THE ROLE OF MICROBES IN GOSSAN MINERAL TEXTURE FORMATION

Microbe-mineral interactions and biosignature preservation in sulfidic terrestrial oxidized ore bodies (gossans) are prime candidates for astrobiological studies, as gossans have been proposed as analog systems for some Martian environments. Understanding the mineralogic record of microbe-mineral interactions preserved in gossans improves our ability to develop criteria for biogenicity of potential Martian mineral textures. Previous researchers have identified both living cells and microbial fossils in gossans, preserved as hydrous ferric oxide (HFO) coated filaments, and recent research has identified these biogenic filaments in the Iron Mountain, California, surface gossan as smooth- and rough-walled filaments.

Mineralized filaments in goethitic rock are coated with a uniform layer of HFO with radiating mineral cross sections. Ones with smooth surfaces are 13-19μm in diameter, whereas ones with rough surfaces are 7.5-9.5μm in diameter. HFO mineralized filaments in mixed quartz-goethite rocks consist of 2.5-5.5μm diameter, bumpy-wall filaments with a uniform coating of ~1μm particles. Smooth mineral spheres (~11.5μm diameter) only form filaments where they can template bumpy-walls. Rough mineral spheres (~9.9μm diameter) form knurled-surfaces with interiors of concentric rings in cross section and never nucleate on bumpy-wall filaments. The high degree of bending and number of direction changes/mm of these filaments demonstrate that they are more likely to be mineralized microbes than abiotic fibers. The distribution of filaments observed in SEM is being placed into the context of sample porosity from x-ray tomography reconstructions to evaluate the role of water transport and bulk mineral precipitation on the textural variations in coated filaments. In addition, filament-forming microbes were identified with 16S rRNA gene cloning. Results will provide a model for microbial biosignature formation in oxidizing gossans that can be used to understand both the roles of microbial communities in gossan weathering on earth and the potential for biosignature formation in ancient rocks on earth and mars. Specifically, some filament textures are optically resolvable at the resolution provided by the Curiosity Rover’s Mars Hand Lens Imager.