Chemical Weathering Characteristics And Influencing Factors On Hillslopes In The Xiying River Basin,Eastern Of Qilian Mountains

The uplift-weathering hypothesis reveals the crucial role of Tibetan Plateau uplift in the late Cenozoic global cooling,and clarifies the fundamental role of the orogenic belt weathering process in expounding the theory of tectonic-climate interaction.The theoretical premise of the uplift-weathering hypothesis that chemical weathering rate could keep pace with the increase of physical erosion rate,remains controversial in broad environmental conditions.The chemical depletion fraction(CDF)is an effective indicator to characterize the relationship between chemical weathering rate and physical erosion rate.It can directly reveal the characteristics of hillslope weathering process on the scale of landform evolution.At present,there exists two controversies about the global CDF studies.One is whether chemical weathering is affected by supply-limited in low denudation rate regions,and the other is whether chemical weathering is easy to reach kinetic-limited in high denudation rate regions.Quantifying the variations of CDF under arid and cold environments is the key to resolve this controversy.The uplift and expansion process of the Tibetan Plateau has created a series of cold or arid marginal mountains,which represents an ideal geomorphological unit for testing the uplift-weathering hypothesis.However,the chemical weathering on hillslopes has not been determined in Tibetan Plateau.In this study,we focused on the Qilian Mountains located on the northeastern margin of the Tibetan Plateau.Firstly,we quantified the chemical weathering characteristics and its response to environmental factors on hillslopes in the Xiying River Basin,eastern of Qilian Mountains.Secondly,we also quantified the water characteristics in order to explore the contribution of hillslope weathering to catchment chemical weathering.Thirdly,we estimated the in situ soil production rate of hillslopes using the cosmogenic nuclide method,and then inferred the background value of the total denudation rate in the Xiying River Basin.Finally,combined with the CDF study carried out in the Heihe River Basin in the middle Qilian Mountains,we analyzed and discussed the relationship between chemical weathering rate and physical erosion rate of Qilian Mountains as well as the applicability of the uplift-weathering hypothesis in the northeastern margin of the Tibetan Plateau.The main conclusions are as follows:(1)The results of the regolith CDF in the Xiying River Basin show that chemical weathering accounts for 24%to 70%of total denudation in the study hillslopes.CDF is not directly affected by the single climatic(temperature,precipitation and NDVI),topographic factors(slope and local relief)or lithology differences,but shows a trend of first increase and then decrease with the increase of elevation.The variation of CDF on the altitude gradient is consistent with the transition of vegetation band spectrum.(2)The multi water of Xiying River Basin are weakly alkaline.The average mass concentration of cations is:Ca²⁺>Mg²⁺>Na⁺>K⁺,and the average mass concentration of anions is:HCO₃^->SO₄^2->CO₃^2->Cl^-.The total dissolved solids range from 71.3 to442.7 mg/L.The analysis results based on Gibbs diagram,Piper's three-line diagram and ion combination and ratio method emphasize that bedrock weathering,especially carbonate weathering,provides the main source of ions for high-altitude water in the basin.(3)The CDF compilation study of Xiying River Basin and the Heihe River Basin shows that there is no obvious correlation between CDF and denudation rate in Qilian Mountains,northeastern margin of Tibetan Plateau.The CDF fluctuates greatly only in the low denudation rate regions(<450 t?km^-2a^-1),while the CDF is nearly constant in the high-denudation rate regions(>450 t?km^-2a^-1),indicating that chemical weathering is affected by supply-limited and the chemical weathering rate can keep pace with the increase of the physical erosion rate.To sum up,the CDF in the Xiying River Basin exhibits significant vertical zonal characteristics regulated by the spatial variation of vegetation band spectrum,which indicates that the relationship between chemical weathering rate and physical erosion rate in the low denudation environment(33.86?138.18 t?km^-2a^-1)is strongly affected by comprehensive environmental factors.The significant carbonate weathering in high-altitude areas found by water chemistry study also reflects the limitation of low-temperature environment on silicate weathering.The above studies collectively reveal that chemical weathering in low denudation environments is highly susceptible to environmental factors,which is consistent with the large fluctuations of average CDF at the catchment scale in low denudation rate regions.Nevertheless,the constancy of the CDF in the high denudation rate regions indicates that chemical weathering does not reach the kinetic-limited.This result emphasizes the dominant role of tectonics in regulating chemical weathering rate.And the alpine environment of the study area may provide a compensation mechanism that enhances near-surface weathering by promoting the freeze-thaw process and increasing the residence time of water in regolith,thereby ensuring that the relationship between chemical weathering rate and physical erosion rate is not decoupled with the increase of uplift rate.The above results emphasize that the theoretical premise of the uplift-weathering hypothesis is still applicable at arid and cold environments in the northeastern margin of the Tibetan Plateau.