting lithologies are notably different. Then their relative contributions to the sediment yields are different as well. Environment has been intensively changed influenced by human activity in the past 50 a, and their relative contributions may vary as well in response. Lithologies underlain the catchment of large reservoir might be more prone to represent the general lithological characteristic of the alternating granite (or gneiss) and limestone in Yimeng Mountain Region than that of small pond. Choosing the Xujiaya Reservoir as the representative example of large reservoir in Yimeng Mountain Region, the objective of this research is therefore to calculate the relative contributions to sediment yields from the granite (or gneiss) and limestone areas in the catchment in recent 50a by examining the reservoir deposits, and reveal the contribution responses to environmental changes in the catchment. To achieve these goals, the following works would be done: establishing chronologies of the reservoir deposits using 137Cs, 210Pbex tracer methods; associating the variations of 137Cs, 210Pbex, SOC, and particle size in the sediment profiles with the history of environment change in the catchment by interpreting these variations in view of the land use / land cover change and historical precipitation in the catchment; calculating the deposit volume in the reservoir for different periods according to reservoir capacity curve; apportioning catchment scale sediment sources from the two main lithologies, viz. granite and limestone, for deposits in different depth range using composite fingerprinting technique, in other words, calculating the proportion and volume of sediments deposited in the reservoir deriving from the granite and limestone underlain area in catchment for different periods; elucidating the sediment source variations in recent 50 a considering environmental changes in the catchment, and reconstructing the history of sediment source variations in responses to environmental changes; and revealing the temporal patterns of recent sediment sources from the alternating granite and limestone catchment in the Yimeng Mountain Region.

泥沙来源研究对认识流域产沙规律和治理水土流失有重要意义。沂蒙山区花岗岩与石灰岩呈复区（交错）分布，近50a在人类活动影响下环境演变剧烈，两种岩性源地来沙均有响应。水库沉积记录了流域来沙的历史过程，相对小塘库，大型水库流域岩性更能体现该区花岗岩与石灰岩复区分布的一般特征。本研究拟选取沂蒙山区代表性大型水库—许家崖水库，查明不同沉积层中花岗岩与石灰岩源地的来沙贡献，分析近50a泥沙来源变化对流域环境演变的响应。以137Cs和210Pbex示踪法进行沉积物断代，建立沉积物剖面137Cs、210Pbex、有机碳、粒度变化与流域近50a土地利用/覆被变化、降雨历史等相关环境事件的对应关系；参照库容曲线计算不同时期淤沙量；应用复合指纹识别法计算不同沉积层泥沙来源，即查明花岗岩与石灰岩源地不同时期的来沙比例及来沙量；探明泥沙来源变化对流域环境演变的响应过程，揭示花岗岩与石灰岩复区流域泥沙来源的近期规律。

水库沉积物记录了流域来沙历史，通过沉积物可探明泥沙来源的近期特征。研究水库泥沙来源对认识流域侵蚀产沙规律和有针对性地治理不同泥沙源地的土壤侵蚀有重要的理论和现实意义。沂蒙山区花岗岩与石灰岩呈复区分布，近50a在人类活动影响下环境演变剧烈，不同岩性源地来沙均有响应。以沂蒙山区代表性大型水库—许家崖水库及其流域为研究对象，结合侵蚀产沙反演，查明了近50a水库泥沙来源对流域环境演变的响应特征和规律。以137Cs和210Pbex示踪法，应用CIC、CRS和C-CRS模型确立了近50a以来沉积物年代序列；基于土地利用/覆被变化和降雨等因素，分析了沉积物剖面137Cs、210Pbex、TOC、TN、C/N和粒度的变化特征；参照流域环境演变历史阶段划分，应用库容法计算了不同时期淤沙量进而产沙模数，同时厘清了流域侵蚀产沙与水库养分沉积速率的关系；以Mg等地球化学元素为指纹因子，应用复合指纹识别法计算了不同沉积层来自流域花岗岩、石灰岩和页岩地区的来沙比例，即水库泥沙来源；在此基础上分析了泥沙来源对流域环境演变的响应，以及颗粒分选对泥沙来源的影响。总体而言近50a以来，产沙强度变化趋势反映了不同岩性背景下人类活动对侵蚀产沙的影响，降水对侵蚀产沙变化无根本影响。花岗岩地区来沙比例呈增加趋势而石灰岩地区则相反，这反映了两种岩性区在人类活动影响下侵蚀产沙的不同特征，花岗岩地区沙源丰富易于侵蚀，来沙比例随人类扰动强度增加，而石灰岩地区沙源少，加之表层岩溶带限制，来沙比例难以增加。页岩地区情况与石灰岩地区相似。尽管花岗岩分布面积大于石灰岩与页岩之和，但前者来沙比例分别低于后二者，此因花岗岩地区土壤颗粒更粗，来沙多沉积于上游；而石灰岩与页岩地区土壤颗粒细，易被搬运至主库区沉积，导致来沙比例高，可见颗粒分选对水库泥沙来源有重要影响。此外在喀斯特洼地初步尝试了更长时间尺度侵蚀产沙反演，为下一步深入研究打下坚实基础。