Chemical weathering in the Himalayan system in response to the evolution of the Indian summer monsoon since 37 ka

Based on clay mineralogy, siliciclastic grain sizes and geochemical records from deep-sea sediments in the central Bay of Bengal (BoB), the evolutionary history of the Indian summer monsoon and associated weathering process in the Himalayan system since 37 ka were reconstructed. Clay minerals and Th-La elements indicate a mixture of sediments originating from the Ganges-Brahmaputra (G-B) Rivers, Indo-Burman ranges/Irrawaddy River and Godavari-Krishna (G-K) Rivers. The Th-La quantitative reconstruction revealed that sediment supply from the G-B Rivers was reduced during a series of millennial-scale cold climate events (i.e., Younger Dryas and Heinrich Events) and the Last Glacial Maximum. By removing the control of sea-level factors on the percentage of clay minerals in the sediments, we suggest that the marked decrease in the illite chemistry index and the increase in the (illite+chlorite)/smectite ratio during cold climate periods reflect enhanced physical weathering and weakened chemical weathering in the Himalayan system. Comparing the contribution from the G-B Rivers and the sedimentation rates in different regions of the BoB, it was inferred that the sediments of the G-B Rivers were mostly deposited in the upper and middle Bengal Fan. During the Holocene, the Active Channel played a crucial role in delivering sediments from the G-B Rivers to the lower Bengal Fan. The decreased chemical index of alteration (CIA) and increased K₂O/TiO₂ ratio indicated the weakening of the Indian summer monsoon. At that moment, the Himalayan system was involved in cold climatic conditions accompanied by glacial extension, which led to enhanced physical weathering and increased sediment contributions from the highlands.

本研究基于孟加拉湾中部（BoB）深海沉积物的黏土矿物学、陆源碎屑粒度与地球化学记录，重建了37 ka以来印度夏季风的演化历史，以及喜马拉雅体系的相关风化作用过程。黏土矿物与钍-镧（Th-La）元素特征显示，沉积物主要来源于恒河-布拉马普特拉河（G-B）、印缅山脉/伊洛瓦底江以及哥达瓦里-克里希纳河（G-K）的混合沉积。钍-镧定量重建结果表明，在一系列千年尺度冷气候事件（即新仙女木事件与海因里希事件）及末次盛冰期期间，恒河-布拉马普特拉河的沉积物供给量有所减少。通过去除海平面因素对沉积物中黏土矿物百分含量的控制作用，本研究提出：冷气候时期伊利石化学指数的显著降低，以及（伊利石+绿泥石）/蒙脱石比值的升高，反映了喜马拉雅体系物理风化作用增强、化学风化作用减弱。对比恒河-布拉马普特拉河的沉积物贡献与孟加拉湾不同区域的沉积速率，研究推断恒河-布拉马普特拉河的沉积物主要沉积于孟加拉扇中上部区域。在全新世时期，活动水道在将恒河-布拉马普特拉河的沉积物输送至孟加拉扇下部的过程中发挥了关键作用。化学风化指数（CIA）的降低与氧化钾/二氧化钛（K₂O/TiO₂）比值的升高，指示印度夏季风出现减弱。此时喜马拉雅体系处于伴随冰川扩张的冷气候环境中，进而导致物理风化作用增强，高原地区的沉积物贡献量提升。