沉积物Li同位素示踪大陆风化的可靠性：中国东南典型小河流研究

In comparison to marine carbonates, the Li isotopic composition of detrital sediments has unique advantages that recording the regional weathering conditions and not being affected by biological fractionation. However, both source rock composition and hydrodynamic sorting have great influences on the Li isotope of the sediments. By far, the reliability and validity of this continental weathering tracer has not been fully assessed. Unlike the large river basins that integrate different weathering regimes and comprise complex rock types, the small catchments have relatively homogeneous climatic and lithological settings. These small rivers can thus be regarded as natural sedimentological laboratories for verifying the significance of Li isotope of detrital sediments. In this research, Li isotopic compositions of bank and suspended sediments will be analyzed from two small rivers, the Mulanxi River and the Zhuoshuixi River, which have characteristic climatic and lithological settings. Based on our previous mineralogy and elemental geochemical studies, we will discuss the influence of provenance lithology, hydrodynamic sorting, and chemical weathering intensity on Li isotopic composition of sediments from small catchments. This work will help us to assess the reliability and validity of using Li isotope to trace the weathering of detrital sediments, and finally leads a better understanding of Li isotope behaviors in the earth surface processes.

碎屑沉积物Li同位素因为不受生物分馏作用影响，且可以反映区域化学风化信息，因而在示踪大陆风化研究中较海区生物自生碳酸盐记录具有独特优势。然而，碎屑沉积物Li同位素组成受源岩类型与水动力分选等因素影响显著，其示踪大陆风化的可靠性仍未得到有效验证。相比大河流域复杂的源岩类型和化学风化过程，小流域源岩组成、气候类型和沉积“源—汇”过程较简单，是检验沉积物Li同位素指示意义的“天然实验室”。福建木兰溪和台湾浊水溪都是典型的小型流域，具有源岩类型单一，沉积物输运简单的特点。本课题拟选取这两个典型小河流域，比较研究不同粒级的沉积物Li同位素组成。在前期矿物学与元素地球化学研究基础上，探讨源岩类型、沉积旋回过程、水动力分选、化学风化强度等因素对小流域沉积物Li同位素组成的控制作用。本研究对评估河流沉积物Li同位素示踪大陆风化的可靠性具有重要的借鉴意义，有助于深入理解地球表生过程的Li循环特征。

本项目系统研究中国东部河流沉积物中主量元素、微量元素及Sr-Nd-Li同位素组成的不均一性，讨论了控制河流沉积物地球化学组成的自然因素和人类活动因素，揭示常用化学风化指标的多解性以及如何剥离出河流沉积物中现代风化信息。.定量研究硅酸盐沉积物风化强度的前提是理解风化信号的保存和传播机制，以及厘清水动力分选和原岩继承性的作用。项目提出了A-CNK-Si图解，以厘清化学风化、水动力分选和物源继承性等对河流沉积物地球化学组成的控制作用。该方法考虑了矿物分选和石英稀释效应，因此有助于解译保存在沉积记录中的硅酸盐风化信号，从而服务于表生地球化学研究。.项目还利用Nd同位素组成研究了黄河沉积物的物源组成。黄河沉积物中Nd同位素的空间不均一性与沉积物源汇过程对应很好，反映了人类活动对沉积源汇过程的影响。 在全球河流面临强烈的人为影响的背景下，沉积源汇系统改变对河流沉积物地球化学的影响值得更多的研究。.此外，本项目基于质量平衡原理，建立了基于化学相态分析与HF-HNO3消解计算SiO2浓度的新方法。该方法需要样品量更小，比传统方法更加省时省力，还避免了碱熔法添加助熔剂所产生的金属元素污染。.项目在Chemical Geology, Water Resources Research, Journal of Geochemical Exploration等SCI刊物发表SCI论文多篇，项目开展将对利用沉积地球化学方法进行古气候和古环境重建提供方法参考。