全新世长江沉积物旋回过程对流域风化的影响：Li-Nd同位素示踪研究

Knowledge of river sediment recycling provides important constraints on the continent weathering and earth surface processes. The weathering product of source-to-sink system transfer record signals of external environmental forcing, as well as internal dynamics of the sedimentary system. Most previous studies analyzing the general driving mechanism of silicate weathering focused on external driving including tectonic and climatic factors after ignoring the internal dynamics, such as sedimentary transport cycle, in river basin, which would lead to large error for paleoenvironmental interpretation and misunderstanding of biogeochemical circulation in marginal sea. The Changjiang Delta and inner shelf of the East China Sea areas have accumulated large amount of clastic sediments during the Holocene. This research program aims to build up tracer proxies using the Li-Nd isotopic records of depth profile samples collected from Changjiang Estuary and two cores taken from the Changjiang Estuary and inner shelf of the ECS. Combined with a mass budget approach, this study is devoted to reconstruct the Changjiang sediment recycling transport process, estimate unweathered rock fragments and “real-time” weathering products in the sedimentary records, restore the chemical weathering process in Changjiang basin during the Holocene. And on this basis, explore the implications of the evolution of sediment recycling transport process (sediment transport rate and detention time variation) for the weathering records in the ECS basins. The results through this proposal would improve knowledge of large river source to sink system and regional biogeochemical cycle.

大河流域源区风化物质经历多次“搬运‒沉积‒再旋回搬运”过程最终进入海区埋藏，沉积记录中除保存物源区风化信号外，还叠加沉积输运过程中所经历的后期风化信息。过去研究主要集中于构造和气候如何影响流域风化，而对沉积输运旋回过程等河流内部动力因素如何影响化学风化认识薄弱，导致沉积区物源判别和古环境解释的偏差，以及在边缘海生物地球化学通量与循环研究中出现不平衡等问题。本课题拟选取长江河口深度剖面时序样品和长江口‒东海内陆架全新世钻孔为研究对象，利用Li‒Nd同位素示踪及质量平衡模型方法，重建全新世长江入海沉积物输运旋回过程，并辨析沉积记录中“即时”风化和继承性风化物质来源及相对贡献，还原全新世长江流域化学风化过程。在此基础上探讨流域沉积物输运旋回过程（输运速率和流域滞留时间变化等）对流域风化的影响机制，深化对大河流域源汇系统以及地球表生地球化学循环的研究认识。

长江口及东海内陆架泥质区作为长江流域风化剥蚀产物的沉积“汇”，记录了冰后期季风气候和人类活动对长江流域化学风化与沉积物源汇过程的影响。末次冰消期晚期至早全新世，随着全球气候转暖，长江流域化学风化强度加强；中全新世海平面快速上升至接近现今高度，长江沉积中心向流域内部迁移增加了沉积物在流域内的滞留时间，使得长江入海沉积物在早中全新世具有较高的化学风化强度；约4.0 ka以来东亚夏季风强度减弱，降低了长江沉积物的搬运能力，同时长江河流沉积物沉积中心向下游强风化作用地区迁移，硅酸盐岩化学风化加强；晚全新世，人类活动开始影响长江沉积物源汇过程，上游弱风化物质进入河口地区以及沉积物在流域内滞留时间减少，使得长江口沉积记录化学风化强度降低。.Li同位素因其在示踪大陆风化作用方面的独特优势，成为近几年研究的热点。广受关注的河口区域是陆地和海洋系统的结合地带，物理化学条件变化复杂。目前对于河口地区Li同位素地球化学行为的研究鲜有报道，限制了Li同位素在流域风化研究中的应用。本研究对长江口断面表层悬浮颗粒物、底层河床砂样品进行主微量元素、Li同位素测试，结合二元模型计算长江口样品中风化产物的比例，探讨了长江口地区Li同位素地球化学特征及其影响因素。研究结果发现由陆向海，长江口河床砂的δ7Li有升高趋势，悬浮颗粒物的δ7Li有降低趋势，与沉积物化学风化程度指标CIA值的变化趋势相似；河床砂的风化产物比例与粒度替代性指标Al/Si有良好的线型相关性，而悬浮颗粒物中并无以上关系。本研究认为长江口底层河床砂与表层悬浮颗粒物两者在最大浑浊带之前有相对稳定的明显分层，最大浑浊带处两者逐渐混合。长江口底层粗颗粒沉积物是不同粒径沉积物的混合，由最大浑浊带的水动力条件等物理作用将水体混合导致；而细颗粒悬浮物的风化产物比例变化受粒度、水动力影响较小，并不是由水体混合导致的，推测是由长江口物理-化学因素（pH、盐度和氧化还原电位）改变引起的。