Source Of Sediment In The Small Catchment Of Composite Lithology In The Three Gorges Area

Purple sandstone and limestone are widely distributed in the Three Gorges Reservoir Region(TGRR),and the processes of soil erosion and sediment deposition are quite different under the two lithological conditions.Studying the contribution ratio of purple sandstone and limestone slope to sediment,that is,sediment source,is beneficial to understanding the law of erosion and sediment yield and sediment deposition characteristics of purple sandstone and limestone slope,and can provide scientific basis for the application of soil and water conservation measures in two lithologic environments.Related research has not been reported.Mjiawan compound lithologic Small catchment in the TGRR is selected as the research object,and its potential source areas are divided into purple sandstone zone and limestone zone,and the silt dam downstream of the watershed is the sediment area.166 surface samples from the source area and 158 stratified samples from the check dam core were collected.Among the samples,22 geochemical elements and 17 soil color indexes were determined.The composite fingerprint tracing technique is used to calculate the contribution rate of sediment in two sources,and the chronological scale of sediment is obtained by combining radionuclide dating method.By calculating the deposition rates in different stages,the variation trend of sediment sources in Mjiawan compound small watershed in recent 100 years is obtained.The results show that:(1)Through extreme range test and mean range test,the fingerprint factors with preservation are screened.A total of 9 factors in geochemical indicators passed the range test,and 8 factors in soil color indicators passed the range test.Kruska1-Wallis H test was used to screen the fingerprint factors of two potential sources,which have significant differences and can distinguish different sediment sources(P<0.050).Seventeen factors,including geochemical elements and soil color,show significant differences.Stepwise discriminant analysis showed that the best fingerprint combination of geochemical elements was Sr,Mo,K,V,Fe,Lu and Li,with the cumulative discriminant rate of 96.6% and the correct rate of sample classification of 100%.The best combination of soil color fingerprints is: G,RI and IRGB,with a cumulative discrimination rate of 89.3% and a correct classification discrimination rate of 94.6%.Geochemical elements and soil color meet the application requirements of fingerprint tracing technology,and they are agreed to be substituted into multivariate mixed model.(2)Using the multivariate mixed model,running 1000 Monte Carlo simulations,and combining with the best fingerprint factor combination of geochemical elements,the sediment contribution rates of two potential sources in Mjiawan compound small catchment are calculated,and the sediment contribution rates in purple sandstone area and limestone area are73.5% and 26.5% respectively,and the goodness of fit is 0.89.The best fingerprint combination of soil color was brought into multivariate mixed model calculation,and the sediment contribution rates in purple sandstone area and limestone area was 58.4% and 41.6%respectively,and the goodness of fit was 0.77.(3)Using the best fingerprint combination of geochemical elements,combined with multivariate mixed model and Monte Carlo simulation,the sediment contribution rate of stratified samples with vertical depth of 9 cores is calculated,and it is found that the contribution rate of two lithology has no obvious change in depth direction.The activity of radionuclide in core sediment samples was measured,and the time scales in 1920,1954 and1963 were obtained by analysis.Combined with the depth information,the historical deposition rate of Mjiawan small catchment was calculated to reflect the change of erosion intensity.The results showed that the deposition rate of nine cores reached the lowest in recent100 years during 1963-2020,on the contrary,the deposition rate reached the highest during1954-1963,and the deposition rate of purple sandstone area was significant.Through the analysis of the above results,it is found that the sediment contribution rates of purple sandstone slope is higher than that of limestone slope.From the bottom of the core to the top,generally speaking,the sediment contribution rate of purple sandstone slope shows an increasing trend,while that of limestone slope shows a decreasing trend.In the past 100 years,the deposition rates of the two lithology increased first and then decreased to the lowest.This reflects the different characteristics and laws of erosion and sediment yield of two kinds of rocky slopes under the joint influence of soil development,geological conditions and human activities.Under the control of two kinds of lithology,the soil formation rate in purple sandstone area is fast,the soil layer is deep,and the cultivation intensity is high.On the contrary,the soil formation rate in limestone area is slow,and the development of rocky desertification leads to rock outcrop,which restricts the further development of cultivation activities.This is the reason why the contribution rate of sediment in sandstone slope is higher than that in limestone slope,and shows an increasing trend.The changes of sediment deposition rates in the two lithologic zone reflect the changing rules of erosion and sediment yield under the influence of human activities in the past 100 years.The process of rocky desertification restricts the erosion and sediment yield in limestone area,while there is no process of rocky desertification in purple sandstone area.By comparison,the erosion and sediment yield are not restricted by lithology.