Evidence for Biogenic Iron in Ediacaran Limestones of the Yenisei Ridge Vorogovka Series (South-Western Framing of the Siberian Craton).

The composition and origin of magnetic minerals are used for reconstructing paleogeography and changes in paleoenvironments. Ediacaran limestones of the Vorogovka Series were studied in the Yenisei Ridge. Rock magnetism of the upper part of the Vorogovka Series completely depends on terrigenous admixture of iron. Non-carbonate residue consists of quartz, plagioclase, muscovite, Fe-Mg chlorite, smectite and pyrite. A prominent positive correlation is observed between the Fe content (1550…9350 ppm) and the amount of insoluble residue. Magnetization of the lower part of Vorogovka Series is due to authigenic iron. There are several indirect indications of the presence of magnetotactic bacteria and seaweed remains in these limestones. While Fe content is high (1200…8330 ppm), Fe-hydroxides, pyrite and other minerals indicating secondary transformations were not detected. Thus, iron was included in the carbonate material during sedimentation. There is no correlation between iron content and the amount of insoluble residue, which indicates that iron could not have been taken from terrigenous admixture. Study of the dependence of magnetic susceptibility from temperature in inert atmosphere (argon) revealed magnetic minerals with demagnetization temperatures of 320°C and 580°C. The former may correspond to greigite (Curie Temperature 307°C) or hexagonal pyrrhotine (325°C). The latter indicates magnetite (Curie Temperature 586°C) which may be authigenic or newly formed during the experiment. Both magnetite and greigite, are major biogenic minerals produced by magnetotactic bacteria. The main environment of modern magnetotactic bacteria is microaerophilic habitat. Therefore, their abundant presence in the Ediacaran sediments is quite natural. All of these are preliminary conclusions until bacteria can be visualized by electron microscopy, which is an ongoing. The work was supported by the Ministry of Education and Science of Russia 5.2324.2017/4.6 and the RFBR 15-05-01428.