汉江上游河谷风成黄土堆积及其成壤改造对全新世气候变化的响应规律

The upstream areas of the Hanjiang river, located at the boundary between the temperate zone and subtropical zone in China, is sensitive to environmental change. So far, eolian loess in the areas was absence of researches. Pre-study discover that loess-like sediment over the first level river terraces of the Hanjiang valley is typical eolian loess and have an excellent record information on environmental change. This project will study the material source, strata and its pedogenic modification of the eolian loess in the upper reaches of the Hanjiang river valley. This research involves geomorphology, quaternary sedimentary, soil genesis, mineralogy, geochemistry, chronology and environmental change science fields. Using sedimentary, geochemistry, soil genesis and chronology of multi-disciplinary approach, the upper reaches of the Hanjiang river valley and the reaches of the Danjiang river valley were systemically surveyed. Combination of excavating pits and trench the marked features of the eolian loess is identified by detailed observation. The distribution characteristics and stratigraphic sequence of the eolian loess and its spatial variations, etc. are observed. The evolution of environmental change and human activities in the study areas are analyzed. The typical eolian loess profiles are selected as research material, and then sampled systematically. Particle-size distribution, mineral assemblage, major/trace elements, Nb-Sr isotope, magnetic susceptibility, micromorphology are analyzed in laboratory. Based on above data, sedimentology, micromorphology, geochemisty, and physical and chemical properties indicators are established, which could trace material sources of the eolian loess and determine pedogenic modification and weathering intensity of the eolian loess. The stratigraphic sequence and stratigraphic age of the eolian loess are established using various technologies of dating. Using various tracing methods, including Nb-Sr isotope, mineral and elements assemnlages, grain size distribution etc. determine the material source of the eolian loess. According to physical and chemical properties, micromorphology and composition characteristics of the eolian loess and their variation, analyze and describe the pedogenic modification intensity, process and mechanism of the elolian loess under the sub-tropical environment in the south of the Qinling Mountains; reveal the relationship between the pedogenic modification change of the eolian loess and Holocene climatic change; and clarifies the law between the evolution of the pedogenic modification of the eolian loess and the monsoon advance and retreat, global change and land use change; and provide basic data to sustainable development and utilization of the regional land and water resources, soil and water conservation, disaster prevention and mitigation. This studies are great theoretical significance to reveal the law of the regional soil /climate system response to the global change.

汉江上游地处温带亚热带分界线，是一个环境变化敏感带。汉江上游河谷风成黄土的研究是个空白，本项目拟就这些黄土的物源、地层、成壤改造等问题开展研究。采用沉积学、地球化学、土壤发生学和年代学多学科交叉的方法，通过野外考察观测、筛选典型剖面、采样和室内实验测试，获得黄土沉积学、微结构、组成和理化性质数据。结合多种技术测年，确定黄土成时代，建立风成黄土的地层序列和年代框架；运用多种示踪方法，确定风成黄土的来源和途径；依据沉积物组构和理化性质特征及其变化，分析秦岭南侧亚热带环境当中黄土风化成壤强度、变化过程和发生机制，揭示黄土成壤改造演变与全新世气候变化的相互互动关系；阐明黄土成壤改造演变与季风进退变化、全球变化及土地利用演变关系的规律，为区域土地资源和水资源的持续开发利用、水土保持和防灾减灾提供基础支撑数据。对于揭示区域土壤/气候系统对于全球变化的响应规律具有十分重要理论意义。

汉江上游是一个环境变化敏感带，区内分布的风成黄土蕴含有丰富的古环境信息，但对其的研究仍是一个空白，本项目在这样的背景下进行相关研究。主要研究了汉江一级阶地形成的年代、汉江上游地区风成黄土的分布特征、黄土的地层层序和地层年代学、黄土的风化成壤特征以及研究区域晚更新世以来（55 kaBP）气候变化规律及对全球变化的响应。研究认为：（1）汉江下切作用在晚更新世开始加剧，于55 ka BP前后形成了汉江Ⅰ级阶地。（2）黄土主要分布郧县盆地、安康盆地、西乡盆地、汉中盆地、商丹盆地等，其中汉江第一级和第二级河流阶地是黄土主要分布的地貌位置，黄土厚度分别在5 -10米和15-20米。（3）汉江低级地上黄土是55 Ka BP以来连续堆积形成，地层从下向上依次分为：L1-L3 (55.0 - 28.5 ka BP) → L1-S2 (28.5 - 24.5 ka BP) → L1-L2 (24.5 - 23.0 ka BP) → L1-S1 (23.0 - 21.0 ka BP) → L1-L1 (21.0 - 11.5 ka BP)→Lt (11.5 - 8.5 ka BP) → S0 (8.5 - 3.0 ka BP)→L0 (3.0 - 0 ka BP)。（4）汉江上游地区黄土的常量元素含量具有SiO2＞Al2O3＞Fe2O3＞K2O＞MgO＞Na2O＞CaO特征，其中SiO2、Al2O3和Fe2O3三者总含量达767.32 g/kg。黄土风化成壤过程中Na、Ca、Mg、Si发生不同程度的淋溶，Fe、Al、K相对富集。汉江黄土与洛川黄土元素组合特征高度一致，含量十分接近且UCC标准化变幅均小于0.25。以黄土为母质发育的土壤具有A-AB-Bt1-Bt2-BC-C剖面构型，具有“黏磐”诊断层，在系统分类中的可归为黏磐湿润淋溶土。（5）5.5ka BP以来在44.5 - 41. ka BP、37.5 - 34.0 ka BP、28.5 - 24.5 ka BP、24.5 - 21 ka BP、8.5 – 3.0 ka BP出现五次较暖湿的阶段，其中44.5 - 21 ka BP期间的暖湿可与深海同位素曲线MIS-3的气候特征进行比较。本项目的成果是汉江上游地区有关地学问题的最新研究成果，可为区域第四纪问题的研究、为准确理解区域土壤/气候系统对于全球变化的响应规律提供基础资料。