A study on the weathering and soil formation process of clastic rocks in the Duoyitang Formation of the Qingbaikou system in the Hanshou area of the Dongting Lake, Hunan, China

The weathering of rocks, as an important geological process on the Earth′s surface, plays a key role in shaping surface morphology, providing nutrients required by the ecosystem, and regulating the global climate. In this paper, we systematically collected samples from the weathering profile of clastic rocks in the Duoyitang Formation of the Qingbaikou system in the Hanshou area. Using the X-ray diffractometer (XRD), scanning electron microscopy-energy spectrometer (SEM-EDS), X-ray fluorescence spectrometer (XRF), inductively coupled plasma mass spectrometer (ICP-MS), and multi- receiver inductively coupled plasma mass spectrometer (MC-ICP-MS), we have analyzed mineral compositions, content of major and trace elements and Sr isotopic ratios of samples from the weathering profile, aiming to reveal the weathering geochemical processes of clastic rocks in the critical zone. The results show that clastic rocks of the weathering profile were moderate chemically weathered (CIA values of 73.12–76.57). With the progress of chemical weathering, the insoluble mineral of K-feldspar was transformed into illite, and illite was transformed into chlorite at the same time. In addition, during the weathering process of clastic rocks, the acidic environment formed by the oxidation and decomposition of pyrite can further promote the dissolution of minerals, resulting in the loose of mineral cementation increase of porosity, and gradual increase of 87Sr/86Sr values of weathered rocks in the profile. The water-rock chemical reaction resulted in the enrichment of all major elements for weathered rocks in the fracture zone, while the strong enrichment of trace elements for weathered rocks in the fracture zone is mainly related to the adsorption effect of iron and manganese oxides or partly related to the precipitation of sulfates formed by the combination of S and some elements. The research shows that the content of carbonate is not increased in the weathering and soil formation process of clastic rocks. This means that the weathering and soil formation process of clastic rocks can not cause the carbon sink effect.