变化条件下雅鲁藏布江碳来源与通量变化及其生物地球化学过程研究

Riverine carbon cycling is a frontier of multiple research areas such as global change, surface processes, water resources and environmental sciences. However, some crucial issues, such as the plateau uplift effects on atmospheric CO2 balance; the relationship between material transport of highland rivers and Land-Sea interaction, still remain poorly understood. To address the key objective “Quantitatively identify the transportation process and flux variation of biogenic matter under runoff regulation” of the NSFC Major Research Plan, this project focuses on sources and fluxes and biogeochemical processes of riverine carbon in the Yarlung Zangbo River (YZR). The YZR drains the Tibetan Plateau characterized by complex geological structure, intensive erosion and fragile ecological environment. It is very sensitive to climate change and human activities, providing an idea region for studies on riverine carbon cycling under changing ambient environment. This project will employ multiple isotopic tracers including δ13C, Δ14C, δD, δ18O, δ34S, δ15N, 87Sr/86Sr and δ7Li, combined with elemental geochemistry, to examine the geochemical and isotopic compositions, occurrence state and temporal-spatial patterns of each process involved in the riverine carbon cycling in the YZR. Quantitatively identifying of sources, transferring and transformation, as well as fluxes of the riverine carbon in the YZR will be conducted in the proposed project, aiming to evaluate the effects of lithology, topography, climate and runoff changes on carbon dynamics in basin scale, especially their feedback to global change and damming on rivers. The results of this project will not only improve our theoretical knowledge of mechanisms of riverine carbon cycle, but also provide scientific supports for assessment and prediction of the effects of climate change and human activities on the plateau environment.

青藏高原河流碳循环是当前国内外研究热点，目前对青藏高原隆升风化侵蚀与大气二氧化碳平衡、高原河流物质输送通量变化的陆-海相互作用影响等重大科学难题尚缺乏确切认知。本项目围绕重大计划“径流变化下生源物质迁移转化及其通量变化的定量解析”核心问题，以雅鲁藏布江中上游为重点，建立以水化学、微量元素、常规和非常规同位素（δ13C、Δ14C、δ34S、δ15N、δ18O、87Sr/86Sr、δ7Li等）多要素耦合的高原河流碳循环多元示踪体系，查明雅江干流与支流、湿地、冻土等主要环境单元中碳及有关地球化学指示特征，定量解析河流碳的不同来源及其变化规律，全面辨识风化侵蚀、冻融、沉积、生物作用等对河流碳来源与生物地球化学过程的影响，系统揭示气候变化和人类活动干扰情况下高原河流碳通量变化规律及其作用机制，为重大计划预期目标的实现提供理论基础和科技支撑。

本项目围绕重大计划“径流变化下河流生源物质迁移转化及其通量变化的定量解析”科学问题，以雅鲁藏布江流域为研究对象，建立了以水化学、元素和同位素地球化学手段为基础的河流碳来源和迁移转化多指标联合示踪体系，查明了雅江流域河流各形式碳含量及同位素时空分布特征与变化规律，取得了河流碳的来源及其物质贡献量的定量解析结果；揭示了河流碳的来源、迁移转化过程及其控制机制，以及河流碳在不同储库之间的输入/输出通量的定量认识；揭示了径流变化条件下河流碳的含量组成和来源变化及其响应关系与控制机理。.本项目研究基于对雅江干流、主要支流和典型子流域的高密度空间和高频次时间序列的系统观测与分析研究发现：1）雅江河流各种碳含量（DIC、DOC和POC）具有高度的时间与空间变化异质性，并表现出稳定碳同位素δ13C异常偏正、且14C年龄严重偏老的独特特征；2）河流有机碳主要来自陆地植物，主要受土壤侵蚀过程的控制。河流无机碳主要来源于流域岩石的化学风化过程，因其独特的地质和构造背景，明显受到了硫酸参与的碳酸盐岩风化地质碳和热泉等深部碳来源的贡献；3）基于高频时间序列观测，建立了研究河流物质含量和径流变化（即C-Q）定量响应关系，发现径流变化不仅显著影响河流各种碳的含量和通量，还明显改变了河流碳的来源。并运用水岩反应化学计量学和元素-同位素地球化学方法，阐明了径流变化下雅江碳来源与迁移转化过程的控制机理，并取得了不同径流变化条件下化学风化过程对河流碳来源贡献的定量认识；4）取得了各主要控制断面各形态碳在不同储库间的输移通量的定量结果，构建了雅江流域河流碳的传输过程与碳循环源汇收支平衡模式。该项研究的开展与成果认识的取得促进了对青藏高原流域风化侵蚀与碳循环过程的理论认识，为完善大陆风化碳循环和气候变化理论提供新的依据，并为雅江梯级水库适应性开发提供了科学支撑，具有科学和应用上的双重意义和价值。