Iron ore mining began at Soudan in 1882 as an open pit and proceeded underground near the turn of the 20th century. Cut-and-fill mining continued until U.S. Steel abruptly terminated operations in 1962. Shortly thereafter, the mine was donated to the State of Minnesota and has been open as a state park ever since, attracting thousands of visitors each year. Unlike deep-sea sites that can only be reached through organized expeditions costing millions of dollars, Soudan is a uniquely accessible window into the deep terrestrial subsurface.
Researchers from the University of Minnesota have been conducting fieldwork at Soudan since the early 2000s, when they demonstrated the first use of 454 pyrosequencing on environmental samples . In addition to housing high-energy physics research (http://www.soudan.umn.edu), Soudan's lowest level (Level 27, 713 m depth below surface) contains abandoned exploratory diamond drill holes. These fractures act as conduits that continuously produce anoxic, metal-rich calcium chloride brines with circumneutral pH but undetectable concentrations of organic carbon (L. Briscoe & B. Toner, unpublished data). Horizontal boreholes along the east drift of Level 27 give rise to magnificent iron "beards," shown at right, which result from both abiotic oxidation of Fe(II)-rich fluids upon contact with mineshaft air, as well as microbial Fe(II) oxidation.
Some vertical boreholes constantly evolve methane gas, and ongoing research focuses on what fraction, if any, of this gas is biogenic as well as the residence time of the fracture fluids. With sequencing support from DCO's Census of Deep Life, a University of Minnesota team is starting to unravel how microbial communities nonetheless survive in this deep terrestrial habitat. For example, amplicon and shotgun metagenomic sequencing revealed only one putatively methanogenic microbial population, Methanolobus, whose metabolism suggests that a currently unidentified a methylated carbon source drives methanogenesis. However, the community members involved in synthesizing potential C1 substrates are unclear.
In August 2016, a team of University of Minnesota researchers led by Dr. Jon Badalamenti (BioTechnology Institute) conducted field sampling across multiple levels of the Soudan Mine. In addition to collecting brines from Level 27, the team, which included Assistant Professor Cara Santelli & colleagues (Department of Earth Sciences), explored other levels of the mine for novel fungi that could be playing a role in metal redox transformations and/or sequestration, such as a copper-rich pool fed by groundwater intrusion on Level 10. With support from MnDRIVE and the Minnesota Environment and Natural Resources Trust Fund, ongoing work at Soudan seeks to understand the structure and function of native microbial and fungal populations, as well as their potential for biotechnological applications such as sorption and bioremediation of toxic metals.
From left to right: Carla Rosenfeld, Elizabeth Roepke, Kelly Duhn, Cara Santelli, and Jon Badalamenti
Field report and photos contributed by Jon Badalamenti and Cara Santelli.