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    Biogeochemical Factors Impacting the Habitability of the Soudan Formation: Iron and Sulfur Cycling, Biofilms, and Fracture Surface Alteration
    (2023-12) Schuler, Christopher
    Fractured-rock networks in the continental crust constitute one of the largest microbial habitats on earth. These environments are generally low in oxygen and are cut off from surface-derived carbon; bacteria and archaea instead derive their energy from the oxidation and reduction of inorganic substrates sourced from rock-water interactions. Because of this, biogeochemical cycles governed by local mineralogy can have a major impact on the distribution and characteristics of subsurface organisms. Life in these environments mainly consists of surface-attached and biofilm-hosted microbial communities, and the impact that fine-scale mineralogical variation has on their abundance and composition is not yet fully understood. This dissertation characterizes the rock and groundwater chemistry of the Soudan formation, an Archean banded iron formation in northeastern Minnesota, to better understand both the rock-water interactions driving crustal biogeochemical cycling and the interactions between microbes and minerals in the subsurface. The differing rock and water chemistries of boreholes drilled into this formation are compared both to each other and to other sites where deep subsurface life has been studied. Mineral precipitate forming within these boreholes is collected and characterized; biofilm on the mineral precipitate surface is examined as well. Finally, thin sections are prepared from drill cores to identify both the bulk mineralogy of the Soudan formation and the alteration phases forming at fracture edges. The Soudan formation was found to contain isolated groundwaters with high Ca-Na-Cl salinity. Both the subsurface microbial community and hematite in the banded iron formation drive an active, cryptic S cycle with the potential to support both sulfur oxidizing and sulfate reducing microbes. In some fractures within the formation, Fe and S reduction lead to the precipitation of the iron sulfide minerals mackinawite and greigite. The mineral precipitates become covered in dense biofilms hosting morphologically diverse microbial communities. However, the extent of precipitate formation differs from borehole to borehole, showing that subsurface activity can differ extensively from fracture to fracture even within a geographically confined area. Fracture edges accessed via drill cores show signs of both the high-temperature deposition of chlorite minerals and the low-temperature precipitation of calcite. Carbonate minerals are undersaturated in most water samples from Soudan, though, suggesting that water chemistry at the fracture surface differs from that measured in well-mixed borehole effluent. Overall, this work describes Fe and S cycling in the Soudan formation in detail and illustrates that understanding fracture mineralogy at the micron scale is necessary to get a full picture of subsurface ecosystems.