| 摘要: |
| 铀（U）同位素作为一种新的地球化学示踪手段，被逐渐用于研究陆地和海洋沉积物的搬运过程。然而，这一新技术能否有效指示不同环境中各类沉积物的搬运还需更多流域数据的支持。本文选取位于帕米尔高原东北部具有显著海拔梯度和气候差异的盖孜河冰川流域作为研究对象，通过该流域河流沉积物细颗粒中的U同位素的活度比（ (234U/238U) AR ）的空间变化，探索U同位素指示内陆冰川流域沉积物搬运的可行性。流域内河流沉积物的矿物组成以石英和长石（占51%—77%）为主，表明较弱的风化作用。流域内受冰川侵蚀控制的上游山区支流康西瓦河和木吉河沉积物的(234U/238U) AR 范围分别是0.990—1.017和0.988—1.009，盖孜河中下游段河流沉积物的234U/238U) AR 则为0.913—0.997。从上游山区至中下游段显示了一个明显的下降趋势，指示了沉积物搬运过程中(234U/238U) AR 的确发生了系统的变化。然而，盖孜河流域碎屑颗粒比表面积和分形维数计算得到的反冲损失参数太低，未能利用U同位素破碎年龄模型获得合理的沉积物搬运时间，该模型如何用到冰川流域尚需更多的研究工作。 |
| 关键词: 冰川流域 河流沉积物 234U/238U活度比 破碎年龄模型 搬运时间 |
| DOI：10.7515/JEE182074 |
| CSTR：32259.14.JEE182074 |
| 分类号: |
| 基金项目:国家自然科学基金项目（41403111）；中国科学院战略性先导科技专项（XDA20070102）；中国科学院西部之光项目（Y529061299） |
| 英文基金项目:National Natural Science Foundation of China (41403111); Strategic Priority Research Program of Chinese Academy of Sciences (XDA20070102); CAS “Light of West China” Program (Y529061299) |
|
| Uranium isotopic compositions of fine detrital particles in a glacial catchment of the Pamir Plateau and its implication for sediment transfer |
|
XU Yang, ZHANG Fei, JIN Zhangdong
|
|
1. State Key Laboratory of Loess and Quaternary Geology, Institute of Earth Environment, Chinese Academy of Sciences, Xi’an 710061, China
2. University of Chinese Academy of Sciences, Beijing 100049, China
3. Institute of Global Environmental Change, Xi’an Jiaotong University, Xi’an 710049, China
|
| Abstract: |
| Background, aim, and scope The uranium (U)-series isotopes are recently used in the study of the transport processes of terrestrial and marine sediments as one of new geochemical approaches. However, how to use this new approach effectively to trace sediment transport processes still needs more supporting data from various geological and climatic settings. In this study, we analyzed mineralogy and U-series isotopes of fine detrital particles of river sediments within a glacial catchment at the northeastern Pamir Plateau, in order to understand sediment transfer mechanism and to test feasibility of U-series comminution model in a glacial environment. Materials and methods The study was performed within the Gaizi River catchment in the northeastern Pamir Plateau, owing to its significant elevation gradient and climate difference. The upper reaches of the Gaizi River catchment are extensively covered with glaciers. Sixteen river sediment samples were collected along the major tributaries and mainstream of the Gaizi River. The mineralogy of samples was determined by XRD, 234U/238U activity ratio ((234U/238U) AR ) of fine detrital particles by MC-ICP-MS and particle BET surface areas by nitrogen adsorption-desorption instrument. Results The mineralogical compositions of river sediments are dominated by quartz and feldspar (accounting for 51%—77%), indicating a weak weathering. The measured (234U/238U) AR are high, ranging from 0.988 to 1.017 in river sediments from the upstream Kangxiwa and Muji tributaries, whereas the ratios decreased in the middle and lower reaches of the Gaizi River, ranging from 0.913 to 0.997. However, the recoil loss factors f α calculated by particle specific surface areas of the typical samples are much lower than the theoretical minimum values. Discussion The measured (234U/238U) AR of fine detrital particles in the upstream mountains are slightly larger than or close to the values of fresh rocks in secular equilibrium ((234U/238U) AR =1.00±0.01), reflecting rapid erosion process within a glacial environment. On the one hand, the (234U/238U) AR of fine detrital particles show a decreased trend from the upstream to the lower reaches within the Gaizi River catchment, indicating the release of 234U during sediment transport. On the other hand, similar to some high-erosion catchments, the low particle S BET specific surface areas of detrital particles in a glacial environment lead to an unreasonable transport time by the U-series comminution model. Conclusions Due to rapid erosion within the Gaizi glacial catchment, the measured (234U/238U) AR of fine detrital particles in the upstream mountains can be served as an endmember of initial (234U/238U) AR of fresh rocks. Although transport time cannot be obtained from the comminution age equation owing to low particle surface area, a decrease in (234U/238U) AR ratios from the upstream to the lower reaches within the Gaizi River catchment still proves the robustness of the U-series isotopes in tracing sediment transport processes. Recommendations and perspectives We argued that the assumption of secular equilibrium of (234U/238U) AR =1.00±0.01 may need further work to estimate for the endmember of initial (234U/238U) AR . The fine detrital particles in glacial catchments may not be suitable for the application of the U-series comminution model, but further work is worthy to test its feasibility in various geological and climatic settings. |
| Key words: glacial catchment river sediments 234U/238U activity ratio comminution age transport time |
