Microbial iron reduction in the formation of iron ore caves


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Traditionally, it was thought that caves formed only through the dissolution of rock by groundwater; however, the unusual chemistry of sulfuric acid caves suggested that microorganisms could also play an important role in cave formation. By studying this biogeochemistry, researchers discovered that caves could form through microbial oxidation of hydrogen sulfide gas, which accounted for as much as 25% of cave formation worldwide. They have hypothesized a new potential mechanism for microbially-driven cave formation, based on microbial respiration of iron(III) minerals in iron-rich rocks (known as banded iron formations; BIF). The identification of cave forming processes in BIF is significant as it dramatically expands the environments in which caves can form, which provide critical subterranean habitats for many rare and endangered animal species. There is also a strong correlation between the location of these BIF caves and the presence of iron ores of global economic significance, providing the source material for the production of steel. This correlation suggests that the cave forming processes may be linked to the creation of these important iron ore deposits. By gaining a better understanding of the microbial processes that form caves, it may be possible to selectively identify the processes that lead to ore formation. This may allow for more precisely targeted identification and mining of iron ore deposits, limiting the environmental impact that prospecting for such ores often generates. Investigators' preliminary research has demonstrated the presence of active Fe(III) reducing microbial communities within BIF caves, abundant dissolved Fe(II) in pore fluids, and textural evidence of reductive dissolution of Fe(III) phases. Together, these observations suggest that microbial Fe(III) reduction may be responsible for driving mass separation, while groundwater flow may be responsible for Fe(II) removal to create the cave voids. Investigators will therefore test the hypothesis that the activities of Fe(III) reducing microorganisms are responsible for the formation of iron ore caves. To test this hypothesis they will use an approach that integrates biogeochemistry, environmental microbiology, laboratory microcosms, kinetic studies of Fe(III) bioreduction, and field-scale empirical data. Models of Fe(III) reduction rates and Fe(II) transport will be used to integrate this empirical data with potential cave forming processes across a range of scales, from microscopic to regional. Together these data should allow them to constrain the mechanisms and rates of iron cave formation and determine the role that microbes play in iron cave speleogenesis.


Caves, Iron

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NSF award abstract