SRA

Embedded deep within northern Minnesota's expansive Iron Range, the Soudan Iron Mine (47°49'24? N, 92°14'14? W) transects massive veins of hematite and Archaean (2.7 Gy) banded iron formations, reaching a depth of 713 m (2,341 ft) below the surface. Now an actively maintained State Park serving to preserve Minnesota's oldest and deepest underground mine, Soudan provides year-round access to the deep terrestrial subsurface and the unique microbial communities it hosts. Before mining ceased in 1962, vertical and horizontal exploratory cores were taken at its lowest level (Level 27, 713 m depth) to track the iron formation. Today these boreholes act as conduits for anoxic, low flow, saline groundwater in an otherwise dry mine. Calcium chloride brines emanate from seeps with ionic strengths up to three times saltier than seawater, low oxidation-reduction potentials, circumneutral pH, and low concentrations of organic electron donors. Despite carbon limitation, anoxic brines contain reduced metals and continuously evolve methane gas, raising questions about the microbes responsible for primary production and carbon flux in this subsurface ecosystem.