Decadal-scale Erosion Based On Sediment Flux And Environmental Radionuclides In Qilian Shan Mountains

Previous studies have presented a theoretic coupling and feedback between tectonics, climate, and erosion in long-term landscape evolution. Therefore, erosion rates on different spatial-temporal scales and the current topography are significant in understanding this coupling and feedback system, by inference, the landscape evolution. Most of the studies focused on tectonically active mountain ranges, in which the climatic gradient caused by the strikingly steep topography represents a significant variation both in topographic and climatic conditions. In this coupling and feedback system, the critical issues are the competing role of tectonic and climatic influence on erosion rates, whereas agreements have never been reached.The Qilian Shan Mountains, located along the northeast margin of the Tibetan Plateau, have been experiencing intense tectonic movement since the late Cenozoic. Three inland rivers, originated from the high altitude areas of the Qilian Shan Mountains, incise the tectonically active margin and then flow into the arid Hexi Corridor basin. These rivers, down cutting through the active deformation, together with the strikingly steep topography, contribute a major part in controlling the sediment flux and shaping landform. Therefore, Qilian Shan Mountains is a potential place to study the feedback mechanism between tectonics, climate and erosion in arid-semiarid areas. However, in the study areas, only geological-scale exhumation rates, millennium-scale catchment-avaraged denudation rates and river incision rates have been reported. So we should pay more attention to complete the erosion database, especially in decadal-scale. Tectonics and climate could control the type and intensity of erosional processes individually and collectively, and thereby could modify topography. In decadal-scale, current topography and climatic conditions also could influence the erosional processes and rates, which are important to explore the relationships between tectonics, climate, topography and erosion in these areas.Therefore, this paper is designed to produce a better understanding how the topography and climate influence decadal-scale erosion rates both in catchment-level and slope-level. Firstly, we estimated the catchment-averaged erosion rates throughout the Qilian Shan Mountains and analyzed the major controls on erosion rates; secondly, we estimated the hillslope erosion rates in Xiying Basin, located in the Northeastern Qilian Shan Mountains and analyzed the major controls on hillslope erosion rates; thirdly, based on the field observations and the stream power model, we identified the predominant erosion processes in the study basin.In catchment-level, the sediment yield records of11drainage basins were collected from the Qilian Shan Mountains. The calculated decadal-scale erosion rates has a wider rate distribution (0.02mm/a-0.20mm/a) with an average erosion rate is about0.08mm/yr. Correlation analyses indicate that mean local relief controls the spatial distribution of catchment-average erosion rates. But, for individual drainage basin, the annual erosion rates increase with increasing discharge.In slope-level, we used the environmental radionuclide137Cs to measure erosion on soil-mantled slopes with varying climatic regimes and topographic characteristics in Xiying Basia. The calculated erosion rates also has a wider rate distribution (0.05mm/a-0.26mm/a) with an average erosion rate is about0.14mm/yr. Our results suggest a strong positive correlation between transect averaged erosion rates and gradients, however, vegetation cover influences the erosion distribution at fine scale. Field observations suggest that overland flow is common in many areas of the basin, and the21study slopes characterized by varied land cover could represent the general spatial variability of erosion processes and rates within the catchment. A1-D stream power model was also employed to predict lateral sediment flux to identify the main erosion processes at the study slopes. The results suggest that the study slopes are strongly influenced by sheet wash, which is consistent with the runoff-dominated annual variation in sediment flux from this and other neighboring basins. In the Qilian Shan Mountains, the decadal-scale erosion rates in two different spatial scales (catchment-slope), both show strong topographic controls.The linear relationship between mean local relief and decadal-scale catchment-average erosion rates agreed with previous studies. To connect the two spatial scale erosion rates, we predicted the catchment-level erosion rates by the strong positive correlation between gradient and slope-level erosion rates. The results represented comparable catchment-level erosion rates with rates estimated by total sediment load in the11study basins. However, as the local relief/slope of a landscape reaches the critical status, decadal-scale erosion rates will underestimate the long term condition, because of missing measurments for extremely episodic erosive events and sediment delivery. However, local relief analysis indicated that the landscape of study area is below the critical status.The consistency between the decadal-scale and millennial-scale erosion also indicated the topographic control plays the most important role on erosion rates. The spatial distributions of river incision rates and geological-scale exhumation rates both show the similar pattern with topography. Given the erosion rates in varied temporal and spatial scales showing the identical distributions with topography, we suggested that the current topography plays the major control on spatial distribution of erosion rates.Based the topography analysis, we deduced that the current topographic characteristics in the Qilian Shan Mountains might be governed by the tectonic uplift and river down cutting. Accordingly, it seems that the landscape evolution in the Qilian Shan Mountains corresponds with a coupling and feedback processes between tectonics, topography and erosion. In long-term landscape evolution, the tectonic uplift dictates the topographic characteristics, which in turn presents its influence on erosion rates. The spatial variability of current topography and the associated erosion rates might be caused by spatial variability of uplift rates in the Qilian Shan Mountains. Therefore, the landscape evolution of the Qilian Shan Mountains corresponds to an uplift-incision-erosion model which generally happened at a young state.