Microbial Biosignature Preservation at Iron Mountain, California

Physical biosignatures record the presence of life, even if that life is no longer in the environment. and can range from macroscopic fabrics (e.g. spring deposits and microbialites) to individual, microscopic microbial cells preserved in the rock record. Interpreting a mineralogic feature as a biosignature requires exploration of the controls on biosignature formation and preservation in a specific environment, including the presence of filamentous nucleation site, active mineral precipitation, and physical and geochemical similarity to biosignatures in other environments. To document how certain mineral structures record biosignatures in an iron-dominated system, I investigated how modern mineral filament biosignatures are formed and preserved in a geochemically restricted pipeline precipitate (Chapter 1) and how pre-modern mineral filament biosignatures are formed and preserved in an older, naturally weathered, massive sulfide deposit (gossan) (Chapter 2). Lastly, I evaluated the microbial community in the gossan as a source of filamentous nucleation sites for the formation of mineral filament biosignatures (Chapter 3). To investigate how modern biosignatures are formed and preserved, I systematically documented mineral filament biosignatures, microbial filaments, and other mineral textures in a geochemically restricted acid mine drainage (AMD) pipeline precipitate at Iron Mountain, CA. The pipe precipitate formed within months to years. Comparison of mineral textures and structures in the pipe precipitate to established biosignature criteria indicates that the mineral filaments are mineral-coated microbial filaments that serve as biosignatures and record the presence of microbial filaments. The identification of these mineralized filamentous features as biosignatures further improves the ability to identify other cryptic biologically-influenced structures detectable in the ancient and modern rock record. To evaluate how biosignatures are formed and preserved in an older gossan, I systematically documented mineral filament biosignatures, microbial filaments, and other mineral textures in the naturally weathered Iron Mountain, CA, gossan. The gossan rocks are many thousands of years old. Comparison of mineral textures and structures in the gossan rocks to established biosignature criteria indicates that the mineral filaments are mineral-coated microbial filaments that serve as biosignatures and record the presence of microbial filaments. Identification of mineral filaments as biosignatures supports the effort to recognize similar microbial fabrics in other environments on and beyond Earth, such as on Mars. Individual mineral filaments are below the camera resolution on Mars rovers, such as the Mars Hand Lens Imager (MAHLI) onboard the Mars Curiosity rover; however mineral filaments forming mat-like macroscopic textures are resolvable with the MAHLI camera. With additional investigation, mineral filament fabrics like these could be identified on Mars as similar to mineral filaments on Earth, and potentially biogenic. To investigate the gossan microbial community as a source of filamentous nucleation sites for the formation of mineral filament biosignatures, I characterized the molecular phylogeny of the microbial community in the Iron Mountain gossan using Sanger sequencing techniques. The results indicate that the microbial community is dominated by non-acidophilic soil- and rock-dwelling organisms. Few cloned organisms from the gossan were related to the acidophilic organisms commonly found in AMD environments and few clones were known filament forming organisms. These results suggest that the microbiome in the gossan is transitional between soils and AMD systems. This interpretation has repercussions for the understanding of acid mine water production and the associated environmental impacts, as moderately acidic regions adjacent to severe acid mine drainage do not necessarily contain a majority of acidophilic organisms and therefore may not provide significant contributions to AMD. The systematic characterization of environments where mineral filament structures form biosignatures further improves the ability to identify other cryptic biologically-influenced structures detectable in the ancient and modern rock record, on both Earth and Mars.