矿床地球化学国家重点实验室知识库

The late Ediacaran (551-541 Ma) oceanic environment could represent a significant preparation for the enrichment of redox sensitive elements during the early Cambrian and even for the evolution of life. However, the marine environment dur-ing the late Ediacaran has not been thoroughly characterized to date. To better understand late Ediacaran oceanic redox conditions, we investigated the Fe and S isotopic compositions of cherts from the slope (Yinjiang) and basin (Silikou) sections of the Nanhua basin in South China. The slope (Yinjiang) section is characterized by a wide range of delta Fe-56(T) values (-0.12 parts per thousand to 1.06 parts per thousand) and by continuously decreasing delta S-34(py) values from 40.4 parts per thousand to 4.1 parts per thousand. The variable delta Fe-56(T) values in the Yinjiang section primarily reflect the various oxidation degrees of dissolved Fe2+ in ferruginous seawater. The homogeneous delta Fe-56(T) signatures (0.02-0.40 parts per thousand) along the Silikou section and the decreasing trend of delta Fe-56(HR) values from 0.86 parts per thousand to near zero primarily depend on the mixing of Fe3+ hydroxide/oxide precipitates in a ferruginous water column, possibly followed by partial reduction, the authigenic framboidal pyrite trapped within an intermittent euxinic condition and detrital silicate Fe minerals. A 20-30%e discrepancy in delta S-34(py) values between the Yinjiang and Silikou sections may be ascribed to the formation of diagenetic pyrites and framboidal pyrites, respectively. Therefore, the continuously decreasing delta S-34(py) values in the Yinjiang and Silikou sections may reflect a significant increase of sulfate diffusion into sediments at the slope location and the enlarged marine sulfate reservoir at the basin location. These new results indicate that the deep water in the Nanhua basin is primarily characterized by a ferrug- inous condition with intermittent euxinic conditions in the basin location during the late Ediacaran.