Investigating the Impact of Banded Iron Formations on Subsurface Lithotrophy

Abstract:

Microbial communities in the deep continental biosphere are isolated within subsurface aquifers; consequently, they depend on lithotrophic metabolisms, deriving energy from substrates sourced from the rock surrounding them. Understanding and characterizing the ways in which local mineralogy supports a given community in the deep biosphere, then, is an essential component of understanding how life in the subsurface functions. We characterized the specific contributions of an Archaean banded iron formation to subsurface habitability, contrasting this iron-rich lithology with other lithologies found at the same site. This study focuses on archived cores from the Soudan Underground Mine State Park, located in the southernmost portion of the Canadian shield. The cores were drilled from a depth starting over 700 m below the surface and access an aquifer containing an isolated, anoxic brine. In regions of the core adjacent to subsurface fractures, thin sections were made; these thin sections were taken to a synchrotron-based X-ray fluorescence microprobe to characterize their elemental composition and mineralogy. µ-X-ray diffraction showed that the banded-iron formation contains an abundance of oxidized iron, in the form of hematite, as well as reduced iron, in the form of pyrite. Chlorites, with the potential to contain both mixed-valence iron and manganese were found in both the banded iron formation and in the surrounding schist. X-ray absorption spectroscopy and electron microprobe analysis will be used to characterize these chlorites in more detail, to better understand their potential to be reduced or oxidized. Tender X-ray fluorescence will be used to characterize sulfur-bearing minerals across the region. Understanding the mineralogy of this site in great detail enables us to model the energetics of microbial metabolic pathways; in conjunction with genomic data, these mineralogical redox relationships provide a comprehensive picture of life in the deep biosphere.