MINERALOGICAL HETEROGENEITY AND THE DEEP CONTINENTAL BIOSPHERE

The metabolic backbone of many isolated crustal environments is thought to be lithotrophy, and as such redox-sensitive elements in the surrounding rock may exert a great deal of control on subsurface microbial communities. To better understand the contributions of local mineralogy to the habitability of the deep biosphere, we characterized select portions of rock extracted from boreholes in the Soudan Underground Mine State Park. Located in Minnesota’s Iron Range, the mine allows access to a deep saline aquifer within a 2.5 Ga banded iron formation in the southernmost portion of the Canadian Shield. Analyses of the water flowing from the legacy boreholes indicate an anoxic, highly reducing environment that has long been isolated from surface conditions. The cores from which rock was selected were obtained from depths ranging from 10 to 100 m into the borehole, almost 800 m below the surface. Petrographic thin sections prepared from areas of interest within the cores were analyzed via synchrotron-based X-ray fluorescence microprobe. The banded iron formations were found to contain a variety of iron minerals, largely a mix of hematite and iron-rich chlorites, with a great degree of spatial heterogeneity. µ-X-ray diffraction provided evidence of reduced iron sulfide deposits throughout. Additionally, investigations into the mineralogy abutting pores in the rock show areas with high manganese concentrations, roughly 150 µm in diameter. Tender X-ray fluorescence microprobe data will also provide insight into how sulfur is distributed throughout the rock and will enhance our understanding of the speciation of redox sensitive elements in the deep biosphere. Integrating geochemical investigations of the deep biosphere with previous research on microbial communities within deep aquifers will expand our understanding of how life exists under extremely energy-poor conditions. A comprehensive understanding of how rock, water, and microorganisms coexist in the subsurface is necessary to truly describe these environments.