Uranium Isotopic Compositions Of Fine Detrital Particles In A Glacial Catchment Of The Pamir Plateau And Its Implication For Surface Environment

Located in active tectonic uplift zones,characterized by significant elevation and climate gradients,and rapid erosion rates,inland glacial catchments provide ideal conditions for studying the surface erosion-weathering processes and the controlling mechanism of chemical weathering rates in high erosion environment since the late Cenozoic.The uranium-series?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 elements of bulk river sediments,specific surface areas and U-series isotopes of fine detrital particles of river sediments collected from a glacial catchment at the northeastern Pamir Plateau,in order to understand sediment transfer mechanism and to test feasibility of U-series comminution age model in a glacial environment.Such research on the transport processes of fine detrital particles of river sediments within an inland glacial catchment is significant for further understanding glacier erosion rate and its relationship with tectonic activity and climatic change.The study was performed within the Gaizi river catchment in the northeastern Pamir Plateau,owing to its significant elevation gradients 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 mainstreams of the Gaizi river.The mineralogy and elements of samples were determined by XRD and XRF,respectively.The(²³⁴U/²³⁸U)activity ratios[(²³⁴U/²³⁸U)_AR]of fine detrital particles were determined by MC-ICP-MS and particle BET surface areas by nitrogen adsorption-desorption instrument.The results show that mineralogical compositions of bulk river sediments are dominated by quartz and feldspar?accounting for 51%⁷7%?,indicating a weak chemical weathering as a result of dry-cold climate in the Pamir Plateau.At the same time,the river sediments have lower contents of easily weathered minerals?such as biotite,chlorite?than those in the surface sediments in Kala Kul Lake.This difference indicates that the river sediments have undergone relatively stronger chemical weathering or sorting processes than the surface sediments in Kala Kul Lake,though chemical weathering is weak and physical weathering is dominant within the catchment.The major and trace elements of river sediments show some differences among tributaries and mainstreams of the Gaizi river catchment.The SiO₂ contents of river sediments are ranged from 50.9%to 71.1%,with a good negative correlation with CaO contents.The Rb/Sr ratios are ranged from 0.78 to 0.96 in the upstream tributary?Kangxiwa river?,but the ratios decreases to 0.35 in the lower reaches of the Gaizi river,with a good negative correlation with Na/K,further indicating a relatively weaker weathering in the upstream compared to the low reaches.The weak weathering within the catchment is further supported by low CIA of fine particles in river sediments,in consistent with the dry cold climate in the study area.The CIA of the bulk sediment is ranged from 58.2 to 76.8,with 58.2⁶4.4 for the 20-43?m fine particles.Large difference of Zr and Ti contents between the upstream and the lower reaches may relate with sorting and/or different sources in the tributaries.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.The(²³⁴U/²³⁸U)_AR ratios of fine detrital particles in river sediments are ranged from 0.988 to 1.017 at the upstream Kangxiwa and Muji tributaries.Therefore,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.On the one hand,the(²³⁴U/²³⁸U)_AR of the fine detrital particles show a decreased trend from the upstream to the lower reaches within the Gaizi river catchment,with the(²³⁴U/²³⁸U)_AR ratios ranging from 0.913 to 0.997 at its middle and lower reaches,indicating the release of ²³⁴U during sediment transport.On the other hand,similar to some high-erosion catchments,the low particle S_BET surface areas of detrital particles in a glacial environment lead to much lower value of the recoil loss factor than the theoretical minimum one.Furthermore,the low cannot obtain a reasonable transport time of detrital particles using the U-series comminution age model.Even though,a decrease in(²³⁴U/²³⁸U)_ARR 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.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.In summary,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 other geological and climatic settings.