Bacteriomorph Structures in Nodules, a Characteristic of Euxinic Conditions of Nodule Formation

Resumo

Electron microscopic, spectroscopic, and geochemical study of Middle Ordovician shamosite, Wenlockian calcite, and Lower Carboniferous siderite nodules revealed the presence of organic matter and traces of microbial organisms involved in the formation of iron oxides and hydroxides and mineralized microbial films. The structure and composition of the films show high similarity to mineralized glycocalyx, which is a product of bacterial metabolism. Diversity of the framboidal micropyrite and bacteriomorph structures in size and shape indicates the formation of nodules under anoxic bottom conditions and abundance of sulfate-reducing, iron-reducing, and iron-oxidizing bacteria. The presence of authigenic minerals in the nodules, which is atypical for sedimentary rocks, suggests the influence of bottom gas–fluid seeps and, therefore, local hydrosulfuric contamination, fluctuations in salinity, and intensive development of bacterial communities.