A Preliminary Study Of Environmental Geochemistry Of Weathering Of Lower-Cambrian Black Shales In Hunan, China

Surficial weathering is becoming the focus of Earth and Environment Science research recently. Comparing with other rocks, black shales are actually a series of sedimentary rocks enriching sulfide minerals, organic matters and trace elements such as V, Ni, Mo, Cu, Zn etc. Thorough studies of black shales weathering have important theoretical, economic and environmental values. But in contrast of the research of chemical weathering of other rocks, such as granites and basalts, the research of chemical weathering of black shales both in width and depth still remains much to advance.After reading and investigating widely, we selected lower-Cambrian black shales and their weathering profiles in Hunan province as research objects of our work at first. Then, based on field investigation of weathering status of lower-Cambrian black shales and it's relationship with soil composition and human activities in Hunan, we studied changes of chemical composition such as major and trace elements before and after weathering of black shales by comparison. After that, we discussed mechanisms of element enrichment, mobility and transformation during black shales weathering. Finally, we discussed the relationship between black shales weathering and regional environment quality. Based on our above investigation, research and discussion, we can offer some information about black shales weathering as follows:(1) Titanium is one of the most immobile elements and can be used as reference-frame to calculate the mobility of other elements by mass-balance methods during black shales weathering.(2) CIA can not reflect the degree of chemical weathering of black shales precisely, and A-CN-K diagram can not reflect chemical weathering processes of black shales completely.(3) Negative electricity, radius, complex numbers, electrical values of ions, maturity of weathering profiles, distribution of elements in parent rocks, medium conditions, absorption of clay minerals and organic matters are important factors controlling geochemical behavior of elements during black shales weathering.(4) We defined SSEE factor( surficial soil element enrichment factor) and WPEM factor (weathering profile element mobility factor), and applied them to evaluate the degree of effects of black shales weathering on soil, water bodies and their sediments composition. The results are as follows: trace elements such as Cd, Sb, Zn and U can change residual soil composition deeply, elements such as REE, Y, Cr and Cu can change water bodies and their sediments composition deeply, while elements such as V,Ni, Mo and Tl can change residual soil, water bodies and their sediments composition deeply during black shales weathering.(5) In some regions covered by black shales continuously, black shales weathering might be the cause of variations of regional background values, thus the cause of some endemics and other geochemical diseases.