Enhanced terrestrial Fe(II) mobilization identified through a novel mechanism of microbially driven cave formation in Fe(III)-rich rocks

Abstract

Most cave formation requires mass separation from a host rock in a process that operates outward from permeable pathways to create the cave void. Given the poor solubility of Fe(III) phases, such processes are insufficient to account for the significant iron formation caves (IFCs) seen in Brazilian banded iron formations (BIF) and associated rock. In this study we demonstrate that microbially-mediated reductive Fe(III) dissolution is solubilizing the poorly soluble Fe(III) phases to soluble Fe(II) in the anoxic zone behind cave walls. The resultant Fe(III)-depleted material (termed sub muros) is unable to maintain the structural integrity of the walls and repeated rounds of wall collapse lead to formation of the cave void in an active, measurable process. This mechanism may move significant quantities of Fe(II) into ground water and may help to explain the mechanism of BIF dissolution and REE enrichment in the generation of canga. The role of Fe(III) reducing microorganism and mass separation behind the walls (outward-in, rather than inward-out) is not only a novel mechanism of speleogenesis, but it also may identify a previously overlooked source of continental Fe that may have contributed to Archaean BIF formation.

The Influences of O2 Availability on the Microbial Activities and Fe Transformations in the Iron Formation Caves of the Southern Espinhaço Range, Brazil

“…This was supported by the identification of active FeRM within the caves [4,14] and the presence of a Fe(III)depleted material (termed sub muros) behind the IFC walls [14]. Sub muros was found behind an Fe(III)-(hydr)oxide crust, which forms as a result of oxidation of the outermost exposed Fe(II) in the sub muros and the precipitation of Fe(III)-(hydr)oxide phases to form the crust [14]. This Fe(III)-(hydr)oxide crust is thought to create an O 2 barrier, limiting O 2 intrusion into the sub muros and allowing Fe(III) reduction to proceed, with the resulting Fe(II) being removed via groundwater.…”

“…Recent evidence has demonstrated that Fe(III)-reducing microorganisms (FeRMs) may be responsible for the reduction in poorly soluble Fe(III)-oxides in the host rock to Fe(II), supported by organic carbon from the terrestrial surface [4,[8][9][10][11][12][13][14]. This was supported by the identification of active FeRM within the caves [4,14] and the presence of a Fe(III)depleted material (termed sub muros) behind the IFC walls [14].…”

“…Recent evidence has demonstrated that Fe(III)-reducing microorganisms (FeRMs) may be responsible for the reduction in poorly soluble Fe(III)-oxides in the host rock to Fe(II), supported by organic carbon from the terrestrial surface [4,[8][9][10][11][12][13][14]. This was supported by the identification of active FeRM within the caves [4,14] and the presence of a Fe(III)depleted material (termed sub muros) behind the IFC walls [14]. Sub muros was found behind an Fe(III)-(hydr)oxide crust, which forms as a result of oxidation of the outermost exposed Fe(II) in the sub muros and the precipitation of Fe(III)-(hydr)oxide phases to form the crust [14].…”

“…The collapse of the crust leads to the oxidation of Fe(II) in the freshly exposed sub muros, creating a new Fe(III)-oxide crust in place and facilitating another cycle of reductive solubilization of Fe(III). This novel cave genesis process has been termed exothenic biospeleogenesis [14].…”

Prospecting and evaluating antimicrobial resistance and metal tolerance of microorganisms isolated from cave soil in Serra de Carajás – PA

“…Research into cave microbiota has proven vital for the exploration of underground biodiversity, however current knowledge about the distribution of the diversity of microorganisms and their functionalities in natural cave environments in Brazil is incipient, especially regarding ferruginous caves (PARKER et al, 2022).…”