| 摘要: |
| 流域风化是理解大陆岩石化学风化对全球气候变化的反馈机制的重要途径，目前的研究集中在大河流域，小流域特别是高寒地区流域化学风化的影响因素尚不确定。本文选择岩性相对单一、人为活动干扰少的青藏高原尼洋河流域为研究对象，开展一个水文年的采样分析，阐明高寒地区流域岩石化学风化的季节变化特征及其影响因素。研究结果显示：尼洋河河水离子主要来源于碳酸盐岩风化和硅酸盐岩风化，对河水中阳离子贡献量分别达到60%和29%，风化速率分别为0.20—19.00 t∙km−2∙month−1和0.09—0.80 t∙km−2∙month−1，年平均值分别为11.90 t∙km−2∙a−1和4.38 t∙km−2∙a−1。在一个水文年内，碳酸盐岩风化明显受到季节变化的影响，而硅酸盐岩风化对季节变化的响应不明显，总体表现为雨季风化速率增加，旱季风化速率降低。地表径流是控制尼洋河流域化学风化的重要因素，径流增大，促进碳酸盐岩和硅酸盐岩风化，但岩石的溶解动力特征会限制径流对风化速率的促进作用。碳酸盐岩溶解速率大，径流增大能持续有效促进碳酸盐岩风化；而硅酸盐岩溶解速率小，随着径流量增大，风化速率增速降低。温度升高能有效促进尼洋河流域的岩石风化，提高岩石矿物的溶解速率。温度也能通过影响径流变化，间接影响流域的风化。位于高寒地区的尼洋河流域气候因素之间相互影响，也影响着物理风化和化学风化。为此，在高寒地区流域展开监测周期更长、采样密度更高的工作，将有助于更好地理解气候因素对风化作用的影响规律。 |
| 关键词: 尼洋河 化学风化 季节变化 气候因素 |
| DOI：10.7515/JEE192029 |
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| 基金项目:国家自然科学基金项目（91647205，41661144042，41930863）；长安大学中央高校基本科研业务费专项资金项目（300102278302） |
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| Seasonal variations of chemical weathering and its controlling factors of Nyang River in the Tibetan Plateau |
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MENG Junlun, GUO Jianyang, WU Jie, ZHAO Zhiqi
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1. State Key Laboratory of Environmental Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang 550081, China
2. School of Earth Science and Resources, Chang’an University, Xi’an 710054, China
3. University of Chinese Academy of Sciences, Beijing 100049, China
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| Abstract: |
| Background, aim, and scope Chemical weathering of continental silicate rocks and carbonate rocks is closely related to global climate change. Study of weathering in watersheds is an important way to understand how continental weathering responds to global climate change. Previous researches focused on large river basins, and few of them worked on small river basins. In particular, the influencing factors of chemical weathering in river basins in high and cold regions are still uncertain. The Nyang River Basin on the Tibetan Plateau with relatively simple lithology and less human disturbance was selected as the research object. A hydrological year sampling analysis was performed to explain the seasonal variation characteristics of rock chemical weathering rate and its influencing factors in the alpine region. Materials and methods In the present study, a hydrological year (2017—2018) sampling analysis was conducted at the lowest stream of Nyang River Basin. Annual variation of chemical compositions of surface river water were analyzed, including the major cations (Na+, K+, Mg2+, Ca2+), soluble silicon and major anions (F-, Cl-, SO42-, NO3- and HCO3-). Based on chemical composition analyses, contributions of four end-members (silicate, carbonate, hotspring and atmosphere) to riverine cations were estimated. Coupled with discharge data calculated by a hydrological model and water temperature measured in the field, weathering rates of silicate and carbonate as well as their responses to discharge and temperature were investigated. Results Results show that the water ions of the Nyang River are mainly derived from the weathering of carbonate rocks and the weathering of silicate rocks, contributing 60% and 29% of cations to the river water, respectively, and the weathering rates are 0.20—19.00 t·km−2·month−1 and 0.09—0.80 t·km−2·month−1, with annual averages of 11.90 t·km−2·a−2 and 4.38 t·km−2·a−1, respectively. In a hydrological year, the weathering of carbonate rocks is obviously affected by seasonal changes, while the response of silicate rock weathering to seasonal changes is insignificant. In general, the weathering rate increases in the rainy season and decreases in the dry season. Discussion Discharge is an important factor controlling chemical weathering in the basin. The increase of discharge promotes the weathering of carbonate and silicate rocks, but the dissolution dynamics of rocks will limit the effect of discharge on the weathering rate. The increase of discharge can continue to effectively promote the weathering of carbonate rocks because of the large dissolution rate, while for the silicate rocks with small dissolution rate, the effect of discharge on weathering is weakened when discharge continues to increase. Different dissolution dynamic characteristics of rocks are important reasons for the different responses of carbonate rock weathering and silicate rock weathering to seasonal changes. Increased temperature can effectively promote rock weathering in the Nyang River Basin by increasing the dissolution rate of rock minerals. Temperature can also import an effect on weathering of the watershed by influencing discharge. Conclusions From above, it can be concluded that, in Nyang River basin, discharge can be a primary control on chemical weathering, and temperature is relatively minor control, but temperature can also affect glacier activities as well as discharge in this alpine area. The different dissolution kinetics of carbonate and silicate account for the discrepant weathering behaviors of the two kinds of rocks during a hydrological year. Recommendations and perspectives The climatic factors in the alpine area like Nyang River Basin interact with each other and also affect physical and chemical weathering. For this reason, the longer time monitoring and higher sampling density in alpine watersheds will help to better understand the response of weathering to climate change. |
| Key words: Nyang River, chemical weathering, seasonal variations, climate factor |
