Hydrologic Alteration and Enhanced Microbial Reductive Dissolution of Fe(III) (hydr)oxides Under Flow Conditions in Fe(III)-Rich Rocks: Contribution to Cave-Forming Processes

Abstract

Previous work demonstrated that microbial Fe(III)-reduction contributes to void formation, and potentially cave formation within Fe(III)-rich rocks, such as banded iron formation (BIF), iron ore and canga (a surficial duricrust), based on field observations and static batch cultures. Microbiological Fe(III) reduction is often limited when biogenic Fe(II) passivates further Fe(III) reduction, although subsurface groundwater flow and the export of biogenic Fe(II) could alleviate this passivation process, and thus accelerate cave formation. Given that static batch cultures are unlikely to reflect the dynamics of groundwater flow conditions in situ, we carried out comparative batch and column experiments to extend our understanding of the mass transport of iron and other solutes under flow conditions, and its effect on community structure dynamics and Fe(III)-reduction. A solution with chemistry approximating cave-associated porewater was amended with 5.0 mM lactate as a carbon source and added to columns packed with canga and inoculated with an assemblage of microorganisms associated with the interior of cave walls. Under anaerobic conditions, microbial Fe(III) reduction was enhanced in flow-through column incubations, compared to static batch incubations. During incubation, the microbial community profile in both batch culture and columns shifted from a Proteobacterial dominance to the Firmicutes, including Clostridiaceae, Peptococcaceae, and Veillonellaceae, the latter of which has not previously been shown to reduce Fe(III). The bacterial Fe(III) reduction altered the advective properties of canga-packed columns and enhanced permeability. Our results demonstrate that removing inhibitory Fe(II) via mimicking hydrologic flow of groundwater increases reduction rates and overall Fe-oxide dissolution, which in turn alters the hydrology of the Fe(III)-rich rocks. Our results also suggest that reductive weathering of Fe(III)-rich rocks such as canga, BIF, and iron ores may be more substantial than previously understood.

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

“…We have demonstrated enhanced porosity by FeRM activities in flow-through experiments of columns packed with crushed canga 33 , which matches the increased porosity demonstrated in the CT scan (Fig. 3 ).…”

“…This pulsed water delivery, which is consistent with rainfall patterns in the rainy season, increases FeRM activity and porosity, accelerating dissolution and the formation of Fe-depleted sub muros (Fig. 8 ) 33 . Over time, this weakening of the rock matrix causes a collapse of the Fe(III)-depleted sub muros material into the cave void (Fig.…”

“…( d ) During the next dry season, the lack of pulsed flow causes the Fe(II) to begin to accumulate on surfaces again, slowing Fe-reduction and increasing FeRM abundance ( d.i ). As this cycle repeats annually, the reduction of Fe(III) increases porosity (as demonstrated in 33 ), and eventually the Fe-oxide crust becomes unstable, and the collapse and Fe-reduction cycle repeats. Over time, repeated collapse leads to wall retreat and formation of the cave passages and observed morphology.…”

“…( c ) During the wet season, water enters the porous canga, introducing a pulsed flow that removes passivates (as demonstrated in Ref. 33 ). These conditions ( c.i ) also bring in surface-derived organic compounds (shown as CHO) and favor Fe-reduction.…”

“…While fermentation has not previously been associated with large-scale Fe-reduction, Parker et al 31 , 32 demonstrated that it dramatically accelerated Fe-reduction when compared to respiratory Fe-reduction. Nonetheless, the reduction experiments of Parker et al 31 , 32 were carried out in batch incubations, where passivation of Fe-oxides by Fe(II) and the closed system could reduce Fe-reduction rates and influence the drivers of FeRM metabolism 31 , 33 . To better reflect the conditions experienced in canga, we demonstrated that under flow conditions bacterial fermentation Fe-reduction led to an accelerated rate of dissolution, which enhanced permeability 33 .…”