藏东南大峡谷地区黄土-古土壤-红粘土沉积序列研究

There are numerous studies of the stratigraphy and paleoenvironmental significance of the loess-paleosol-red clay on the Chinese Loess Plateau and its adjacent regions. Emphasis has been placed on their use as a paleoclimate archive, especially for the study of changes in the East Asian monsoon. In contrast, however, there have been few investigations of the loess-paleosol-red clay sedimentary sequence in the gorge region of the Yarlung Tsangpo River, southeast Tibetan Plateau. In this project, we will make the gorge area, where only dominated by the Indian Monsoon, as the focus of the investigation. The loess-paleosol-red clay sedimentary sequence in area will be the key research object. The mineral compositions, geochemical compositions, ages, sensitively substitutive indicator of environments, isotopic tracing index of the loess-paleosol-red clay sedimentary sequence and their potential source materials will be examined. Based on all of the analytical data, we will (1) identify the genesis of red clay, (2) investigate the temporal and spatial variability for the loess-paleosol-red clay sediments, (3) reveal the provenance and transport mechanism for the parent materials of the loess-paleosol-red clay sediments, (4) build the chronosequence of the loess-paleosol-red clay sediments, (5) and reconstruct the changes of the Indian monsoon. Furthermore, the interrelationship between the Indian monsoon and the East Asian monsoon will be explored based on a comparison with loess-paleosol-red clay on the Chinese Loess Plateau. Moreover, the knowledge about the sedimentary processes of the loess-paleosol-red clay in the gorge region is also of interest for understanding the evolution of monsoonal temperate glacier and the dammed processes of the Yarlung Tsangpo River by glacier and/or moraines.

在我国的黄土高原及周边地区，黄土-古土壤-红粘土的地层及古环境意义已得到深入研究。作为古环境记录，这些风尘堆积在东亚季风演化历史的重建中发挥了重要作用。相对而言，对于藏东南雅鲁藏布江大峡谷地区黄土-古土壤-红粘土沉积的调查和研究却十分薄弱。本项工作，拟以受印度季风单一主导的藏东南大峡谷地区为重点调查区域；以大峡谷地区的黄土-古土壤-红粘土沉积体系为重点研究对象。基于矿物组成、地球化学组成、年代学、物源示踪及环境替代指标的研究；确认大峡谷地区红粘土的成因，阐明该地区黄土-古土壤-红粘土沉积的时空分布，揭示其物源及风尘传输过程；建立黄土-古土壤-红粘土沉积的年代序列，并重建研究区印度季风的演化历史。进而，通过与东亚季风控制区黄土-古土壤-红粘土沉积序列的对比研究，探讨印度季风与东亚季风的相互关系。最终，探索大峡谷地区黄土-古土壤-红粘土沉积过程与海洋性冰川进退、冰川-冰碛物堰塞湖发育的关系。

在雅鲁藏布江流域，末次冰期印度季风变化及其所导致的海洋性冰川进退、冰川-冰碛物堰塞湖发育、风尘过程与风尘堆积等一系列问题仍然不清楚。本项工作，以受印度季风主导的藏东南大峡谷地区为重点调查区域；以大峡谷地区的黄土-古土壤沉积体系为重点研究对象；结合雅鲁藏布江中游堰塞湖沉积-黄土沉积序列的对比研究。以粒度、磁化率、有机碳和无机碳含量（TOC/TIC）、常量及微量元素（包括稀土元素）分析为基础，结合Sr-Nd同位素地球化学示踪及AMS14C定年等方法，对上述问题开展了深入的调查和研究。研究结果指示：（1）藏东南大峡谷地区黄土-古土壤是山谷风短距离搬运的近源沉积。风尘主要来自尼洋河谷早期的堰塞湖沉积和河流沉积。（2）藏东南黄土-古土壤沉积以碳酸盐缺失为特征。物源区碳酸盐组分的缺乏及风尘沉积后的淋溶改造是黄土-古土壤中缺失碳酸盐的主要原因。（3）磁性矿物的粒度分布及成土环境对黄土-古土壤磁学特征有着重要的影响。在低海拔成土环境下，古土壤发育程度是低频磁化率和频率磁化率变化的主要控制因素。相反，在高海拔成土环境下，黄土-古土壤低频磁化率和频率磁化率主要受原生磁性矿物的粒度分布所控制。（4）印度季风在末次冰期存在千年尺度上的气候不稳定性和冷暖事件的强烈振荡循环。古土壤的发育特征指示了印度夏季风的异常增强。这一现象类似于Dansgaard–Oeschger事件。此外，末次冰期印度夏季风增强事件可能比东亚夏季风更为显著。（5）在末次冰期，雅鲁藏布江中游发育了堰塞湖，河谷被堰塞湖所占据，从而导致该区域缺失末次冰期风尘堆积。而在大峡谷地区的尼洋河河谷，早期堰塞湖的消亡及大量堰塞湖沉积物的出露，为风尘堆积的发育奠定了基础。因此，印度季风的波动及其导致的雅鲁藏布江冰川（冰碛物）堰塞事件，影响和控制了河谷风尘堆积的发育，并导致局地气候-风尘的脱耦。本项工作为深入理解青藏高原的气候、水文、湖泊、生态环境变化和河流地貌演化的历史和机制提供了科学依据。