Microbial influence on the structural design, microtexture and geochemistry of hydrothermal iron-deposits

Large amounts of Fe-hydroxides are deposited by low-temperature hydrothermal venting distal to a high-temperature vent area at the southwestern part of the Mohns Ridge in the Norwegian-Greenland Sea. The Fe-deposits occur as yellow to rust coloured mounds and small chimney-like structures along faults and fissures in the rift valley floor, for distances of several hundreds of metres. The deposits have a stratified structure of mm-cm thick yellow to dark brown laminated layers that are separated by mm-cm sized cavities. The individual layers and lamina have a highly porous microtexture of various filamentous particles. The dominating morphology in brown layers are 1 µm thick tubular and flat branching and twisted filaments resembling stalks of iron-oxidising bacteria like Gallionella sp and the newly described Mariprofundus ferrooxidans that show different degrees of encrustation. In light yellow lamina much thinner (200-300 nm), straight to curved fibers of uncertain origin dominate. The fibers are most frequently solid, but lamina of hollow fibers, sometimes partially to completely filled by an inner core, also occur. Microanalyses of the filaments and fibers demonstrate that the major component is Fe, but that they also contain significant amounts of Si, and highly variable concentrations of P. Hollow fibers have the highest P concentration and the core material frequently contain more P than Si.
Patchy accumulations of manganese are present within the majority of layers. The REE composition of the deposited material shows a similar pattern to that of the basaltic crust, indicating formation from low-temperature hydrothermal fluids, derived from interactions between the basaltic crust and circulating seawater. Sediment particles of diatoms, basaltic glass, and high-temperature vent minerals like baryte within lamina in the discrete layers show that they sequentially formed at the surface of the deposits at the different stages of growth. This, together with the architecture of alternating layers of biogenic filaments and cavities suggest that the formation of these deposits were controlled by flexible biofilms of Fe-oxidising microorganisms that developed at the interface between the reduced vent fluid and the surrounding oxidized seawater. Successive nucleation and precipitation of Fe-hydroxide and silica on the microbial stalks resulted in the formation of the mounds and chimneys.