Study Of Weathering Intensity In The Critical Zone And Its Influence Factors Across Heihe Drainage Basin

The weathering of bedrock reflects the combined influence of tectonics and climate on landscape evolution processes.The intense tectonic uplift since the late Cenozoic and the arid climatic conditions made the Qilian Mountains became a representative area to study the weathering process and landscape evolution processes.In this study,the spatial variation characteristics of weathering intensity of bedrock in the critical zone and its influencing factors were analyzed in Heihe drainage basin,which is originated from the Qilian Mountains.Through analyzing the hardness of bedrock(R-value)and the chemical elements of weathering crust,the spatial variation of physical weathering and chemical weathering in Heihe drainage basin is obtained.Based on the analysis of potential topographic and climatic factors,the main factors affecting weathering process are clarified,and the geomorphologic evolution significance revealed by the results is discussed.In this study,35 sampling areas were selected in the upper and middle reaches of Heihe drainage basin to collect the R-value,and the weathering samples on slope surface are collected simultaneously from 20 sampling areas.The correlation between R-value and total denudation rate,lithology,topographic factor and climatic factor,and the correlation between Chemical Depletion Fraction(CDF)and the total denudation rate,topographic factor and climatic factor were analyzed.The results show that:(1)The R-value can represent the spatial variation characteristics of the total denudation rate in the study area approximately,and landscape development in accordance with "weathering-limited".Rock type,mean slope,local relief,NDVI and annual mean precipitation have limited influence on R-value.The relationship between annual mean temperature between 4 ? and-3 ? and between-3 ? and-8 ? has changed.R-value was also changes with elevation at 2 400 m and 3 800 m.These two relations fully reflect the vertical zonal characteristics of R-value.(2)The variation characteristics of CDF with total denudation rate may be in accordance with the law of “Diminishing returns”iminismineral supply and retention time are two factors that cause CDF to increase first and then decrease with the change of mean slope and local relief.The CDF of Heihe drainage basin shows obvious vertical zonal change,which indicates that the spatial variation of chemical weathering intensity is influenced by environmental factors such as temperature,vegetation,precipitation and soil development.Based on the above conclusions,we further discussed the geomorphologic evolution significance indicated by the weathering characteristics of the Heihe drainage basin.Firstly,we used the frost cracking model to calculate the frost cracking intensity,FCI.We found that the change trend of mean FCI with altitude since LGM can well explain the change trend of R-value at altitude.Therefore,we believe that the weathering procesess of bedrock in Heihe drainage basin is controlled by the frost cracking weathering.Climate change regulates geomorphological process through temperature,and there was a lag in the response of geomorphological process to climate change in “weathering-limited” areas.In the view of the obvious vertical zonality of both R-value and CDF,we compare and analyze the change trend of R-value,CDF and FCI with altitude.We found that the physical denudation rate inhibits the chemical weathering rate when the slope evolution of Heihe drainage basin is carried out under the condition of "weathering-limited",the chemical weathering process in the region should conform to the “kinetic-limited” model.When the slope evolution is carried out under the condition of "transport-limited",the two promote each other,the chemical weathering process in the region should be in accordance with the model of supply-limited.This means that the uplift-weathering hypothesis in the arid and semi-arid orogenic belt is valid in the elevation range subject to "transport-limited".In the wider "weathering-limited" region,the theoretical premise of the uplift-weathering hypothesis is not valid.