The present study aimed to identify the seasonal changes in the hydrochemical characteristics of the Huixian karst wetland and the underlying mechanism. Conventional ions and isotopes of 130 groundwater samples collected during the wet and dry seasons were analyzed. The hydrochemical characteristics of groundwater in the Huixian karst wetland were clarified using mathematical statistics and hydrochemical methods, including Durov diagrams, ionic ratios, Gibbs diagrams, and H and O isotopes. The hydrochemical evolution and sources of major ions in the Huixian karst wetland were also investigated. The results showed that cations and anions in groundwater in the study area were dominated by Ca2+ and HCO3−, respectively, sourced mainly from calcite weathering. The ions of some groundwater samples were regulated by weathering of dolomite, dolomitic limestone, and pyrite, resulting in relatively high concentrations of Mg2+ and SO42−. K+, Na+, SO42−, NO3−, and Cl− in groundwater originated from precipitation, Na+ and Cl− originated from domestic sewage, K+ was related to the application of potassium fertilizer in agriculture, and NO3− mainly originated from chemical fertilizer. Groundwater ions were primarily controlled by rock weathering, followed by evaporative concentration. The sources of major ions were dependent on the dissolution and precipitation of carbonate rock, as well as the chemical weathering of silicate rock and evaporite. Samples from the various water sources were scattered on both sides of the local meteoric water line (δD = 3.13δ18O − 13.9), which indicated isotopic composition was affected by evaporation and precipitation.
Cap carbonate of the early Ediacaran period represents an abrupt climate shift from severe Marinoan glaciation to a post-glacial warm environment. Some consensuses on the distinctive negative excursion of δ 13 Ccarb and the origin of its unique sedimentary structures have been reached over the past decades, but certain aspects such as the distribution and redox conditions of cap carbonate still remain controversial. To resolve these issues, we collected detailed samples of the Doushantuo cap carbonate from the Ganlong-Fanyang section and the Poutoushang reservoir section in the Danzhai County, southeast Guizhou Province, respectively. We conducted carbon and oxygen isotope analysis and trace element analysis in this study. The Ganlong-Fanyang section was possibly situated in a submarine highland because of its higher carbon isotope than that of the Potoushang section according to the carbon isotopic gradient. Negative carbon isotope excursions at the basal Doushantuo cap carbonate from the two sections correlate well with those from other equivalent cap carbonates and were corresponding to the first one (SN1 or EN1) of the carbon isotopic curve during the Ediacaran period, implying a remarkable global event after the post-Marinoan glaciation. Geochemical features of trace elements and rare earth elements support an overall oxygenrich condition with syn-depositional or pene-contemporaneous hydrothermal activities and methane seeps during the deposition of cap carbonate. We reconstruct a new sedimentary model that the Doushantuo cap carbonates were mainly precipitated in relatively shallow-to deep-water shelf environments without disturbance by abundant terrestrial materials, which may be on isolated platforms over the early developed grabens in a deep-water basinal condition, rather than within the entire oceanic basin.
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