西南喀斯特洼地泥沙沉积速率反演及来源复合指纹示踪

Karst peak-cluster depressions are typical landforms and mainly composed of limestone and dolomite in southwest of China. Due to the differences in fissure and soil depth, limestone and dolomite regions were probably experienced different erosion processes. However, few studies were performed to compare the sediment deposition rates between limestone and dolomite depressions. Moreover, the relative contributions of different sediment sources from a karst limestone or dolomite depression is not yet clear, which need further and systematic study. In this study, taking typical limestone and dolomite karst peak-cluster depressions as the field experimental site, a sediment column from depressions and soil samples from the three potential sediment sources (surface soil, trench walls soil, and fractured earth) were collected and radionuclides (137Cs、210Pbex), soil texture, soil organic matter, magnetic susceptibility, total phosphorus (TP) and metallic elements (e.g. Al, Fe) were analyzed. The profile distribution of 137Cs, 210Pbex and clay content were used to date the deposited sediments in the karst depression bottom and to further reconstruct the soil erosion history. The relative contributions from different sources for the depressions were determined using the composite fingerprinting techniques. The aims of this study are to identify recent (50 a) changes of sediment deposition rates and sediment sources for the two depressions, and to explore the reason which is responsible for these changes. The results would provide helpful advices for optimal soil and water management and the control of rock desertification.

西南喀斯特峰丛洼地分布最广的两类岩石是石灰岩和白云岩，两种地质背景区裂隙发育和土壤厚度的不同，可能导致洼地泥沙沉积速率及来源存在差异。然而，目前对于石灰岩与白云岩洼地泥沙沉积速率的对比研究并不多见，同时由于喀斯特洼地泥沙来源的多维性和多源性，各泥沙来源的贡献率也尚不清楚。为此，本研究拟在西南喀斯特典型石灰岩与白云岩峰丛洼地小流域，采集、测定洼地和三类源地土壤（坡面表层土、岩缝裂隙土、沟壁土）的核素、粒度、有机质、磁化率、全P和金属元素等指纹示踪因子，应用137Cs、210Pbex示踪技术进行洼地沉积物断代，结合粒度分析，反演两类洼地泥沙沉积的历史过程，并运用复合指纹识别法量化三类泥沙源地的泥沙贡献比，查明两类洼地的泥沙来源，探究其近50年的变化趋势及原因。通过对比研究，揭示石灰岩与白云岩峰丛洼地侵蚀产沙的不同特征和规律，同时为该区两种地质条件下水土优化配置和石漠化治理提供科学依据。

西南喀斯特区为我国典型生态脆弱区，由土壤侵蚀及其造成的石漠化已成为制约该区可持续发展最严重的生态环境问题。峰丛洼地是该区分布很广的一种地貌形态，由封闭的洼地与周围的峰丛组成，该地貌特征决定了流域的产流产沙具有明显的向心聚集性，洼地是坡面产沙的汇集中心，可视为天然的大型沉沙池或塘库，记录了流域侵蚀产沙的历史过程，因而是进行喀斯特流域侵蚀产沙及泥沙来源研究的良好场所。为此，在典型喀斯特洼地选取三个剖面，测定137Cs、210Pbex、粘粒、粉粒、砂粒、各颗粒中值直径、磁化率、有机碳、总氮和总磷等，利用137Cs和210Pbex示踪技术进行西南喀斯特洼地小流域沉积泥沙断代，发现自1949年至2015年土壤侵蚀速率呈现相关学者所猜测的“倒抛物线”形状，其中土壤侵蚀速率由1965-1971年的682 t km−2 yr-1骤减至退耕还林后1999-2015年的80 t km−2 yr-1，减沙效率达88%。但是该区土壤成土速率极低，仅为紫色土的1%，纯碳酸盐地区形成1 cm厚的土壤往往需要2000-8000年，允许土壤流失量仅为30–68 t km-2 yr-1。因此，虽然自1999-2000年退耕还林还草措施实施以来土壤侵蚀得到一定程度的控制，但侵蚀量仍然高于土壤允许流失量，因此识别喀斯特流域土壤侵蚀的主导因素，对该区水土流失进行有效防治仍然需要引起足够的重视。同时，对喀斯特洼地的三类泥沙来源（农地、林地、岩缝裂隙土）样品进行采集，运用复合指纹识别法量化了各泥沙来源的比例，发现洼地泥沙主要来源于农地（69.3%），来源于岩缝裂隙土和林地的比例分别为22.1%和8.6%。据我们所知，本研究是第一次量化喀斯特洼地的泥沙来源，对于该区土壤侵蚀的研究具有重要意义。该项目共计发表SCI论文9篇，其中第一标注1篇（一区1篇），第二标注3篇，第三标注3篇，第四标注2篇。协助培养博士生1名，硕士生1名。