| In this study,10 laterite profiles from Shilin karst area in Yunnan Province were seleceted as the research object.The characteristics of elements migration and distribution,mineral composition,particle size distribution and silicon,lithium isotopic composition were used to reveal the evolution and material source of red weathering crust,explore the influencing factors of Si and Li isotopes fractionation in carbonate profile.Meanwhile,the influence of micro-landfroms on elemental geochemical characteristics druing weathering process was revealed.The main conclusions are as follows:The study area has the typical characteristics of laterite developed under the subtropical climate,which has experienced intense weathering and leaching process,and the elements appear significant depletion or enrichment.The migration coefficient of alkali metals is close to-1,while Al,Fe and Ti are gradually enriched in the profiles.The migration coefficients of heavy rare earth elements are higher than those of light rare earth elements,with(La/Yb)N ranging from 5.08 to 13.53.There are significant Ce anomalies in the study profiles,and Fe and Mn oxides are important factors controlling Ce anomalies.Futhermore,heterogeneity of carbonatite and soil pH also affect Ce anomalies.The combination of Si and Li isotopes can provide competent evidence for the genesis of laterite in karst area.The linear correlation of δ30Si and δ7Li indicates that the fractionation of Si and Li isotopes in the profiles is not the result of mixing of multiple endmembers,and the overlying soil is developed from the underlying carbonate rocks without the addition of exogenous substances.Combining mineralogy and geochemistry evidence,it can be concluded that the laterite in the study area is formed by in-situ weathering of carbonate rocks,and the main material source is the underlying bedrock.Significant fractionation of Si and Li isotopes were found during the weathering process of carbonate rocks.The δ30Si values range from-0.9‰ to 1.1 ‰,and δ7Li values range from-0.28‰ to 3.72‰.Silicon isotope is affected by soil weathering intensity,organic matter content,clay mineral content and redox conditions,while lithium isotope is only related to organic matter and clay mineral content.Karst micro-landform is an important factor affecting element distribution and migration,chemical weathering characteristics and soil thickness.Due to the steep slope promoting soil erosion and limiting chemical weathering,the average contents of stable elements Al,Fe,Ti and trace elements in the profiles from karst doline micro-landform are lower than those in depression and upland microlandform.The order of weathering degree of soil profiles is karst upland>karst depression>karst doline.Meanwhile,the element geochemical characteristics of the study profiles indicate that there is a trend of landform evolution among doline micro-landform,depression micro-landform and upland micro-landform.The calculation of carbon sink of carbonate profiles shows that the carbon sink of carbonate weathering is mainly concentrated in the primary stage:the stage of carbonate rocks decomposition,the profiles from different micro-landforms have similar CO2 consumption.This study deepens the understanding of the evolution of elements in the red weathering crust,provides new evidence for the genesis of laterite through the combination of Si-Li isotopes,and provides a reference for the subsequent study of carbonate weathering carbon sink. |
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