天山北坡流域的化学风化与水碳通量研究

Nowadays, the CO2 concentration in the atmosphere is strongly increasing, which leads to the global warming. How to alleviate the environmental impacts of this global event is an important scientific problem for the society. The chemical weathering process of the continental drainage basins is one of the most important ways of CO2 consumption in the atmosphere, and may play an important role in the inhibition of global warming, especially the drainage basins in the High Asian orogenic belt. However, it is still not clear what is the role on climate of the chemical weathering process in the drainage basins of the Central Asian Orogenic Belt (CAOB) in the hinterland of Asia. The North Slope of Tianshan Mountains (NSTM) is located in the central part of the CAOB, with a great diversity in climatological, geological and hydrological backgrounds. The on-applying project intends to take the natural water bodies of these basins as research object and use the isotope geochemistry, combined with the natural environment parameters, as method to explore the characteristics of chemical weathering processes and their products in the drainage basins of the NSTM; to identify the driving mechanisms of different types of chemical weathering processes in the basins and the related environmental influencing factors; to quantify the consumption rate of the silicate weathering process on the atmospheric CO2 and the “water carbon” (or weathering carbon) flux of the silicate weathering products in a watershed scale; and to identify the contribution of the water carbon flux from the NSTM even the CAOB on global carbon cycle, based on the comparisons with other drainage basins in the low and middle latitudes of the Northern Hemisphere. The research findings will provide a substantial evidence for understanding the potential role of the Central Asia drainage basins in the global climate change. In addition, the research will be theoretically helpful for understanding the role of silicate weathering on global climate change, to which a hypothesis “tectonic-and-climate forcing → chemical weathering → increasing consumption of atmospheric CO2 → negative feedback to climate”, namely the Raymo hypothesis, is prosed.

现今大气圈CO2浓度剧烈增加并导致全球增温，如何缓解这一环境效应是社会各界面临的重要科学问题。大陆流域的化学风化过程是大气圈CO2消耗的重要途径之一，在抑制全球变暖方面可能有潜在作用，尤其是亚洲高海拔地区的造山带流域。但亚洲腹地的中亚造山带（CAOB）流域有怎样的风化过程及其与气候的关系目前还不清楚。天山北坡位于CAOB的中央，具有气候、地质和水文背景的多样性，项目拟以这里的流域水体为研究对象，利用同位素地球化学等手段并结合流域的自然环境要素，探索CAOB流域化学风化过程的特征和风化产物；识别不同类型化学风化作用的驱动机制及其环境影响因素；量化流域尺度上硅酸岩化学风化对大气CO2的消耗速率及其风化产物中的“水碳”（风化碳）通量，并与北半球中、低纬度流域进行对比，识别天山乃至CAOB水碳对全球碳循环的贡献。研究结果将为了解中亚流域在全球气候变化中的潜在作用提供科学证据。

亚洲腹地中亚造山带(CAOB)地区的流域具有怎样的化学风化潜力及其水碳通量尚未知。项目系统收集与整理了天山北坡及周边不同流域的最新文献资料数据，并开展了研究区不同流域的野外考察、样品采集、实验分析和第一手远传数据的获取，并综合资料数据和原创数据开展了流域风化的研究。主要研究内容包括：（1）探讨了天山北坡不同流域天然水体的水文地球化学背景特征及其成因；（2）评估了天山北坡乃至北疆流域天然水体的水质特征、污染状况及其影响因素；（3）明确了玛纳斯河流域天然水体的来源及其补给机制；（4）阐明了北疆流域的化学风化特征、岩石风化类型（碳酸盐岩、硅酸岩、蒸发岩等风化）和影响因素；（5）量化了北疆流域化学风化速率和风化通量，CO2的消耗速率和水碳通量，并进行区域（尤其是中纬度地区）对比。上述研究获得了“天山北坡乃至北疆水系的河流矿化度普遍高于我国季风区和湿润地区的河流，也高于世界平均水平”、“天山北坡及河流的碳酸岩风化强度远高于硅酸岩风化，蒸发盐溶解也是流域风化产物的重要来源”、“中纬度流域几乎都以碳酸盐岩风化为主，具有一致性和高效性”、“天山北坡流域化学风化过程受限于运输限制机制”、“北疆流域风化速率介于0.116～93.6 t/km2/yr之间（平均18.6 t/km2/yr），略低于世界平均值（24 t/km2/yr）”、“天山北坡流域CO2吸收速率介于0.19~101×103 mol/km2/yr之间（平均值10.7×103 mol/km2/yr）低于世界平均水平（246×103mol/km2/yr）”、“北疆流域水碳通量（0.93×106 t/a）约占中国河流总量（1.41×107 t/a）的6.57%，全球河流总量（2.52×108 t C/a）的0.37%”等新认识。在上述研究工作中，鉴于中亚造山带CAOB的地理广布性、水文地质背景多样性和地表过程的多样性，项目还针对中亚造山带中部的河西走廊地区、阿拉善高原地区等其他流域的地表过程（包括风化过程）等进行了对比研究工作。截至2021年底，我们已经完成了研究区大部分样品的实验分析工作，并撰写和发表了相关的学术论文12篇，完成了计划书中预期的研究工作内容和成果。